M See messier numbers

McDonald Observatory Major US optical observatory 720 km (450 mi) west of Austin, Texas, in the Davis Mountains, on the twin peaks of Mount Locke (2070 m/ 6790ft) and Mount Fowlkes (1980 m/6500 ft). It is a facility of the University of Texas at Austin, and operates the 9.2-m (360-in.) hobby-eberly telescope and several smaller instruments, all equipped with state-of-the-art instrumentation. These include the 2.7-m (107-in) Harlan J. Smith Telescope, opened in 1968, and the 2.1-m (82-in.) Otto Struve Telescope, dating from 1939. The observatory has one of the first and most productive lunar ranging stations.

Mach, Ernst (1838-1916) Austrian physicist and philosopher. He was the first (1877) to discover the shock waves produced by projectiles moving faster than the speed of sound, and his name is best known in connection with the Mach number, the ratio of an object's velocity to the velocity of sound. His philosophy of science freed einstein from the theoretical restrictions imposed by Newtonian spacetime and helped him develop his general theory of relativity. Mach's principle holds that all the inertial properties of a piece of matter are in some way attributable to the influence of all the other matter in the Universe.

MACHO See massive compact halo object

McMath-Pierce Solar Telescope World's largest solar telescope, located at kitt peak national observatory, incorporating three separate optical instruments with apertures of 1.6 m (63 in.) for the main telescope and 0.9 m (36 in.) for the two auxiliaries. Unusually, these instruments have all-reflective optics with no windows, lenses or central obscuration, and produce a very high image quality. The McMath-Pierce is operated by the national solar observatory. Completed in 1962, it is used mainly for solar spectroscopy, polarimetry and imaging, but also for planetary work and observations of comets. It also provides a unique facility for monitoring the Earth's atmosphere.

McMathPierce Solar Telescope The main shaft of the McMathPierce Solar Telescope is 152 m (500 ft) long. Light is reflected from a heliostat down the shaft to a 1.5-m (5-ft) mirror, back up to a second mirror and then down to the underground observation room, to form an image of the Sun 85 cm (34 in) across.

Madler, Johann Heinrich (1794-1874) German astronomer who collaborated with amateur astronomer Wilhelm beer to produce Mappa selenographica (1837), the first truly accurate and comprehensive map of the Moon. They constructed their map by making numerous micrometer measurements of feature positions and sizes, using only a 3.75-inch (95-mm) refractor. They also produced the first map of Mars to show its albedo features (1830). From 1840 Madler directed Dorpat Observatory, Estonia, observing and cataloguing hundreds of double stars with the fine 9.6-inch (245-mm) refractor there.

Maffei Galaxies Two relatively nearby galaxies in Cassiopeia. Lying close on the sky to the plane of the Milky Way, they are almost completely obscured by galactic dust and can only be detected at red or infrared wavelengths. They were discovered in 1968 by the Italian astronomer Paolo Maffei (1926- ) as two infrared sources in line of sight close to the bright galaxy IC 1805. Their nature as faint, extended objects with the attributes of nearby galaxies raised the exciting possibility that they were newly discovered members of the local group.

Maffei 1 (RA 02h 36m.3 dec. + 5939') is now known to be a giant elliptical galaxy 4 million l.y. away on the edge of the Local Group. It has an absolute magnitude of 20.

Maffei 2 (RA 02h 41m.9 dec. + 5936') lies far beyond the Local Group, 20 million l.y. away, and is an Sb spiral galaxy.

Magellan national aeronautics and space administration (NASA) mission to map the planet venus at high resolution, using synthetic aperture radar (SAR). The spacecraft was launched from the Space Shuttle Atlantis on 1989 May 4 and inserted into a near-polar elliptical orbit around Venus, with a periapsis altitude of 294 km (183 mi), on 1990 August 10. Each mapping orbit typically imaged an area 20 km (12 mi) wide by 17,000 km (10,600 mi) long. The raw SAR data were then processed into image strips that were assembled into mosaics.

Magellan During the first phases of NASAs Magellan mission, its orbit around Venus was highly elliptical (blue), and it used radar to map almost the whole planet. After three years, the orbiter used aerobraking techniques to reach a lower, circular orbit (red); it then mapped Venus gravity field.

The mission was divided up into 'cycles'. Each cycle lasted 243 days, the time necessary for Venus to rotate once under the Magellan orbit. The first three cycles (1990 September to 1992 September) were dedicated to radar mapping of the surface. At the completion of this phase, 98% of the surface had been imaged at resolutions better than 100 m (330 ft), and many areas were imaged several times. The fourth and fifth cycles were mainly devoted to obtaining gravity data. During the final stages of the mission, scientists carried out a 'windmill' experiment to study the effects of atmospheric drag on the spacecraft.

Although its primary objectives were to map the surface of Venus and determine its topographic relief, Magellan also collected radar emissivity, radar reflectivity, slope and radio occultation data.

Magellan revealed an Earth-sized planet with no evidence of Earth-like plate tectonics. At least 85% of the surface was found to be covered with volcanic flows, with the remainder marked by highly deformed mountain belts. More than 1000 impact craters and 1100 volcanic features were discovered, as well as sinuous valleys and unique geological structures known as coronae and arachnoids. A strong correlation was found between the gravity field of Venus and the surface topography, suggesting that processes deep in the interior play a major role in influencing the distribution of highlands and lowlands. Radio contact with Magellan was lost on 1994 October 12. It burned away in Venus' atmosphere.

Magellanic Clouds Two small GALAXIES that are the nearest external galaxies to our own. Both clouds are easily visible to the naked eye, appearing like isolated offshoots of the MILKY WAY with apparent diameters of about 6 (the LARGE MAGELLANIC CLOUD) and 3 (the SMALL MAGELLANIC CLOUD). Their distances are estimated to be about 170,000 and 190,000 l.y. respectively. The clouds are sufficiently close to the Earth for detailed observations to be made of stars and nebulae; such observations are not possible for more distant galaxies. For example, the PERIODLUMINOSITY RELATIONSHIP for CEPHEID variables was first established in 1912 by Henrietta LEAVITT'S studies of the Small Magellanic Cloud.

Magellan Telescopes Major southern-hemisphere optical astronomy facility consisting of two identical 6.5-m (256-in.) telescopes at LAS CAMPANAS OBSERVATORY in Chile. Its construction was a collaboration between the CARNEGIE OBSERVATORIES (which operate it) and other US universities, including the University of Arizona, whose STEWARD OBSERVATOR Mirror Laboratory fabricated the unusual optics. The spun-cast primary mirrors are steeply curved, with a FOCAL RATIO of only f?1.25. The two telescopes can be used independently or in combination, and are named after the astronomer Walter BAADE and the benefactor Landon T. Clay (1926 ). They were completed in 2000 and 2002.

Maginus Lunar crater (50S 60W), 177 km (110 mi) in diameter, with rim components reaching 4270 m (14,000 ft) above its floor. An ancient crater, it has been severely modified by impact erosion, having walls nearly level with the surrounding terrain. The rim is difficult to identify in places, while the EJECTA blanket has been completely eroded away. The walls are deeply incised by impact scars. The floor contains several small central peak remnants and later impact craters.

magma Molten rock, predominantly silicate in composition; upon solidification it yields IGNEOUS ROCKS.

magnetic field One of the fundamental forces of nature. Magnetic flux density, symbol B, is measured in units of tesla (T). Magnetic field strength, H, is related to magnetic flux density by the magnetic permeability, j, of the medium containing the magnetic field, with B = j H. It is measured in A/m. On Earth a weak magnetic field exists capable of swinging a compass. In astronomical objects the field can be more than 1012 times stronger, and it then can control the motions of gases and the shape of objects. Planetary magnetic fields are often discussed in units of Gauss (G); 1 G = 0.0001 T.

magnetic monopole Elementary particle that contains only one pole of a magnetic field. The existence of magnetic monopoles was predicted by symmetry considerations in elementary particle models and they were thought to be a single pole of a magnetic field. The fact that magnetic monopoles have never been detected in the Universe presented a minor problem for the BIG BANG THEORY. The theory of INFLATION, if correct, eases the constraints on the detection of magnetic monopoles and explains why they have yet to be seen.

magnetic reconnection Mechanism for the exchange of energy from a magnetic field to a plasma, quoted as a possible acceleration mechanism in a wide variety of astrophysical phenomena, from planetary MAGNETOSPHERES, SOLAR FLARES and CORONAL MASS EJECTIONS, to ACCRETION DISKS and ACTIVE GALACTIC NUCLEI. Two regions of space or astrophysical plasma containing magnetic fields with different orientations are generally separated by a thin current layer; for example, the magnetopause separates a planetary magnetosphere from the SOLAR WIND. The two regions are thus not generally able to mix. Magnetic reconnection is thought to occur on such a current layer if the fields on either side are close to antiparallel. Under these circumstances, the fields on either side break and reconnect with their counterparts on the other side of the boundary. A magnetic link is created through the boundary, which allows the mixing of plasmas from each side along the reconnected field lines. In addition, magnetic energy is liberated from the system, and appears as thermal and kinetic energy of the plasmas on the reconnected field lines. The actual physical process involved in magnetic reconnection is not well understood. However, observations of plasmas at energies compatible with the reconnection process are widespread within the terrestrial magnetosphere, providing strong circumstantial evidence of its occurrence within astrophysical plasmas.

magnetic star Variety of star that has a strong magnetic field, well beyond the typical level of the Sun. The most common examples are the magnetic AP STARS, which have odd chemical compositions (enhanced silicon, strontium and rare earths) and starspots; their related fields run from a few hundred into the tens of thousands of gauss (the Sun's field has a strength of 20001000 gauss over active regions).

Two classes of degenerate stars are strongly magnetic. A small percentage of DA (hydrogen-rich) WHITE DWARFS have fields in the megagauss (or even hundreds of mega-gauss) range. These stars tend to be relatively massive and are suspected to be the evolved descendants of the Ap stars. NEUTRON STARS are by their nature even more magnetic. Typical field strengths for newly born pulsars are a million million (1012) gauss. A small percentage of neutron stars, suspected to derive from higher mass super-novae, the 'magnetars', have fields a hundred times greater, up to 1014 gauss.

magnetohydrodynamics (MHD) Study of the interactions between a PLASMA and a MAGNETIC FIELD. The science of magnetohydrodynamics was founded largely by the pioneering work of Hannes Olof Gosta ALFVEN. A plasma is an ionized gas. It contains free electrons and positively charged nuclei. In most astronomical plasmas, the nuclei will be predominantly free protons. A plasma may also contain a proportion of neutral atoms, but their behaviour, in particular their motions, will be governed by that of the plasma as a whole. The plasma is highly electrically conducting so that if it moves across a magnetic field, an electric current is induced within it in exactly the same way that a current is induced in the copper wire of a dynamo moving through its magnetic field. The electric current in the plasma sets up its own magnetic field, which interacts with the original magnetic field in such a way that the relative motion between the field and plasma is opposed. The interaction may be sufficient to halt almost completely the relative motion of the plasma across the magnetic field lines. The magnetic field is then said to be 'frozen-in' to the plasma. The movement of the plasma is then constrained to follow the magnetic field lines or to move with the magnetic field if it should change; or the magnetic field is dragged with the plasma as it moves.

Most of the material in the Universe is in the form of plasmas; therefore, magnetohydrodynamical behaviour is involved wherever there is also a magnetic field. On the Sun, for example, the shapes of both quiescent and active PROMINENCES clearly show linkage to magnetic fields; SUNSPOTS are cool and appear dark because the magnetic field within them inhibits convective transfer of heat; solar FLARES arise through magnetic field-plasma interactions, although there is still some uncertainty in understanding the exact process. There is also some uncertainty in exactly how the solar sunspot cycle arises, but it is again clearly linked to magnetic field interactions. Other magnetohy-drodynamic behaviour occurs in the formation of planetary MAGNETOSPHERES, within ACCRETION DISKS and STELLAR WINDS. Oscillations in the plasma and/or magnetic field produce ALFVEN WAVES. These move at the where B is the magnetic field strength, p is the plasma density and u is the magnetic permeability of the plasma. Such waves may be involved in transferring energy out to the solar CORONA and are thus involved in maintaining its enormously high temperature layer of electrical current known as the MAGNETOPAUSE. In the solar direction, the magnetopause occurs at the point where the pressure associated with the planetary magnetic field balances the pressure of the impinging solar wind flow. As the solar wind pressure is highly variable, these boundaries expand outwards and contract inwards in response to these variations. In the terrestrial system, however, the subsolar points on the bow shock and magnetopause stand at average distances of 15 Earth radii (RE) and 11 RE towards the Sun respectively. On the nightside, the deflected solar wind flow effectively creates a vacuum, which the planetary magnetic field expands to fill. MAGNETIC RECONNECTION occurring on the dayside magnetopause results in magnetic flux being peeled from the dayside and added to the nightside region. These two effects create the magnetospheric tail, or magnetotail. In the terrestrial case, this feature is approximately cylindrical, with a diameter of order 50-60 RE and a length that extends many thousands of Earth radii.

A planetary magnetosphere contains a number of regions with distinct plasma or energetic charged particle populations. Closest to the planet, these regions include the RADIATION BELTS (the VAN ALLEN BELTS at the Earth) and the PLASMASPHERE. The radiation belts contain trapped energetic particles captured from the solar wind or originating from collisions between upper atmosphere atoms and high-energy COSMIC RAYS. The plasmasphere is populated by the polar wind, an upflow of plasma from the upper IONOSPHERE. Farther out, the PLASMA SHEET occupies the central portion of the magnetotail and contains a hot plasma of solar wind origin. Each of these regions, indeed the magnetosphere as a whole, can be extremely dynamic, especially during times of enhanced solar wind-magnetosphere interaction. Magnetic storms and MAGNETOSPHERIC SUBSTORMS are disturbances that cause global restructuring of these magnetospheric regions and significant energization of the particle populations within them. In addition, there is significant coupling between the magnetosphere and the ionosphere. At the Earth this coupling is achieved by the flow of large-scale electrical currents from the magnetosphere through the polar ionosphere, which results in heating of the ionosphere, particularly at disturbed times. It is also associated with spectacular displays of the AURORA.

Magellanic Clouds The Large Magellanic Cloud in Dorado is the second closest external galaxy. The LMC and SMC are joined to the Milky Way by the Magellanic Stream, an arc of neutral hydrogen gas thought to be dragged from the smaller galaxies through tidal interactions.

magnetopause Boundary separating a planetary MAGNETOSPHERE from the external SOLAR WIND. The magnetopause is a thin sheet of electrical current, only a few thousand kilometres thick. It is a crucial boundary at which the exchange of energy and matter between the solar wind and the magnetosphere occurs. This coupling is particularly strong at times when the solar wind magnetic field is directed antiparallel to the planetary magnetic field just inside the boundary, a situation probably caused by MAGNETIC RECONNECTION at the magnetopause. See also SPACE WEATHER

magnetosphere Region surrounding a magnetized planet that is occupied by the planetary magnetic field; it acts to control the structure and dynamics of plasma populations and ionized particles within it. A magnetosphere can be considered as the magnetic sphere of influence of the planet. The size of the magnetosphere is controlled by the interaction of the planetary field with the SOLAR WIND, which compresses the upstream side while dragging the downwind side out into an extended magnetotail, resembling a windsock. The magnetosphere represents an obstacle to the supersonic solar wind flow from the Sun, with the solar wind having to flow around the magnetosphere. To do this, the wind is slowed, deflected and heated at a BOW SHOCK, standing in the flow upstream of the magnetosphere. The shocked solar wind plasma and associated magnetic field downstream of the bow shock is known as a magnetosheath.

magnetosphere The Earths moving iron core gives it an active magnetic field that extends well beyond the planet. The arrows indicate the direction of the magnetic field. The magnetosphere protects us from solar radiation.

magnetosphere The Earth's moving iron core gives it an active magnetic field that extends well beyond the planet. The arrows indicate the direction of the magnetic field. The magnetosphere protects us from solar radiation.

Each of the other magnetized planets, Mercury, Jupiter, Saturn, Uranus and Neptune, is known to possess a magnetosphere. The magnetic field of Mercury is relatively weak (only one-hundredth of the terrestrial field), and thus it has a very small magnetosphere. However, it still provides a definite interaction with the solar wind, with a well-defined bow shock and magnetopause. mariner 10 observations suggest that magnetospheric substorms occur on timescales of minutes, rather than over one to two hours as occurs at Earth. Mercury has no significant atmosphere or ionosphere, although, curiously, these regions are known to be important for sub-storms in the terrestrial magnetosphere. The planet itself also occupies a large fraction of the magnetosphere, such that the regions that would constitute the radiation belts lie below the surface.

The Jovian magnetosphere is the largest in the Solar System: if it could be visualized from the Earth, it would be seen to subtend a cross-section equivalent to the Moon's diameter in the sky. It is extremely variable in size, extending to distances of between 50 and 100 Rj in the direction of the Sun as a result of the changes in the solar wind. The magnetotail of Jupiter is so large that it extends over more than 5 AU and can interact with Saturn. The high-energy particles in Jupiter's magnetic field also form radiation belts, which are 1000 times more intense than those of the Earth. Seven of Jupiter's innermost satellites and the rings are in this hostile region. These satellites are constantly bombarded by the high-energy particles, which erode the surfaces and alter their chemistry. In addition, the structure of the Jovian magnetosphere is influenced by the moon io, which adds plasma to the inner regions by volcanic processes, creating a cloud of sodium, potassium and magnesium, known as the Io torus, stretching around Io's orbit. The rapid rotation of Jupiter accelerates plasma from the torus to high velocities, and the resulting centrifugal force distorts the magnetosphere outwards, in its equatorial regions, to form the Jovian magnetodisk. Io and Jupiter are also connected by a magnetic flux tube, which carries a current of order 5 million amps across a potential difference of 400,000 volts. This electrical energy plays an important role in the local heating of the surface of Io and the generation of the volcanic activity. Jupiter emits waves at radio frequencies, corresponding to deca-metric and decimetric regions of the electromagnetic spectrum, which are greater in intensity than any extraterrestrial source other than the Sun. The interactions between the charged particles and the Jovian atmosphere produces polar aurorae, which are about 60 times brighter than their terrestrial counterparts.

The Saturnian magnetosphere is intermediate in size between those of Jupiter and the Earth, and, similar to those, it is sensitive to variations of the solar wind. The voyager 1 spacecraft crossed the bow shock five times, at distances ranging from 26.1 to 22.7 Rs. A major difference between the Saturnian magnetosphere and that of Jupiter is the concentration of highly electrically charged dust in the ring plane. It is possible that this material is the source of the high-energy discharges in the rings. At Saturn the large system of rings and many of the satellites have the effect of sweeping a path through the charged particle environment. The rings and the satellites are very efficient in absorbing the protons in the magnetosphere. The Saturnian magnetosphere is a major target of the cassini mission, due to arrive at Saturn in 2004.

Uranus has an unusual magnetosphere in which the dipole magnetic field axis is tilted by an angle of 60 with respect to the planetary rotation axis, with a centre that is offset by 0.3 Uranus radii. At the time of the Voyager 2 encounter the Uranian magnetosphere extended 18 Rd, and the magnetotail had a radius of 42 Rd at a distance of 67 Rd. The extreme tilt of the magnetic axis, combined with the tilt of the rotational axis, causes the field lines in the Uranian magnetotail to be wound up into a corkscrew configuration. Voyager 2 found radiation belts at Uranus of intensity similar to those at Saturn, although they differ in composition. The moons Miranda and Ariel modulate the flux of the trapped radiation. It is possible that this radiation environment is responsible for the creation of the unusually dark material found on the satellites and rings of Uranus. There are also radio emissions from the magnetosphere of Uranus, although weaker than those from Saturn. A further magnetospheric interaction with the atmosphere, which is not known on Earth, is called the electroglow. This is only seen on the dayside of Uranus, where the aurorae are not observed.

Neptune's magnetic field is tilted 47 from the planet's rotation axis, and is offset at least 0.55 Rn from the planet's centre. As a result of this unusual arrangement, Neptune's magnetosphere goes through dramatic changes as the planet rotates in the solar wind. During each rotation, the magnetosphere moves from an orientation similar to that at Earth to one in which the south pole points head on into the solar wind, and then back again. The pole-on orientation of the magnetic field was observed when Voyager 2 flew into the southern cusp of Neptune's magnetosphere in 1989. The spacecraft subsequently remained in the magnetosphere long enough to observe two of these rotation cycles. Voyager 2 detected periodic radio emissions generated in the magnetosphere. These waves provided the first accurate estimation of the rotation rate of the planet's interior. Voyager 2 also detected weak aurorae in Neptune's atmosphere. Because of Neptune's complex magnetic field, the aurorae occur over wide regions of the planet, not just near the planet's magnetic poles.

Finally, it is possible for unmagnetized bodies to exhibit magnetospheric-like behaviour. For example, the ionosphere of Venus and cometary comae also provide obstacles to the solar wind flow. The slowing of the flow in the vicinity of the body creates a draping of the solar wind magnetic field around the obstacle, thus creating an induced magnetotail. Regions of remnant surface magnetization on the Moon and Mars also interact with the solar wind on a small scale, thus creating the 'mini-magnetospheres' detected, for example, by the Mars Global Surveyor and Lunar Prospector missions.

magnetospheric substorm Major reconfiguration of the terrestrial magnetosphere resulting in an explosive release of energy stored in the magnetotail and subsequent deposition of this energy into the polar ionosphere, radiation belts and downstream solar wind.

The 'growth phase' of a magnetospheric substorm begins with a period of enhanced coupling between the solar wind and the magnetosphere, which may last several hours. magnetic reconnection at the dayside magne-topause results in a build-up of magnetic energy within the magnetotail, reaching levels of 1016-1017 Joules. A poorly understood instability occurring within the magnetotail plasma sheet results in this energy being rapidly released at the onset of the substorm 'expansion phase'. This energy is converted into fast plasma jets within the magnetotail plasma sheet and is also dissipated by a shearing off of a large portion of the tail plasma sheet to form a plasmoid, which is ejected away from the Earth and out into the solar wind. In the near-Earth region, energy is deposited as an injection of energetic particles into the radiation belts. A large, substorm-associated current connects the magnetosphere to the ionosphere, and flows across the ionosphere at auroral latitudes. This current dissipates energy by heating the ionosphere, and it is also associated with a major expansion of the nightside aurora, both in the poleward and east-west directions. Spectacular high-latitude auroral displays are thus observed from the ground during substorms. Typically, after about 30-60 minutes activity dies away, and the magnetosphere and ionospheric systems relax to their pre-substorm state during the 'recovery phase'. Magnetospheric substorms are thought to occur at a much faster rate at Mercury, and probably also occur in the other magnetized planets.

magnification Factor by which an optical system increases the apparent size of an object. It is a measure of the increase in the angle subtended by the object at the observer's eye. It is usually designated by 'X' used before or after the magnification power; for example, eight times magnification would be written as X8 or 8X. In the specification of a pair of binoculars two numbers are usually given, separated by X; the first is the magnification. For example, 10X50 binoculars have a magnification of ten times and an entrance aperture 50 mm in diameter.

The magnification of a telescope can be calculated by dividing the focal length of the objective by the focal length of the eyepiece. A telescope with a focal length of 1000 mm would give a magnification of 50X when used with an eyepiece of focal length 20 mm.

magnitude Brightness of an astronomical body. apparent magnitude 1 is exactly 100 times brighter than magnitude 6, each magnitude being 2.512 times brighter than the next. Magnitudes brighter than 0 are minus figures, thus Sirius is 1.4, and the Sun is 26.8. The faintest objects yet photographed are about magnitude 26. See also absolute magnitude; bolometric magnitude; photographic magnitude; photovisual magnitude; visual magnitude

main-belt asteroid One of the many asteroids that occupy the region of space between Mars and Jupiter. Most such bodies are dynamically stable, and they are believed to have remained in that part of the Solar System since their formation more than 4.5 billion years ago. The main-belt asteroids represent chunks of solid material, mostly rock and metal, which failed to form a major planet because of the effect of the gravitational perturbations of Jupiter. Their total remnant mass is small - only about 5% the mass of the Moon.

main sequence Region of the hertzsprung-russell diagram where most stars are found. It runs diagonally from top left (high temperature, high luminosity stars) to bottom right (low temperature, low luminosity stars). Stars spend most of their lifetime on the main sequence, and a main-sequence star is one in which energy is primarily produced from the fusion of hydrogen into helium in its core.

The original line formed by stars shortly after their ignition is called the zero-age main sequence. As compositional changes alter the internal structures of these stars, the points that represent them on the HR diagram will move away from the main sequence and trace out an evolutionary track on the diagram (see also stellar evolution).

Main-sequence stars are the most common type of star in the galaxy: they constitute some 90% of the approximately 1011 stars in the Galaxy and contribute about 60% of its total mass. From a theoretical point of view mainsequence stars are common because stars spend most of their evolutionary lives on the main sequence.

The time for which a star remains on the main sequence is related to the ratio of its mass to its luminosity. Using the mass-luminosity relation, this gives rough main-sequence lifetimes proportional to M3.7 for solar-type stars and M0.6 for very high-mass stars (where M is the mass of the star). Very low-mass stars are fully convec-tive and can continue core hydrogen fusion for much longer than its mass-luminosity ratio would suggest, giving a star of mass one tenth of a solar mass a main-sequence lifetime of around 1013 years.

The internal structure of a main-sequence star consists of a core depleted in hydrogen (to a degree determined by the time it has spent on the main sequence; that is, the time since nuclear reactions first started in its interior), surrounded by a hydrogen-rich envelope. In stars heavier than the Sun the principal nuclear reaction network is the carbon-nitrogen-oxygen cycle and the core is mixed by convection. In lower-mass stars the nuclear energy generation comes from the proton-proton reaction and there is no convection in the core. An outer convection zone, extending in from the surface of the star, increases in depth for lower-mass stars. For stars of spectral type A0 (about 3 solar masses) there is little or no convective envelope; at the mass of the Sun the convection zone reaches in about a quarter of the way to the centre; at about 0.3 solar mass the zone reaches the centre.

The relatively short lifetimes of the hottest main-sequence stars result in their being very rare in the Galaxy, whereas the lifetimes of the lowest mass (spectral type K and M or red dwarfs) are so long that none has evolved from the main sequence since the Galaxy was first formed. This strong lifetime dependence, together with the fact that the processes of star formation produce far more low-mass than high-mass stars, results in the lower-mass dwarfs being much more populous.

Most main-sequence stars have the same abundances of hydrogen and helium as the Sun, and have abundances of the heavier elements which are within a factor of two of those in the Sun. For stars with masses less than 0.08 times that of the Sun, the core temperatures do not become high enough to start hydrogen-burning nuclear reactions; as a result the main sequence terminates at stars with surface temperatures of about 2500 K. However, lower-mass stars are formed, probably in even greater numbers than the M red dwarfs, and become brown dwarfs.

An upper limit to the masses of main-sequence stars is determined by the increase of radiation pressure in their interiors: above about 150 solar masses the outward force exerted by radiation exceeds the attractive force of gravity, preventing such a star from being formed, or making it unstable with a short lifetime if it does form (see also stellar mass). At various places along the main sequence variability of luminosity is found, such as in Beta Cephei, Delta Scuti, oscillating Ap and flare stars (see also variable stars).

As well as the intrinsic properties of main-sequence stars, their space velocities show some interesting correlations with other stellar properties. The dispersion of their velocities increases towards lower masses. This is a consequence of the increase in average age of stars at lower masses: stars of all masses are probably formed with approximately the same dispersion in space velocities, but the mechanisms that act to increase the dispersion have longer to work on those stars that remain as main-sequence stars for the longest time. Thus O and B stars evolve off the main sequence before their velocity dispersions have changed much from their initial values, but among the M main-sequence stars there is a spread of ages from that of the Galaxy itself down to the most recent ones formed. Over long periods of time, encounters with massive interstellar clouds (and, to a lesser extent, other stars) change the orbits of stars in the Galaxy, increasing their velocity dispersion. Thus the oldest M main-sequence stars have greatly increased dispersions. See also high-velocity stars; runaway stars; stellar populations

major planet Name given to the nine main planetary members of the Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The term distinguishes them from the asteroids or minor planets. There is some debate as to whether Pluto, because of its size, warrants classification as a major planet.

Maksutov, Dmitri Dmitrievich (1896-1964) Soviet optician and telescope-maker, born in the Ukraine, who designed the type of reflecting telescope now named after him. He organized a laboratory of astronomical optics at the State Optical Institute in Leningrad (now St Petersburg), where he designed and then built (1941) the first of several maksutov telescopes. The meniscus-shaped optical surface employed by Maksutov was later used to build ultra-fast, ultra-wide-angle systems. Maksutov also was a principal designer of the special astrophysical observatory's 6-m (236-in.) reflector, at one time the world's largest telescope.

Maksutov telescope Modification of the schmidt telescope design, using a spherical corrector plate whose convex side faces the primary mirror. It was devised independently by Albert Bouwers (1893-1972) in 1940 and Dmitri maksutov in 1944. The design has the advantage of being cheap and easy to produce, making it popular for short-tube amateur telescopes. Silvering of a spot in the centre of the corrector plate allows visual and photographic use in a cassegrain telescope configuration. Maksutov telescopes are free from coma and astigmatism, and almost free from chromatic aberration, while their enclosed tubes obviate problems with internal air currents.

mantle Intermediate region of a planetary body that has experienced differentiation; the mantle is the volume between the core and crust. Earth's mantle extends from the base of the crust, at depths of a few tens of kilometres, to the the core-mantle boundary at about 2700 km (1700 mi); it comprises about two thirds of the mass of the planet. The mantle is divided into layers, which increase in density with depth, probably due to changes in composition and pressure-induced changes in crystal structure. Although it is mostly solid, material in Earth's mantle moves slowly (a few centimetres per year) because of convection driven by internal heat; it is this movement that drives continental drift. The other terrestrial planets and the Moon also have silicate mantles.

The term mantle is also used for the thick layer of ice that overlies the core of a differentiated icy body.

many-body problem (n-body problem) Problem in celestial mechanics of finding how a number (n) of bodies move under the influence of their mutual gravitational attraction. The nine planets plus the Sun form a ten-body problem, which is far too complicated to tackle using traditional analytical methods. Instead, the problem of determining Saturn's orbit can be treated as a series of three-body problems: Saturn orbiting the Sun perturbed by Jupiter, Saturn orbiting the Sun perturbed by Uranus, and so on. One instance in which nine bodies (Pluto excluded) are considered together is in the determination of the secular perturbations of their orbits. This problem is simplified by ignoring the rapid longitude motions of the planets, and regarding all eccentricities and inclinations as small. With the advent of powerful computers it is now perfectly feasible to tackle the ten-body problem by numerical integration of their orbits, and this is the way that the current planetary ephemerides are generated.

Maraldi, Giacomo Filippo (1665-1729) Italian cartographer and astronomer, also known as Jacques Philippe, who worked at Paris Observatory (1687-1718). The nephew of Giovanni Domenico cassini, he collaborated (1700-1718) with Jacques cassini to determine the Paris longitudinal meridian. Maraldi was also an astute planetary observer. In 1704 he noticed the Martian polar caps and later mapped their variations, concluding that they were made of some kind of ice. He is known to historians as Maraldi I to distinguish him from Maraldi II - his nephew, the astronomer Giovanni Domenico Maraldi (1709-88), also known as Jean Dominique.

mare Dark plain on the moon; the name comes from the Latin meaning 'sea', and refers to the belief that the dark patches on the Moon were ancient oceans. The maria are now known to represent lava flows. They stand as one of the major geologic units of the Moon, the other one being the highlands. Because the maria and highlands have a different mineral composition, they stand out from one another: the highlands are light, while the maria are dark because of their lower reflectance of sunlight.

Lunar lava formed in the Moon's mantle, where radioactive elements decayed producing heat. When that heat reached a certain level, it melted the surrounding rocks (olivine, pyroxene), producing magmas that were low in silica but high in magnesium and iron (called 'mafic'). These magmas, lower in density than the surrounding rocks, began to rise towards the surface. Upon nearing the crust, the magmas tracked up the deep faults that had been created by large impacts.

The largest impacts on the Moon produced basins, which are immense structures, generally 300-1200 km (190-750 mi) in size, characterized by having two or more ring-like walls surrounding a flat central region. More importantly, these impacts deeply fractured the bedrock, thus providing conduits for ascending magmas and topographically low regions for lavas to pond. Thus, the maria are generally found in basins, and are usually restrained by one of the basin's rings. The extent of flooding was variable, however, and so produced a variety of shapes for the final basin-mare form: (1) the lava may have been restrained by a basin ring (for example, the ring around Mare crisium); (2) the lavas may have flooded over an inner ring(s) producing a 'circular' mare ridge system; and (3) the lavas may have broken through the outer ring to flood adjacent low-lying areas (for example, Mare imbrium, Mare serenitatis).

The set of features that is especially associated with basin rings is the circular mare ridge system, within and roughly concentric to a basin wall. When lava floods a basin, it subsides by compaction and mass loading. As the lava sags, it generates compressive forces that are expressed where the lava thins, such as over basin rims, and there the mare ridges form. Thus, these circular mare ridges mark the underlying basin rings, and they are often the only method available for identifying their location. Examples are visible in Mare Serenitatis and Mare HUMORUM. Significantly, the lavas failed to cover the highest peaks in two of the Imbrium Basin's inner rings (for example, Mons Piton and Mons PICO). Thus, here the inner rings are marked by both mare ridges and individual peaks.

One other volcanic feature often associated with the maria are dark mantling deposits. These are considerably darker areas located around the edges of certain maria. Apollo 17 visited one such site in Mare Sereni-tatis, where astronauts found glass beads in the regolith. This kind of glass is produced by a process called fire fountaining, which occurs when magma containing dissolved gasses ascends. As the confining pressure is released, the gasses come out of solution. If there is enough gas in the fluid magma, it will both disrupt the magma and act as a propellant, lobbing magma droplets high above the surface. Here the droplets rapidly cool, forming a volcanic ash composed of small glass beads. As the lunar variety of glass is high in certain metals (such as titanium), it assumes a dark colour.

Maria Mitchell Observatory Small observatory on Nantucket Island, off the coast of Massachusetts, founded in 1902 to commemorate Maria MITCHELL, the first woman professor of astronomy in the United States. It is now operated by the Maria Mitchell Association, which aims to increase knowledge and public awareness of the Universe and the natural world. The Association supports both public astronomy and astronomical research, and since 1997 has presented the annual Maria Mitchell Women in Science Award.

marine chronometer Mechanical clock capable of keeping time accurately for long periods on board a small ship at sea. While latitude is easily found by observing the Sun or stars, finding longitude at sea requires an accurate knowledge of the time. Although land-based pendulum clocks had achieved the necessary precision by the 17th century, they could not be used at sea because the motions of a ship disturbed the pendulum too much, not to mention variations in temperature and humidity.

In 1714 the British Parliament passed the Longitude Act, which offered a prize of 20,000 for a satisfactory solution to this problem. A useful marine chronometer which depended on a balance-wheel escapement was ultimately devised by John HARRISON. Thereafter, the development of chronometers proceeded rapidly. Chronometers remained essential until the general availability of reliable radio time signals in the 1920s and 1930s. Nowadays, most practical navigation makes use of the GLOBAL POSITIONING SYSTEM (GPS) satellites.

Mariner spacecraft Series of ten US planetary probes, of which seven were successful. Mariner 2 was the first probe to reach VENUS (1962). Mariner 4 achieved the first successful flyby of MARS, sending back pictures of a cratered surface (1964). Mariner 5 made a close flyby of Venus in 1967. Mariners 6 and 7 extended the mapping of Mars, including the south polar region (1969). Mariner 9 went into orbit around Mars, becoming the first spacecraft to be placed in orbit around another planet (1971). Mariner 10 flew past Venus before becoming the first spacecraft to study MERCURY.

Marius, Simon (1570-1624) German mathematician and astronomer whose career was characterized by controversy. In 1608 he learned of Hans LIPPERSHEY's design for a telescope and quickly made one from spectacle lenses. He was the first to use a telescope to observe the ANDROMEDA GALAXY (1612), but could not resolve its spiral structure. In 1614 he claimed to have discovered the four brightest satellites of Jupiter in 1609 November. Because of the five-year delay, he was accused of falsifying his observations by GALILEO, whom the scientific community had credited for this discovery. However, reliable records show that Marius observed the Galilean moons at least as early as December 1610.

Markab The star a Pegasi, visual mag. 2.49, distance 140 l.y., spectral type A0 III. Its name is the Arabic word for 'saddle'.

Markarian galaxy GALAXY found to show excess ultraviolet radiation in a survey carried out at the Byurakan Observatory under the direction of the Armenian astronomer Benjamin Eghishe Markarian (1913-85). Markarian galaxies included the first large examples of SEYFERT GALAXIES, as well as many STARBURST GALAXIES and some QUASISTELLAR OBJECTS and BL LACERTAE OBJECTS.

Mars Fourth major planet from the Sun, popularly known as the Red Planet. Mars is intermediate in size between the Earth and Mercury. Mars' coloration is a function of widespread regions of reddish dust, which on occasions may be raised high into the tenuous Martian atmosphere by winds. A great dust storm was observed at close quarters in 1971 by the American spacecraft MARINER 9 as it approached the planet and eventually went into orbit around it. This storm reached global proportions, taking months to abate. On the surface the dust collects into vast sand-sheets and dune fields.

Mars The atmosphere on Mars is extremely tenuous. This is clearly shown in this truecolour Mars Pathfinder image of the Martian sunset where there is very little refraction of light

Telescopically, Mars shows bright regions at the poles, sometimes bright areas at the LIMB or TERMINATOR, and various dark markings. Most early observers believed the dark features to be seas, but when, in the 19th century, it became apparent that the atmosphere was too tenuous for this to be the case, the consensus view became that they were dried-up sea beds supporting lowly vegetation. Together with the Martian CANALS theory, this supposition has been long rejected by scientists.

The atmosphere of Mars is extremely tenuous, being only 1/150th that of Earth. Ninety-five per cent of it is carbon dioxide, the rest being nitrogen (2.7%), argon (1.6%) and very small amounts of oxygen, carbon monoxide and water vapour (0.03%). The thin atmosphere, coupled with Mars' eccentric orbit and axial tilt, leads to a wide temperature range, from 136 K (over the winter south polar cap) to 299 K (after noon in summer), with a mean of 250 K. Most of the VOLATILES are frozen out of the atmosphere in the polar caps, but there is sufficient water vapour for water ice clouds to form at altitudes from 10 to 15 km (6-9 mi) in the early morning and evening, and over specific topographic features such as the giant volcanoes. Dust storms are also common, though only the largest can be seen from Earth. The most extensive storms occur near perihelion, when the temperature is relatively high and the southern polar cap is rapidly evaporating and releasing volatiles to increase the atmospheric pressure. Wind speeds during these phenomena may reach as much as 400 km/hr (250mi/hr) but average a few tens of km/hr. Many major Martian dust storms have begun in the Hellas, Argyre, Chryse and Isidis basins, and in and around the VALLES MARINERIS complex. Dust deposition causes the contours of the dark regions of the planet to change slightly with time, and colours the Martian sky pink.

Mars The Viking spacecraft gave unprecedentedly sharp images of Mars. The prominent dark area left of centre is Syrtis Major, and the bright area below it is Hellas Planitia.

Since the Mariners of the 1960s, the VIKINGS of the 70s, MARS PATHFINDER in 1997 and the ongoing MARS GLOBAL SURVEYOR, the topography and geology of Mars has become known in some detail. The planet is believed to have an iron-rich core, some 2900 km (1800 mi) across, and it exhibits a magnetic field hundreds of times weaker than Earth's. The MANTLE may be 3500 km (2200 mi) thick, and the crust some 100 km (60 mi) deep.

There is a hemispheric asymmetry to the planet: much of the southern hemisphere is up to 3 km (2 mi) above datum (datum being an arbitrary height at which

MARS: DATA the pressure is 6.2 millibars) and is heavily cratered. The high density of craters suggests that this terrain is as old as the highland regions of Earth's Moon. The two large impact basins HELLAS PLANITIA and ARGYRE PLANITIA are located in this hemisphere. In several places this ancient cratered terrain is cut by gullies and complex channel systems, which appear to have been incised by water. Several tongues of the cratered surface extend into the northern hemisphere.

Most of the northern hemisphere is less heavily cratered and much is below datum. The surface looks altogether 'smoother' on the large scale. The most impressive features of this hemisphere are the vast shield volcanoes of THARSIS MONTES and the extensive valley network known as Valles Marineris, both first revealed in Mariner 9 images. The Tharsis region is in the nature of a huge bulge in the Mars lithosphere; it has a size roughly the same as that of Africa south ofthe Congo river, 4000 X 3000 km (2500 X 1900 mi). The entire Tharsis region rises to some 9 km (6 mi) above datum, but it rises to heights of up to 18 km (11 mi) in the large shield volcanoes ASCRAEUS MONS and PAVONIS MONS. The great volcano OLYMPUS MONS, 1500 km (930 mi) to the west, rises to 24 km (15 mi) above datum. A similar but smaller bulge resides in the region known as Elysium, containing the volcanoes Elysium Mons, Hecates Tholus and Albor Tholus.

These shield volcanoes, in terms of profile, are analogous to Earth's Hawaiian volcanoes, such as Mauna Loa. They are, however, substantially larger and have vast summit depressions (calderas) crowning them. Very extensive lava flows can be traced radiating out from these constructs. Radiating out from the Tharsis Montes and Elysium regions is a vast array of fractures, which must have developed in response to the formation of the two crustal bulges. Mars has no plate TECTONICS, and any volcanic hot-spot will remain in one place. Continued activity over a long period, aided by the low Martian gravity, has enabled high volcanic mountains to form. The planet is probably geologically inactive today. Olympus Mons seems relatively young, perhaps 30 million years old, but the oldest volcanoes are estimated to be up to 3.4 billion years old.

Mars The polar caps on Mars are not permanent. These two Mars Global Surveyor images of the northern polar cap were made exactly one Martian year apart, in early northern summer. Close examination reveals that there was less frost in the later image.

To the east of Tharsis is an impressive canyon system named Valles Marineris. It extends from near the summit of the bulge at 5S l00W for 4500 km (2800 mi) eastwards, eventually merging into an immense area of 'chaotic terrain' between Aurorae Planum and Margaritifer Terra. Valles Marineris marks a vast faulted area up to 600 km (370 mi) wide at the widest point; in places it falls 7 km (4 mi) below the rim.

The oldest recognisable features are the roughly circular impact basins, of which the most prominent is Hellas Planitia. Also prominent are Argyre Planitia and ISIDIS PLANITIA. A number of other, generally less well-defined basins have been recognized on spacecraft images. For instance, CHRYSE PLANITIA is believed to occupy an ancient multi-ringed basin, although the surrounding ramparts are not so clearly defined as those of Argyre and Hellas Planitia.

The most exciting of all Martian features are the 'outflow channels', in no way related to the illusory canali. These channels can be several hundred kilometres long and tens of kilometres wide. They generally start abruptly, without tributaries. Most channels are located north of the great canyon system and converge on the plain known as Chryse Planitia, although others appear to be associated with Elysium's north-west edge. They seem to be related to a period of flooding in the distant past, when the climate was quite different from that of today and liquid water (probably ice-covered) could have existed in quantity on the Martian surface. Standing water could have extended beyond Chryse, but the matter is controversial.

Mars Global Surveyor imaging greatly surpassed the resolution of the Viking Orbiters, enabling several intriguing discoveries to be made. High resolution images revealed many dozens of small channels within craters, each apparently carved by running water. These features are generally more than 30 from the equator. Each feature resembles a terrestrial gully, exhibiting boulders, a winding channel and debris at the bottom of the flow. The lack of associated small impact features implies a geologically recent history. The channels might have been eroded by water suddenly released behind a plug of ice. There are also many examples of what look like small, dried-up Martian lakes, which display apparent sedimentary layering: these have mostly been imaged at low latitudes in canyons and deep craters. In this instance, however, dust deposits could also have masqueraded as sedimentary strata.

The Viking Lander probes took panoramic photographs of the two landing sites, Chryse Planitia and UTOPIA PLANITIA, which are both plains areas. They recorded dune-like features and a reddish dusty surface crowded with dark rocky blocks. Many of the blocks were vesicular and appeared similar in aspect to terrestrial basaltic rocks. Chemical analyses carried out on the surface revealed that the soil was probably the result of weathering and oxidation of basaltic rocks. Pulverization by meteorite impacts has created fine micrometre-sized dusty particles. The deep reddish coloration is due to the presence of iron (III) oxide. Mars Pathfinder's landing site in ARES VALLIS looked broadly similar. Its Sojourner robot vehicle showed by X-ray spectrometry that the boulders were of various types, some similar to terrestrial basalt and andesite, and others apparently of a sedimentary nature. These findings implied that the rocks had been transported from geologically different places, underscoring the long-range impression from orbital imagery that the region is an ancient flood-plain. The Thermal Emission Spectrometer aboard Mars Global Surveyor indicated the presence of the iron-containing minerals haematite and olivine in the basaltic bedrock areas observable in Mars' southern hemisphere.

The Vikings found the Martian soil to be sterile, bereft of organic matter, though long exposure to solar ultraviolet radiation would have destroyed any evidence of such matter in time. The so-called 'microfossils' described in a Martian meteorite (ALLAN HILLS 84001) in 1996 are now thought by a consensus of scientists to be simply crystals of inorganic origin, rather than past Martian life-forms. This meteorite was blasted from the surface of Mars by a nearby impact over 16 million years ago and later picked up in Antarctica.

In addition to the volcanic shields and the surrounding lightly cratered plains, the heavily cratered terrain of the southern hemisphere and the polar caps, there are extensive regions adjacent to the poles that have been dissected to reveal strongly laminated deposits. Such lamination points to a geological history of alternating warmer and colder climates. This can be explained by the fact that the inclination of the Martian axis is known to oscillate from 14.9 to 35.5, and the planet's orbital eccentricity is known to vary from 0.004 to 0.141 over tens of thousands of years.

Mars Because Mars takes longer to orbit the Sun than Earth, the mean period between oppositions (the closest approach) is more than two years. With the Earth at E1 and Mars at M1, Mars is in opposition. A year later the Earth has come back to E1, but Mars has only reached M2. The next opposition occurs only after 780 days when Earth has caught up with Mars, with the Earth at E2 and Mars at M3.

The polar caps are believed to represent the visible summits of more extensive subsurface permafrost. Each consists of a permanent cap perhaps several kilometres thick (water ice in the north, a mixture of water and carbon dioxide in the south) and an overlying seasonal cap of carbon dioxide. The seasonal caps undergo cyclic changes, occupying a diameter of over 60 in latitude in winter, and shrinking in summer to just 10 or less. It is estimated that the complete melting of the polar caps, which would only be possible given an adequate atmospheric pressure, would cover the planet with 10 metres of water - sufficient for the supposed rivers and larger bodies of water of Mars' geological past.

Mars-crossing asteroid Any of a group of asteroids with orbits that cross some part of the orbit of Mars. In general this implies a perihelion distance of less than about 1.67 AU, although some near-earth asteroids may be disqualified from this group because their aphelia lie within Mars' perihelion distance of 1.38 AU. Asteroids with perihelia less than 1.30 AU are classed as amor asteroids, unless they actually cross the Earth's orbit, in which case they are either apollo asteroids or aten asteroids.

Mars Exploration Rover national aeronautics and space administration (NASA) mission, originally known as Mars 2003, to land two roving vehicles on Mars. Although it is possible that NASA will be forced to reduce the mission to one rover, the plan in 2001 involved two launches in 2003 June, with landings in 2004 January and February, using an inflatable airbag method used successfully for mars pathfinder in 1997. The craft could bounce a dozen times and could roll as far as 1 km (0.6 mi). The bags will deflate and retract and petals will open up, bringing the lander to an upright position and revealing the rover. The landings will be made in two separate locations to be identified using mars global surveyor images. The criteria will be that the sites reveal clear evidence of surface water, such as lakebeds or hydrothermal deposits. The identical rovers will carry a suite of instruments, including three spectrometers and a microscopic imager, to search for evidence of liquid water that may have been present in the planet's past. The rocks and soil will be collected by a sampler, while a rock abrasion tool will expose fresh rock surfaces for study. The 1500-kg rovers will be equipped with a 360 panoramic camera, giving three times the resolution of that carried on the Mars Pathfinder. This will be used to enable scientists to select samples. The rovers will be able to travel about 100 m (330 ft) a day with an operational lifetime expected to last 90 Martian days.

Mars Express European space agency (ESA) mission to be launched in 2003 June and to enter orbit around Mars in 2003 December. Mars Express was originally envisaged as part of a wider international effort to return samples of Mars to Earth in 2004 and 2006. The craft was to help track the US ascent vehicles. Such missions are unlikely to take place until the 2010 timeframe. Nonetheless, Mars Express will carry the UK's beagle 2 Mars lander - a late addition to the craft and will be equipped with a suite of seven instruments to study the surface and atmosphere in detail, including a 10-m (33-ft) resolution photo-geology imager and a spectrometer with a resolution of 100 m (330 ft) to map the mineralogy of the planet. The spacecraft will operate in a polar orbit and will complement the new US mars exploration rover and mars odyssey missions.

Mars Global Surveyor (MGS) One of the USA's most successful Mars probes. It was launched in 1996 November and entered orbit around Mars in 1997 September. The spacecraft was still operating in 2001 for special observations after its primary mission had ended. The MGS took over 300,000 high-resolution images of the Martian surface, including those that revealed what seemed to be clear evidence that at one time liquid water flowed across the surface. MGS was equipped with five of the seven instruments that flew on the Mars Observer, which failed to orbit the planet in 1993, probably because of an engine explosion. The images returned by the MGS - and the mars pathfinder, which landed in 1997 - freely and widely available on the Internet, represented a new era in wide public access to and personal involvement with the exploration of space.

Mars Global Surveyor (MGS) The Mars Orbiter Laser Altimeter (MOLA) on the MGS confirmed there is a marked difference between the northern and southern halves of the planet.This false-colour image shows that the north consists mainly of low-lying ground (blue) and the south of highlands (green, yellow, red and white). The volcanoes of the Tharsis region are clearly visible, as is the deep Hellas Planitia impact basin.

Mars Odyssey NASA's 2001 Mars Odyssey is the former Mars Surveyor 2001 craft renamed in honour of the English writer Arthur c. clarke. Launched in 2001 April, Mars Odyssey entered an initial orbit around Mars in 2001 October and for the following 76 days used aerobraking in the upper atmosphere to manoeuvre into its operational orbit. Mars Odyssey is equipped with a thermal emission imager, a gamma-ray spectrometer and a radiation environmental experiment. The spectrometer is identical to the instrument lost when the Mars Observer mission failed in 1993.

Mars Pathfinder First of NASA's 'faster-cheaper-better' Discovery-class missions, mainly designed as a technology demonstration. It was launched on 1996 December 4 and arrived at Mars on 1997 July 4. Airbags were used to cushion the landing. The spacecraft impacted the surface at a velocity of about 18 m/s (60 ft/s) and bounced about 15 m (50 ft) into the air. After bouncing another 15 times it eventually came to rest about 1 km (0.6 mi) from the initial impact site. The landing took place on an ancient flood plain in the Ares Vallis region.

Two days later, a small rover known as Sojourner rolled down a ramp on to the rocky surface. The 10-kg rover carried a stereo camera system and an alpha-proton X-ray spectrometer to study the composition of soil and rocks. Panoramic views showed a landscape of broad, gentle ridges overlain with numerous dark-grey rocks of various shapes and sizes. Close-up images of the rocks revealed pitted, layered and smooth surfaces. It was suggested that some of the rocks were conglomerates - sedimentary rocks that had formed when liquid water existed on the planet's surface.

By the time rover operations ceased on 1997 September 27, Sojourner had travelled about 100 m (330 ft), carried out 16 chemical analyses of rocks and soil and taken 550 images. The lander, which was renamed the Sagan Memorial Station in honour of planetary scientist Carl sagan, relayed an unprecedented 2.3 gigabits of data, including 16,500 lander images and 8.5 million measurements of atmospheric pressure, temperature and wind.

Mars The Martian satellites Phobos and Deimos are thought to be captured asteroids. They orbit close to the planets surface Phobos at 9378 km (5827 mi) and Deimos at 23,459 km (14,577 mi) in almost circular orbits.

Mars sample return missions Attempts to return samples of Mars to the Earth to help answer finally the questions about possible life on Mars. The first sample return mission was to have been made by a Mars Surveyor mission in 2005. The Mars Surveyor class lander would collect a modest 2 kg of samples and place them into a small ascent capsule, which would be fired off the surface and on a course to Earth. The failures of the Mars Climate Orbiter and Mars Polar Landing missions in 1999 were a milestone in Martian exploration, resulting in major re-evaluation of future plans and the realization that such a mission is unlikely to take place until the 2010 timeframe. The high cost and level of technology required for a mission are major stumbling blocks in times of low budgets, cut-price spacecraft and the technological disasters of losing two craft in a matter of months.

Mars-Trojan asteroid asteroid that has the same orbital period as Mars, positioned around one of its lagrangian points,L4 or L5. By late 2001 there were five Mars Trojans known, all of which are associated with L5. See also eureka; trojan asteroid

Martian canals See canals, martian

Martian meteorite meteorite that originated on Mars. Martian meteorites are also known as the SNCs, after the type specimens of the three original subgroups (shergotty, nakhla and chassigny). The collection of additional Martian meteorites from Antarctica and the Sahara Desert has extended the number of subgroups to five. The different subgroups are igneous rocks that formed in different locations at, or below, the Martian surface. The groups have different mineralogies and chemistries, and cannot all have come from a single impact event. At least three craters, with minimum diameters of c.12 km (c.7 mi), are required to produce the variety of Martian meteorite types. As of summer 2001, there are 18 separate meteorites that almost certainly originated on Mars. The Martian origin rests on the age, composition and noble gas inventory of the meteorites.

mascon (mass concentration) Concentration of mass below the Moon's surface. Gravity mapping of the Moon has revealed mascons over many of the lunar basins, including imbrium, serenitatis and nectaris.

The gravity field over a planet is never completely even. Places of large mass concentrations exist, which are usually due to accumulation of either heavy materials (for example high-density minerals) or the piling up of materials (such as mountains). Planetary bodies usually even out these mass concentrations through redistribution of mass, in order to achieve isostatic equilibrium. On the Moon, this occurs through movement (plastic flow) of the asthenosphere, which underlies the rigid lithosphere.

Lunar mascons are always associated with basins, the largest impact structures on the Moon. As enormous amounts of surface material were removed by the crater-ing process, the asthenosphere moved towards the surface to replace the mass loss and to maintain isostatic equilibrium. At a later time, lavas, which are much denser than crustal rock, poured into and filled the basins to depths of 21 km (1.2-2.5 mi). These two processes together formed the large mass concentrations.

Early in the Moon's history, the lithosphere was relatively thin, and mass concentrations were mostly smoothed out. Later, however, the lithosphere thickened because of continued cooling, so that new mass concentrations were maintained.

maser (acronym for 'microwave amplification by the stimulated emission of radiation') Celestial object in which radio emission from molecules stimulates further radio emission at the same energy from other molecules; a maser is analogous to a laser but in the radio region of the electromagnetic spectrum. A maser can be created in an astronomical source when one energy level of a molecule is preferentially populated by the radiation environment, and the molecule cools by stimulated emission. Water (H2O), the hydroxyl radical (OH), silicon monoxide (SiO) and several other molecules can produce maser emission in galactic and extragalactic objects. The name 'maser' is also used for the astronomical object itself. Since the molecular emission is amplified, the objects are easier to detect, and over 10,000 are known in the Galaxy. Masers occur in places where new stars are forming, and in the atmospheres of evolved stars, or variable stars with high mass loss which are in the process of becoming planetary nebulae or supernovae. Some of the first masers to be discovered were the OH-IR stars, which radiate in the 1612 MHz line (see oh). A star will have many masers in its envelope, and they can be tracked using multi-element radio-linked interferometer network or very long baseline interferometry, showing that the conditions in the envelope change over months and years. OH, CH and H2O megamasers have been found in galaxies such as the Large Magellanic Cloud, infrared luminous galaxies, active galaxies, starburst and Seyfert galaxies.

Maskelyne, Nevil (1732-1811) English astronomer and clergyman, the fifth and longest-serving astronomer royal (1765-1811). After studying mathematics at Cambridge University, he became James Bradley's assistant at greenwich observatory. In 1761 he took part in a Royal Society expedition to the island of St Helena to observe a transit of Venus. The voyage inspired him to apply astronomy to navigational problems, and in his British Mariner's Guide (1763) he explained how to determine longitude at sea by comparing the Moon's position as observed aboard ship to its position as observed from a known terrestrial longitude. He travelled to Barbados in 1764 to test John Harrison's newly invented marine chronometer, which proved to be more practical and accurate than the lunar method of finding longitude. Two years later Maskelyne founded the nautical almanac. In 1774 he measured the deflection of a large pendulum erected atop the Scottish mountain of Schiehallion; from the deflection he calculated the mountain's mass and thence the gravitational constant and the Earth's density. His value of 4.87 times the density of water is close to the accepted modern value of 5.52.

mass Measure of an object's inertia. The mass defined in this way is called the inertial mass. Mass may also be defined from the gravitational force that it produces, leading to the gravitational mass. Experiments have shown that the two masses are identical to better than one part in 1012. This has led to the principle of equivalence 'no experiment can distinguish between the effects of a gravitational force and those of an inertial force in an accelerated frame', which underlies einstein's theory of general relativity.

massive compact halo object (MACHO) Dark object, perhaps a brown dwarf or black hole, postulated to explain the missing mass seen in galactic rotation curves.

mass-luminosity relation For stars on the main sequence there exists a one-to-one relationship between luminosity and mass, expressed approximately as L oc Mx. This relation enables mass to be determined from absolute magnitude. The exponent x varies with mass, being 1.6 for stars of around 100 solar masses, 3.1 for stars of around 10 solar masses, 4.7 for stars of a solar mass and 2.7 for low-mass stars of around a tenth of a solar mass. These differences are due primarily to the opacity of the stars caused by their different interior temperatures.

mass transfer Process that occurs in close binary stars when one star fills its roche lobe and material transfers to the other star through the inner lagrangian point. Material can be transferred directly to the other star but is more usually transferred via an accretion disk.

The material flowing through the inner Lagrangian point moves very like a free particle, but when the material impacts the accretion disk around the other star, it stops moving like a free particle, since gas cannot flow freely through other gas, and the energy is dissipated via shock waves.

The observed period of the outbursts from the shock waves gives information about the time it takes material to spiral from the secondary on to the accreting primary. For dwarf novae, the time is typically a few days. These observations suggest that material accretes a lot faster than should be allowed by the natural viscosity of gas. If it is to accrete on to the primary, the material must first lose its original angular momentum. It may be that the gas in an accretion disk behaves like a fluid with a very high viscosity, with the viscosity slowing the motion of the gas.

Masursky main-belt asteroid; number 2685. It was imaged in 2000 January with the camera on board the Saturn-bound cassini space probe. Masursky is c.15 km (c.9 mi) in size.

Masursky, Harold (1922-90) American geologist and planetary scientist who played a major role in most of NASA's 1960s-1980s missions to explore the Solar System. He designed the scientific experiments and helped select the landing sites for the ranger 8 and 9 lunar probes, helped to coordinate the lunar orbiter programme, was responsible for the apollo 8 and 10 geochemical investigations, and coordinated the geological experiments carried out by Apollos 14-16. Masursky led the team that built mariner 9, was largely responsible for selecting the two viking landing sites, and led the Imaging Radar Group for the two pioneer venus orbiters. His career climaxed with his involvement with the voyager missions.

Mathilde main-belt asteroid; number 253. Mathilde was visited by NASA's near shoemaker probe in 1997, as the spacecraft headed for rendezvous with EROS. The information returned shows that Mathilde has a mean density somewhat less than that of stony meteorites, indicating that it may contain voids and may thus be an agglomeration of smaller rocky components held together by self-gravity.

Mauna Kea Observatory World's largest astronomical observatory. It is situated at the 4205-m (13,796-ft) summit of Mauna Kea (which means 'White Mountain'), a dormant volcano on the island of Hawaii and the highest point in the Pacific Basin. It is above 40% of the Earth's atmosphere and 97% of the water vapour in the atmosphere, resulting in very high atmospheric transparency and freedom from cloud. Laminar airflow over the mountain produces superb image quality.

Mauna Kea Observatory The most recent generation of telescopes on Mauna Kea, including Gemini North (foreground), the twin Keck telescopes (right) and the Japanese Subaru (beyond the Keck domes), give results that are second to none.

The observatory, operated by the University of Hawaii, hosts nine telescopes for optical and infrared astronomy and two for submillimetre astronomy. They include the largest single-mirror optical/infrared telescopes in the world, the twin reflectors of the W.M. KECK OBSERVATORY; and the largest submillimetre telescope in the world, the JAMES CLERK MAXWELL TELESCOPE. Also on Mauna Kea is the CALTECH SUBMILLIMETER OBSERVATORY, while a third submillimetre instrument (the SUBMILLIMETER ARRAY) is under construction. The westernmost antenna of the VERY LONG BASELINE ARRAY is nearby.

Astronomy on Mauna Kea began in the 1960s, when the University of Hawaii placed a 0.6-m (24-in.) telescope there, followed in 1970 by their 2.2-m (88-in.) reflector. Three more telescopes were built on the site in 1979: the 3.2-m (126-in.) NASA Infrared Telescope Facility (IRTF), the 3.6-m (142-in.) CANADA-FRANCE-HAWAII TELESCOPE and the 3.8-m (150-in.) UNITED KINGDOM INFRARED TELESCOPE. Mauna Kea's exceptional observing conditions attracted four 8-10 metre class telescopes in the 1990s: the two Keck Telescopes, the Japanese SUBARU TELESCOPE, and the northern telescope of the GEMINI OBSERVATORY. Further development of the site is likely.

Astronomers and technicians working on Mauna Kea must first acclimatize to the altitude. A mid-level facility at Hale Pohaku, at an altitude of 2800 m (9300 ft), provides accommodation and other facilities; it is named the Ellison Onizuka Center for International Astronomy to honour Hawaiian astronaut Ellison Shoji Onizuka (1946-86), who died in the Challenger disaster.

Maunder, (Edward) Walter (1851-1928) British solar astronomer and a founder of the BRITISH ASTRONOMICAL ASSOCIATION (BAA). He was appointed Photographic and Spectroscopic Assistant at the Royal Observatory, Greenwich in 1873, where he used various instruments to observe sunspots, faculae and prominences. Over the next thirty years, he compiled the most complete record of sunspot activity, supplementing the Greenwich data with observations from overseas. By plotting the mean heliographic latitude of sunspot groups against time, Maunder created the first BUTTERFLY DIAGRAM and determined that the Sun showed DIFFERENTIAL ROTATION. He also researched historical sunspot records, discovering the paucity of sunspot activity during the years 1615-1745 now known as the MAUNDER MINIMUM.

Maunder organized and participated in many expeditions to observe total solar eclipses, on which he was often accompanied by his second wife, Annie Scott Dill Maunder (1858-1947), who also worked as an astronomer at Greenwich. Annie Maunder took fine photographs during these eclipses - on one image the corona could be traced to a distance from the limb of six solar radii. The Maunders were instrumental in the formation of the BAA (1890), which soon became the world's foremost group of amateur astronomers.

Maunder minimum Period between 1645 and 1715 when very few SUNSPOTS were observed. It has been named after Walter MAUNDER who in 1922 provided a full account of the 70-year dearth in sunspots, first noticed by Gustav Friedrich Wilhelm SPORER in 1887-89. The scarcity of sunspots during this period has been substantiated by other indicators of low solar activity, such as the amount of radioactive carbon-14 in old tree rings and the occurrence of low-latitude aurorae. The Maunder minimum coincided with years of sustained low temperatures in Europe, from about AD 1400 to 1800, known as the Little Ice Age.

Mauritius Radio Observatory Location of the Mauritius Radio Telescope, constructed to make a southern-hemisphere survey to complement the 6C (sixth Cambridge) radio survey of the MULLARD RADIO ASTRONOMY OBSERVATORY. It also observes selected southern pulsars.

Maurolycus Ancient lunar crater with deeply incised walls (42S 14E), 105 km (65 mi) in diameter. Continued meteoritic erosion by meteorites has worn away most elements of its EJECTA. Its central peaks, produced by rebound of the floor, are still visible. Maurolycus lies over several older craters, the rims of which are visible to the south-west and north-west. At high Sun angles, the bright ray system of TYCHO can be seen on Maurolycus' floor.

Maury, Antonia Caetana (1866-1952) Pioneer American woman astronomer at the HARVARD COLLEGE OBSERVATORY. Maury, a niece of Henry Draper, helped to compile the henry draper catalogue, named as a memorial to him. This work, carried out at Harvard from 1887 under the direction of Edward C. PICKERING, required Maury to examine and classify thousands of stellar spectra, in the course of which she refined the classification system to account for the sharpness of spectral lines. In 1905 Ejnar HERTZSPRUNG showed that Maury's modification could be used to distinguish between giant and dwarf stars. Maury also became expert at identifying spectroscopic binaries, helping Pickering to prove the true nature of the first star to be identified as such, Mizar ( Ursae Majoris), and in 1889 calculated its 104-day period.

Maximum Aperture Telescope (MAXAT) Former name of the GIANT SEGMENTED-MIRROR TELESCOPE

maximum entropy method (MEM) Mathematical technique applied to the inverse problem of how to make reliable deductions from noisy and uncertain data. It derives its name from the concept of an increase in entropy as equivalent to a decrease in the level of structure in a system. The MEM deduction is therefore the one that has the least amount of structure within it (that is, the maximum entropy) and yet is still consistent with the data. The technique is mostly applied to the interpretation of noisy images.

Max-Planck-Institut fur Astronomie (MPIA) Institute located in Heidelberg, Germany, which conducts research in astronomical instrumentation, stellar and galactic astronomy, cosmology and theoretical astrophysics. The institute is responsible for operating the calar alto observatory.

Max-Planck-Institut fur Astrophysik (MPA) Institute located at Garching, near Munich, Germany, which undertakes research in theoretical astrophysics. Its scientists work closely with astronomers from the european southern observatory, whose headquarters are close by. Also nearby is the Max-Planck-Institut fur extrater-restrische Physik (MPE), which conducts observations in spectral regions accessible only from space (for example, far-infrared, X-ray and gamma-ray).

Max-Planck-Institut fur Radioastronomie (MPIfR) Institute located in Bonn, Germany, with the primary purpose of operating the fully steerable 100-m (330-ft) telescope at Bad Munstereifel-Effelsberg, 40 km (25 mi) south-west of the city. The Effelsberg Radio Telescope is a lightweight paraboloidal dish commissioned in 1972, and it is used at centimetre wavelengths both as a stand-alone instrument and as an element of global very long baseline interferometry experiments. The MPIfR collaborated with the steward observatory to build the 10-m (33-ft) heinrich hertz telescope for submillimetre-wavelength observations.

Maxwell, James Clerk (1831-79) Scottish physicist who unified the forces of electricity and magnetism and showed that light is a form of electromagnetic radiation. In 1859 he demonstrated mathematically that Saturn's rings could not be stable if they were completely solid or liquid, and most likely consisted of small solid, particles - as was proved by James keeler in 1895.

Maxwell-Boltzmann distribution Law determining the speed of particles in a gas. Where N(v) is the number density of particles with velocities in the range v to v + dv, N is the total number density of the particles, m is the particle mass, T the temperature and k is boltzmann's constant. It gives a bell-shaped distribution, the peak of which moves to higher velocities as the temperature increases. The onset of degeneracy (see degenerate matter) in, for example, white dwarfs is indicated by deviations of particle speeds from the Maxwell-Boltzmann distribution.

MaxwellBoltzmann distribution The velocity of atoms within a gas is determined by the temperature and density. MaxwellBoltzmanns distribution shows that for any given density, atoms move more rapidly as the temperature increases.

Mayer, (Johann) Tobias (1723-62) German mathematician, astronomer and cartographer who produced precise tables of lunar positions, taking into account lunar libration, that were used by later astronomers to reckon longitude at sea. He produced the first map of the Moon based upon micrometer measurements (1750). From his observations he concluded that the Moon has no appreciable atmosphere. In 1753 Mayer began publishing his lunar tables, and he developed a mathematical method of estimating longitude to within half a degree. He also derived an improved method for calculating solar eclipses.

Mayall Telescope See kitt peak national observatory

Mayan astronomy See native American astronomy mean anomaly See anomaly

mean solar time Local time based on a fictitious, or mean, Sun which is defined as moving around the celestial equator at a constant speed equal to the average rate of motion of the true Sun along the ecliptic. Because the true Sun does not appear to move across the sky at a uniform rate, the length of the solar day as measured by apparent solar time varies throughout the year by up to 16 minutes. For the purposes of establishing a uniform civil time, it was therefore necessary to invent a mean sun. The difference between apparent solar time and mean solar time is the equation of time.

mean Sun Fictitious Sun conceived to provide a uniform measure of time equal to the average apparent solar time. The mean Sun is deemed to take the same time as the real Sun to complete one annual revolution of the celestial sphere, relative to the first point of aries, but it moves along the celestial equator at a constant speed equal to the average rate of motion of the true Sun along the ecliptic. See also mean solar time.

Mechain, Pierre Frangois Andre (1744-1805) French astronomer at France's Marine Observatory, best known as a colleague of Charles messier who contributed to the famous messier catalogue of 'nebulae', and for his own observations of comets. He discovered ten comets between 1781 and 1799, often computing their orbits himself. One of these discoveries (1786) was the famous Comet encke. Like Messier, Mechain began cataloguing celestial objects that appeared 'nebulous' in order to avoid misidentifying them as new comets. He discovered 21 of the nebulous objects that Messier included in his final list published in 1781, and with Messier discovered the six objects added in the 20th century as M104-109.

medieval European astronomy Astronomy as practised in Western Europe between about ad 400 and 1500. Modern scholarship is fundamentally changing the belief that no serious astronomy took place in Europe in this period. But medieval astronomy differs from that of later ages in its attitude to new knowledge. Medieval

MENSA (gen. mensae, abbr. men) scientists looked back to the writers of antiquity for their definitive standards in observation and explanation, generally believing that the Greeks had already uncovered the great truths of nature. Preservation rather than progress was the aim, yet much useful observation and invention came from them.

Between the end of the Roman world in the 5th century AD and the 12th century, only fragments of PTOLEMY, ARISTOTLE and other Greek scientific writers were available in the West. Astronomy was learned from encyclopedic digests, such as those by Pliny the Elder (AD 23-79) and Boethius (c. 480-524). Nonetheless, the basic structures of the classical cosmos were familiar to all educated people. The Earth was not flat, but a sphere, set motionless at the centre of a series of 'crystalline' spheres that carried the Moon, Sun, planets and stars, and rotated around us at different speeds; the Moon's sphere in 28 days, Saturn's in 29 years. The stars were all the same distance away, and were gathered into Ptolemy's 48 constellations, including the 12 zodiacal signs.

One of the main reasons for the cultivation of astronomy in medieval Europe was the refinement of the calendar, and in particular, the accurate determination of the date of Easter, the most sacred of Christian festivals, which was calculated from a formula governed by the 'Paschal' or full moon following the vernal equinox. 'The Venerable' BEDE became Britain's first astronomer when he developed superior techniques for calculating Easter. Throughout the medieval period, the requirements of the calendar, and, to a lesser extent, of observing the daily motions of the stars to determine the times for monastic prayers, kept astronomy and the Church closely wedded. Not until 1582, by which time astronomers had sufficient data on calendrical errors, and established the Earth's rotation period to within seconds of the modern value, could they refine calendrical calculations to produce the GREGORIAN CALENDAR we still use today.

It was only after the mid-12th century that astronomy came to be extensively cultivated in Europe, partly as a result of contacts with ISLAMIC ASTRONOMY in Spain and Palestine. The Arabs had already translated Ptolemy, Aristotle and other writers into Arabic. These works, in turn, came to be translated into Latin, so that for the first time European scholars had access to complete versions of the leading classical texts. They also acquired Latin translations of the original researches of various Arab astronomers. Europeans, such as the Frenchman Gerbert of Aurillac (c. 940-1003), visited Muslim Spain, and it is Gerbert who is credited with introducing the ASTROLABE into Europe.

The large number of astronomical books being translated into Latin gave astronomy an assured place in the curricula of Bologna, Paris, Oxford, and the other emerging universities of the 12th century. In the quadrivium, students were instructed in astronomy, geometry, arithmetic and music: the four 'sciences' of mathematical proportion. Johannes de Sacrobosco (d.c.1256) wrote the best-selling De sphaera mundi ('On the Sphere of the World') around 1240, which would be a reference for students of astronomy for the next 400 years. Through the universities especially, astronomical knowledge became widespread in educated society. The poet Geoffrey Chaucer (c.1340-1400) wrote Treatise on the Astrolabe (c.1381), the first technological book in the English language, being a practical manual describing the use of the astrolabe.

One of the most adventurous branches of medieval astronomical thought was cosmology. While celestial mechanics was explained in terms of Aristotle's spheres and Ptolemy's epicycles, several theologian-astronomers had some remarkably modern-sounding ideas about time and space. Archbishop of Canterbury Thomas Bradwardine (c.1290-1349), Bishops Jean Buridan (c.1295-c.1358), Nicole de Oresme (c.1323-82) and Nicholas of Cusa (1401-64), and others asked such questions as, could time have existed before God

Small and very faint southern constellation between Hydrus and Volans. Mensa was named Mons Mensae (Table Mountain) by Lacaille in the 18th century, because the southern part of the Large Magellanic Cloud in the northern part of the constellation reminded him of cloud overlying Table Mountain, South Africa. The brightest star is a dim mag. 5.1. created the Universe? Could there be such a thing as an INFINITE UNIVERSE? And was motion relative? Space, time and infinity fascinated medieval scholars. No one was burnt at the stake for asking such questions, for the academic clergy saw them as lying within the legitimate bounds of university discussion. Apart from an ultimately unsuccessful attempt to ban aspects of Aristotelian science in Paris in 1277, the medieval Church had no specific policies on astronomy, and would not have until the 17th century.

From the sheer number of manuscripts and, after 1460, printed astronomical books, astrolabes, dials and artefacts in libraries and museum collections, it is clear that astronomy had a high profile in medieval European culture. It was essential to Church administration, it was a major component of the university curriculum and it even penetrated vernacular literature. It was also suspicious of astrology. Where it differed essentially from the astronomy of the scientific revolution, however, was in its conservative, as opposed to the latter's progressive, approach. Without an already established astronomical culture, the developments of RENAISSANCE ASTRONOMY could not have taken place.

Megrez The star 8 Ursae Majoris, visual mag. 3.32, distance 81 l.y., spectral type A2 V. Its name comes from the Arabic maghriz, meaning 'root' (of the tail), referring to its position in Ursa Major.


meniscus lens Thin LENS usually having one convex and one concave surface and resembling the shape of the meniscus at the surface of a liquid such as water. Common examples are contact lenses. In astronomy, meniscus lenses are used to improve image quality in reflecting telescopes. Examples are the corrector plate in SCHMIDT-CASSEGRAIN, Maksutov-Cassegrain and Maksutov-Newtonian telescopes. In all of these, a large meniscus lens with little optical power is mounted at the entrance to the optical tube; it is often referred to as the corrector plate. As the light enters the telescope its path is altered slightly by the meniscus lens so that it hits the main mirror at the optimum angle for forming sharp images right across the whole field of view. In the Schmidt-Cassegrain telescope the meniscus lens appears to be a flat plate, although it has a mild aspheric shape to one surface. In the MAKSUTOV TELESCOPE the meniscus lens is steeply curved.

Menkalinan The star p Aurigae, visual mag. 1.90, distance 82 l.y., spectral type A1 IV. It is an eclipsing binary of period 3.96 days, undergoing two minima of 0.1 mag. in each orbital cycle. Its name comes from the Arabic mankib dhi al-'indn, meaning 'shoulder of the charioteer'.

Menkar The star a Ceti, visual mag. 2.54, distance 220 l.y., spectral type M2 III. Binoculars show an apparent companion of mag. 5.6, but this is an unrelated background star. Its name comes from the Arabic mankhar, meaning 'nostril' or 'nose'.

Mensa See feature article

Menzel, Donald Howard (1901-76) American solar astronomer, astrophysicist and astronomy administrator who directed Harvard College Observatory (1952-66) and helped to found several modern observatories. Menzel and his predecessor at Harvard, Henry Norris russell, strongly influenced the course of American astrophysics.

At Lick Observatory (1924-32), Menzel analysed flash spectra of the Sun obtained by William campbell, developing quantitative spectroscopy in the process. He used the new quantum physics to interpret stellar absorption and emission lines, and derived the abundances of the chemical elements in the solar chromosphere, paying close attention to the lines of neutral and ionized helium observable in the upper chromosphere and in prominences. Menzel's findings that these lines originated at temperatures greater than 20,000 K, together with his discovery of more ionized hydrogen than had been expected, proved that the chromosphere was a distinct layer of the Sun and not an extension of the photosphere, as had been thought.

Menzel applied his knowledge of astrophysics to the nature of diffuse nebulae. In the mid-1930s he developed the quantitative analysis of the spectra of nebulae, culminating in the publication of Physical Processes in Gaseous Nebulae. He refined the theory of radiative transfer and made important studies of the late stages of stellar evolution and associated phenomena, including planetary nebulae around white dwarf stars. He derived a formula for calculating the temperature of a planetary nebula's central star.

Merak The star p Ursae Majoris, visual mag. 2.34, distance 79 l.y., spectral type A0m. It is the southern and fainter of the two pointers to Polaris, the North Pole Star. The name comes from the Arabic maraqq, meaning 'loins' or 'groin'.

Mercator Telescope See roquede los muchachos observatory

Mercury Series of one-man spacecraft in which the USA first gained experience of space flight. The first flights, by modified Redstone boosters, were suborbital. On Mercury-Redstone 3, Alan shepard became the first US astronaut to fly in space (1961 May 5). The capsule reached an altitude of 186 km (116 mi) and landed 478 km (297 mi) downrange. After a repeat suborbital flight by Virgil Grissom (1926-67), four orbital missions were flown using Atlas D boosters. On 1962 February 20, John glenn became the first American to orbit the planet, completing three Earth revolutions in 4h 55m 23s. The last and longest Mercury mission was flown by Gordon Cooper (1927- ) on 1963 May 15-16.

Mercury Mariner 10 made three sets of observations of Mercury. This mosaic of images shows that the small planet is heavily cratered, indicating it has undergone no resurfacing for many millennia.

Mercury Innermost planet of the Solar System. Mercury is often described as the elusive planet, which is a misconception. Mercury is, in fact, easily picked up with the naked eye around the time of its greatest elongation from the Sun, when it can be followed without optical help for several days. Admittedly the planet is difficult to observe telescopically, mainly because of its small angular size and its proximity to the Sun; indeed it can never be seen more than 28 from the Sun since its orbit is inside that of the Earth. Nevertheless bright and dusky markings are visible in moderate-sized telescopes and observers have synthesized their findings into a reasonably reliable albedo chart of named features.

Mercury is the innermost and smallest of the Solar System's major planets. It is also the densest. The unusual density to radius relationship is inferred to indicate an exceptionally large metallic core, which is thought to be about 1800 km (1100 mi) in radius, only some 600 km (370 mi) beneath the surface. Mercury's core is far larger, by proportion, than those of the other terrestrial planets. The evolution of the planet must, therefore, be strongly influenced by core formation, probably from what is believed to be highly refractory material. The magnetic field is approximately a dipole aligned along the axis of rotation and can probably be attributed to dynamo action in a presumed fluid core. Alternatively, if the core is actually solid, Mercury's magnetic field may -uniquely among the terrestrial planets - be a remnant of magnetization of the crust that was acquired during the planet's formation.

Mercury orbits the Sun in 87.969 days and revolves about its axis in two-thirds of the orbital period, 58.646 days. The slow rotation (see also VENUS) may have come about through the retarding tidal action of the Sun. The two-thirds resonance requires the 'trapping' of Mercury in this state, and a non-hydrostatic bulge is inferred, probably caused by convection in its MANTLE occurring by solid state creep.

The lack of atmosphere, the intensity of solar radiation and the length of the planet's day lead to immense temperature contrasts. On the equator at perihelion the noonday temperature soars to 700 K. At night it plunges to 100 K.

Although very little was known about Mercury prior to the three MARINER 10 encounters in 1974 and 1975, Earth-based photometric data did hint at a rough and uneven surface analogous to that of the Moon. Mariner 10 confirmed the supposition. It revealed an airless world, the surface of which, in the form of craters, bright ray systems, ridges (dorsa), valleys (valles) and smooth lava plains (planitia), showed the imprint of a violent past. Craters are named after artists, authors and musicians; valleys after prominent radio observatories; and the plani-tia after Mercury in various languages and after ancient gods with a role similar to the Roman god Mercury. The rupes or scarps commemorate ships associated with exploration and scientific research.

Mercury Because Mercury is nearer to the Sun than the Earth, it displays phases. At (1) it is new, at (2) it is halfphase, at (3) full, and at (4) half-phase again.

Mariner 10 imaged only 35% of Mercury's surface. Of this, 70% was found to be ancient, heavily cratered terrain. The most significant formation is the CALORIS PLANITIA, which is most likely the result of an impact by a body similar to those that formed the multi-ring basins (circular maria) on the Moon. Characteristic surface features found on the planet are long, sinuous cliffs or lobate scarps. These scarps are steep, with an average height of 1 km (0.6 mi), extending for hundreds of kilometres. They may have formed as the crust cooled and shrank. In places the scarps cut across craters, intercrater plains and smooth plains. No strike-slip faults are seen on Mercury and there is no evidence of plate tectonics. Compressional features are seen, however, which could be explained by a contraction caused by cooling since the planet's formation. Estimates of the contraction required are about one part in 1000 or 10,000 and would be consistent with temperature decreases of a few hundred degrees. Solidification of a once-fluid core would be very effective.

Astonishingly, radar data indicate the possible existence of small polar ice deposits. This is not as improbable as it seems. It is just one of the mysteries that await resolution when the next planned space mission starts to probe the battered surface of our thin-shelled neighbour world.

mercury-manganese star Chemically peculiar late b STAR that has greatly enhanced abundances of mercury, manganese and other elements, including rare earths. In these class 3 CHEMICALLY PECULIAR (CP3) stars, mercury can be enhanced 100,000 times or more. The odd element composition is caused by diffusion in the quiet atmospheres of slowly rotating stars, some chemical elements sinking under the force of gravity, with others being lofted upwards by radiation.

meridian North-south reference line, particularly a GREAT CIRCLE on the Earth's surface that runs through both poles and that connects points of the same longitude. A local meridian is the line that passes through an observer, connecting both poles. The prime meridian is the line that passes from pole to pole through the Greenwich Observatory and is the zero point for measuring LONGITUDE. The CELESTIAL MERIDIAN is the great circle on the celestial sphere that passes through the north and south CELESTIAL POLES, together with the ZENITH and the NADIR.


Merrill, Paul Willard (1887-1961) American astronomer who specialized in spectroscopy, pioneering infrared spectroscopy at Mount Wilson Observatory (1919-52). His most famous discovery was made in the early 1950s, during his studies of S stars, including the variable star R Andromedae. Merrill identified the chemical element technetium in these stars (see TECHNETIUM STAR), providing observational support for the s-process of nucleosynthesis of elements in stars.

mesosiderite One of the two subdivisions of STONY-IRON METEORITES. They are a much more heterogeneous group of meteorites than the PALLASITES, which comprise the second stony-iron subdivision. Mesosiderites are a mixture of varying amounts of iron-nickel metal with differentiated silicates, the whole assemblage of which seems to have been brecciated. Mesosiderites are subclassified on the basis of textural and compositional differences within the silicate fraction of the meteorites. Like main group pallasites, mesosiderites have oxygen isotope compositions similar to HOWARDITE-EUCRITE-DIOGENITE ASSOCIATION achondrites.

mesosphere Layer in the Earth's ATMOSPHERE directly above the STRATOSPHERE, in which the temperature falls with height to reach the atmospheric minimum of 110-173 K at its upper boundary, the mesopause. The mesopause's altitude shows two distinct values, 863km (531.9 mi) and 1003km (621.9 mi), the higher value being encountered near the poles in summer, when there is up-welling at high latitudes. Above the mesopause, in the THERMOSPHERE, the temperature rises again. Within the mesosphere, heating by the absorption of solar ultraviolet radiation by ozone declines from the high value found in the upper stratosphere.

Messenger NASA Discovery programme spacecraft that will resume exploration of the planet MERCURY in 2008 January, after an interval of 33 years since MARINER 10. Messenger, which will make an initial flyby of the planet, will later orbit around Mercury in 2009 September, marking a first in space exploration. The orbit will be 200 km (125 mi) by 15,190 km (9430 mi), with an inclination of 80. The spacecraft, designed to monitor Mercury's surface, space environment and geochemistry, as well as ranging, will study the surface composition, geological history, core, mantle, magnetic field and very tenuous atmosphere of the planet. It will search for possible water ice and other frozen volatiles at the poles over a nominal orbital mission of one year. The path of Messenger to Mercury will involve a gravity assist flyby of the Earth in 2005 August, and two Venus flybys in 2006 October and 2007 June. It will also make two flybys of Mercury, in 2008 January and October, prior to its orbital insertion in 2009.

Messier, Charles Joseph (1730-1817) French astronomer known for his discoveries of comets and his

Messier Catalogue M92 (NGC 6431) is a 6th-magnitude globular cluster in Hercules. It is visible in all but the smallest amateur instruments compilation of the famous MESSIER CATALOGUE of deep-sky objects. Little is known of his life before 1751, when he was hired by Joseph-Nicolas Delisle (1688-1768) of Paris Observatory to record observations in his 'neat, legible hand'. Delisle, who had found Messier a post at the observatory at the Hotel de Cluny, calculated positions for the 1758/9 return of Halley's Comet, and instructed his assistant to search for it. Messier recovered the comet on 1759 January 21.

Messier, inspired by his success with Halley's Comet, searched for more. Between 1758 and 1801 he found about twenty comets (14 of which were sole discoveries), increasing his fame and status - in 1770 he was elected to the prestigious Paris Academy of Sciences, and was dubbed the 'comet ferret' by King Louis XV. But Messier's lasting fame rests on the numbered list he compiled of fuzzy, permanent objects that might be mistaken for comets. He had independently discovered the first two objects on this list, the Crab Nebula (M1) and a globular cluster in Aquarius (M2), in 1758 and 1760. His first sole discovery, the globular M3 in 1764, prompted him to make a systematic search, and by the end of that year he had observed and recorded objects up to M40. The list (up to M45) was first published in 1774, and longer versions appeared in 1780 (to M68) and 1781 (to M103). Today's Messier Catalogue of 110 objects includes seven others known to have been observed by Messier.

Messier Catalogue Listing of nebulae, star clusters and galaxies, numbering over 100, begun by Charles MESSIER. Messier drew up the list so that he and other comet-hunters would not confuse these permanent, fuzzy-looking objects with comets. The first edition of the catalogue was published in 1774, with supplements in 1780 and 1781. Not all the objects in the catalogue, known as Messier objects, were discovered by Messier himself; several were found by Pierre MECHAIN, and others added by later observers.

Objects in the catalogue are given the prefix M, and are still widely known by their Messier numbers. The Messier objects (see the accompanying table, page 258-59), including as they do many of the showpiece deep-sky objects visible from northerly latitudes, make popular targets for amateur observers. An attempt to observe as many as possible during the course of one night is known as a 'Messier marathon'. See also CALDWELL CATALOGUE

Messier numbers Numbers allocated to clusters, nebulae and galaxies listed by Charles Messier in his MESSIER CATALOGUE of comet-like objects.

Me star MSTARthat has hydrogen EMISSION LINES in its spectrum; the 'e' is appended to the luminosity class. Me stars are found at all luminosities. Among the dwarfs (dMe stars), the emissions become more prevalent towards later (cooler) subtypes. PROXIMA CENTAURI (M5 Ve) is a good example. The emissions signify notable chromospheres, which in turn indicate strong magnetic dynamos. As a result, dMe stars can produce powerful solar-like flares involving much or even all of the star. FLARE STARS extend to the K dwarfs as well, and even into class L.

Among the class M giants, hydrogen emission - caused by pulsation-generated shock waves - is seen in both oxygen-rich and carbon-star Mira variables; Mira itself (M7 IIIe), Chi Cygni (S6 IIIe) and R Leporis (Hind's Crimson Star, C7 IIIe) are fine examples. In the most luminous class M supergiants (Mu Cephei M2 Iae, VV Cephei M2 Iaep), the emission is related to powerful enveloping winds; such stars are also IRREGULAR VARIABLES.

metals Term used by astronomers to describe all elements heavier than helium. Thus the metal content of a star, denoted by Z, is the combined mass fraction of all elements heavier than helium. Mass fractions of hydrogen and helium are denoted by X and Y respectively; thus a typical star may have X = 0.75, Y = 0.24 and Z =0.01. While the values of X and Y are relatively constant from one unevolved star to another, Z changes dramatically over a range approximately 10-4 to 0.02. Indeed, stars are postulated (Population III stars) with Z =0 (see POPULATIONS, STELLAR).

The relative abundances of the elements are usually quoted as a ratio to the amount of hydrogen, whose value is arbitrarily set at 12.0. The following are the figures for the solar abundances for the more common elements: H (12.00), He (10.93), O (8.82), C (8.52), N (7.96), Ne (7.92), Fe (7.60), Si (7.52), Mg (7.42) and S (7.20).

These figures probably reflect the relative abundance of the heavy elements on a cosmic scale. The abundance of elements in a star is measured from the absorption lines in its SPECTRUM.

For stars other than the Sun the abundances of the elements are not generally so well known or easily measured. Thus Fe/H, the iron to hydrogen ratio, is often used to measure the 'metallicity' (total abundance of elements heavier than helium) of stars, it being assumed that the other heavy elements will be in the same proportion to iron as found in the Sun. Formally, Fe/H is log (NFe/NH)(star) - log (NFe/NH)(Sun).

The reason for the large variation in metal content, Z, of stars is that the heavy elements are not primordial. Only hydrogen and helium (and very small quantities of lithium) are formed in the BIG BANG. All other elements are synthesized in the NUCLEAR REACTIONS that take place in the centres of stars. This processed material is eventually returned to the interstellar medium by stellar mass loss, of which the most dramatic example is the SUPERNOVA explosion. Thus the interstellar medium is continually being enriched with 'metals'. It also follows that young stars that have recently formed from the interstellar medium will have large Z. Thus Population I stars, which are young, have large Z whereas Population II stars, which formed first and are now old, have low Z values. The cosmic lithium abundance is an important indicator of the nature of the early Universe. Unfortunately, there are many reasons why stellar lithium abundances will not accurately reflect primordial lithium abundances.

The metallicity Z has important consequences for a star. The most important effect is that a larger value of Z gives rise to a greater opacity of the stellar gas. The increase in opacity is due to the many possible lines, free-bound transitions and free electrons produced by ionized and partially ionized heavier elements. This will radically affect the structure of a star at any stage and also its evolution. Probably the most striking example of this is that Population II MAIN-SEQUENCE stars (low Z) are both hotter and more luminous than Population I stars of the same mass. Thus the Population II main sequence lies leftwards of the Population I main sequence in the HERTZSPRUNG-RUSSELL DIAGRAM (HR diagram). Furthermore, Population II stars will spend a shorter time on the main sequence than their Population I counterparts. Thus a good knowledge of Z is essential for judging the age of star clusters from the main sequence. Likewise, Z must be well known if the main sequences of two clusters are to be compared for the purpose of measuring cluster distances. A similar effect occurs on the HORIZONTAL BRANCH in the HR diagram. Thus low metallicity RR LYRAE STARS are thought to be intrinsically brighter than high-metallicity RR Lyrae stars.

The original metallicity of a star can also affect its structure and evolution by modifying the nuclear burning in its centre. The most obvious example of this is that the abundance of carbon and nitrogen will affect the CARBON-NITROGEN-OXYGEN CYCLE for hydrogen burning in massive main-sequence stars.

Some stars have particular elements or groups of elements that are relatively either over- or under-abundant. For example, AP STARS show over-abundances of manganese or europium, chromium and strontium. This is thought to be caused by some diffusion process which brings these elements up from the interior. Similarly AM

meteor Meteors sometimes leave a trail of ionized gas in the atmosphere as they burn up. This is known as a 'train', and may last for some minutes.

STARS show a slight over-abundance of elements near iron and an under-abundance of calcium and scandium. Again, a diffusion mechanism is thought to operate.

Amongst the cool giant stars, CARBON STARS (C stars) show carbon to oxygen ratio four times that of normal stars. SSTARS show an over-abundance of zirconium, yttrium, barium and even technetium, which has a half-life of only 2 X 106 years (see TECNETIUM STAR). Some G and K giants also show over-abundance of barium, strontium and other elements produced by the SPROCESS as well as over-abundant carbon. These stars are called BARIUM STARS.

metastable state Relatively long-lived excited state ( see EXCITATION) of an atom that has only forbidden transitions (see FORBIDDEN LINES) to lower levels. In the rarefied interstellar medium an electron can remain in such a state for its natural lifetime, but under terrestrial conditions atoms are very soon knocked out of metastable states by collisions. See also LASER; TWENTY-ONE CENTIMETRE LINE

meteor Brief streak of light seen in a clear night sky when a small particle of interplanetary dust, a METEOROID, burns itself out in Earth's upper atmosphere. As the meteoroid collides with atoms and molecules of air, a large quantity of heat energy is produced, which usually vaporizes the particle completely by a process of ABLATION. Vaporized atoms from the ablating meteoroid make further collisions, causing first excitation, then ionization as electrons are stripped from air atoms and molecules.

An ablating meteoroid thus leaves behind it a trail of highly excited atoms, which then de-excite to produce the streak of light seen as a meteor. Ionization produces a trail of ions and electrons which can scatter or reflect radio waves transmitted from ground-based equipment, causing a radio meteor. The trail of ionization is only a few metres wide, but may typically be 20-30 km (12-19 mi) long.

Most meteors appear at altitudes between 80-110 km (50-70 mi), where the air density becomes sufficiently high for ablation to occur. The altitude of this meteor layer varies slightly over the sunspot cycle, being greater at times of high solar activity. A typical meteor reaches its maximum brightness at an altitude of 95 km (59 mi). Usually, a visual meteor will persist for between 0.1 and 0.8 seconds. Brighter meteors sometimes leave a faintly glowing TRAIN or wake after extinction, and may show bursts of brightening (flares) along their paths.

Meteoroids enter the atmosphere at velocities between 11 and 72 km/s (7-45 mi/s). At the lower end of this range, the velocity is simply that of a particle in free fall hitting the Earth. The greatest value is obtained by summing the maximum heliocentric velocity of the meteoroid at a distance from the Sun of 1 AU (42 km/s or 26 mi/s) with Earth's mean orbital velocity of 30 km/s (19 mi/s).

A typical naked-eye meteor around magnitude +2 is produced by ablation of a meteoroid 8 mm in diameter, and with a mass around 0.1 g. Over the whole Earth, 100 million meteors in the visual range down to magnitude + 5 occur each day.

Meteors can occur at any time, with the bulk of the annual influx of meteoroidal material (estimated at 16,000 tonnes) comprised of random, background SPORADIC METEORS. At certain times of year, numbers are enhanced by the activity of METEOR SHOWERS, which are produced as Earth passes through streams of debris laid down by short-period COMETS.

Meteor Crater (Arizona Crater, Barringer Crater) First crater to be recognized as being caused by METEORITE impact. It is situated on a flat plateau between Flagstaff and Winslow, Arizona, USA. The crater is a basin-shaped depression some 1200 m (3940 ft) in diameter. The walls surrounding the crater rise 37-50 m (121-164 ft) above the surrounding plain.

The outer slopes are quite gentle, but the inner slopes are steep, being as much as 80 in the southern sector. A whole 600-m (2000-ft) section of the pre-existing sedimentary rocks has been lifted about 30 m (100 ft) to form the south wall of the crater.

The feature first created interest in 1891, when large quantities of meteoritic iron were discovered on the surrounding plain. In 1905 boreholes and shafts were sunk in the centre of the crater in an attempt to find the main mass of the meteorite. After passing through crushed sandstone and rock flour, undisturbed rocks were found at a depth of 185 m (607 ft). In 1920 attention was concentrated on the southern rim without success. It is now known that at times of such impacts, the meteorite is either vaporized or shattered to extents that depend on the characteristics of the particular event. It is, therefore, concluded that no large mass exists.

Over 30 tonnes of iron meteorite, known as Canyon Diablo, have been found around the crater. The meteorites consist mainly of iron with just over 7% nickel and 0.5% cobalt. In addition to the irons, oxidized iron shale balls were found intermingled with the local rock debris. Silica glass and very finely divided white sand (known as rock flour), together with forms of quartz known as coesite and stishovite, all point to the structure having been formed by meteoritic impact.

Studies of the distribution of the meteoritic material around the crater have led to the conclusion that the meteoroid responsible for the crater was travelling from north-north-west to south-south-east. This is consistent with the evidence gained from studies of the tilt of the rock layers forming the rim.

Many attempts have been made to ascertain the age of the crater. Early attempts suggested 2000 to 3000 years; current estimates give an age of about 50,000 years.

meteorite Natural object that survives its fall to Earth from space. It is named after the place where it was seen to fall or where it was found. About 30,000 meteorites are known, of which c.24,000 were found in Antarctica, c.4000 in the Sahara Desert and c.2000 elsewhere.

When an object enters the atmosphere, its velocity is greater than Earth's escape velocity (11.2 km/s or 7 mi/s), and unless it is very small (see MICROMETEORITE) friction-al heating produces a FIREBALL. This fireball may rival the Sun in brightness. For example, a brilliant fireball on 1890 June 25, at 1pm, was visible over a large area of the midwest of the United States; the CHONDRITE fall at Farming-ton, Kansas, was the result. If an object (a meteoroid) enters the atmosphere at a low angle, deceleration in the thin upper atmosphere may take tens of seconds. The fireball of 1969 April 25 travelled from south-east to northwest and was visible along its 500 km (310 mi) trajectory from much of England, Wales and Ireland. As commonly occurs, towards the end of its path the fireball fragmented. Sonic booms were heard after its passage, and two mete-oritic stones were recovered, some 60 km (37 mi) apart, the larger at Bovedy, Northern Ireland, which gave its name to the fall. A meteorite that fell at Pultusk, Poland, in 1868, after fragmenting in the atmosphere, is estimated to have had a total weight of 2 tonnes among some 180,000 individual stones. Large meteoroids of more than c.100 tonnes that do not break up in the atmosphere are not completely decelerated before impact. On striking the surface at hypersonic velocity, their kinetic energy is released, causing them to vaporize and produce explosion craters, such as METEOR CRATER.

Photographic observations of fireballs indicate that more than 19,000 meteorites heavier than 100 g land annually, but, of these, most fall in the oceans or deserts and fewer than 10 become known to science. Photographic observations and visual sightings of meteorite-producing fireballs show that they have orbits similar to those of EARTH-CROSSING ASTEROIDS. It is apparent that most meteorites come from the ASTEROID BELT, but a few come from the Moon (LUNAR METEORITES) or from Mars (MARTIAN METEORITES). Meteorites can be divided into three main types, according to their composition: STONY METEORITES (CHONDRITES, ACHONDRITES); STONY-IRON METEORITES (MESOSIDERITES, PALLASITES): and IRON METEORITES. There is not always a clear-cut distinction between types: for example, many iron meteorites contain silicate inclusions related to chondritic and achondritic meteorites.

Some 95% of the meteorites seen to fall are stony meteorites, being composed dominantly of stony minerals. Iron meteorites constitute the bulk of the remainder, while meteorites composed of equal-part mixtures of iron-nickel metal and stony material, known as stony-iron meteorites, are very rarely seen to fall. However, many more iron and stony-iron meteorites have been found than were observed to fall, which reflects their resistance to erosion and their distinctive appearance relative to terrestrial rocks, rather than a change in the composition of the meteorite flux with time. Since 1969 meteorites have been found in large numbers on the surface of the ice in parts of Antarctica. The small number of Antarctic iron meteorites relative to stony types is similar to the ratio in observed falls.

Although frictional heating during atmospheric flight causes the outside of a meteoroid to melt, the molten material is swept into the atmosphere as droplets. The bulk of the heat is removed with the melt, and the inside of the object stays cold. Only the melt during the last second of hypersonic flight solidifies on the object's surface as it falls to Earth under gravity. The solidified melt is known as fusion crust. On most stony meteorites it is dull black, but on many achondrites it is a glossy black. Iron-nickel metal conducts heat more efficiently than stone, so some of the heat generated in atmospheric flight may penetrate to the interior of an iron meteorite. Stony meteorites, however, preserve a record of their history before their encounter with our planet. Meteorites often record shock or thermal events when they were part of their parent bodies. For example, many L-group ordinary chondrites were shock-reheated 500-1000 million years ago. Many chondrites preserve evidence of conditions in the Solar System of 4560 million years ago, and none has an age or isotopic signature consistent with an origin outside it. They provide important clues to the origin and history of the Solar System, as well as records of conditions in inter-planetary space.

Various ages of meteorites can be measured. The formation interval is the period between stellar processing and the incorporation of an element into a meteorite. Chemical elements heavier than hydrogen and helium are synthesized in stars, and many meteorites preserve a record of these processes. Chondrites often contain the decay products of short-lived radionuclides, such as plutonium, indicating that this element was present in the matter from which the Solar System formed. From the quantity of plutonium that must have been present relative to other elemental abundances, the plutonium must have been formed within about 200 million years of the formation of the Solar System.

Most meteorites or their components, such as CHON-DRULES, went through a high-temperature event early in their history. The age of formation is the time, to the present, since a meteorite first cooled to become a closed chemical system. Uranium, for example, decays to lead at a fixed rate, and the uranium-lead age of a meteorite is the time that has elapsed since the uranium and lead were able to exchange freely, when the body was hot. The lead formed from the decay of uranium can be measured; the quantity is proportional to the uranium content, also measured, and to time, which usually is close to 4560 million years.

When an object is broken from its parent asteroid, it continues to orbit the Sun. Cosmic rays from the Sun, and beyond, bombard its surface. The exposure age is the time during which this bombardment takes place. Radiation damage can be measured in various ways, including the content of a substance produced in nuclear reactions, such as a radioactive isotope of aluminium. (The levels of radioactivity in meteorites are so low that specially prepared, ultra-sensitive counting equipment is required for their measurement.) Exposure ages range from a few hundred thousand years for some stony meteorites to 1000 million years for a few iron meteorites. These ages reflect the susceptibility of stony types to erosion by impact in space, compared with the durability of iron-nickel metal.

The terrestrial age is the time since a meteorite landed on Earth. The meteorite with the longest terrestrial age known is a chondrite within a 460-million-year-old Swedish Ordovician limestone. The second-longest terrestrial age may be that of an iron meteorite found in 300-million-year-old coal in Russia. Apart from these examples, most meteorites have much shorter terrestrial ages. Some meteorites from Antarctica have lain in the ice for up to a million years, but these are exceptional.

meteoroid Small natural body in orbit around the Sun. Meteoroids may be cometary or asteroidal in origin. The distinction between a large meteoroid and a small ASTEROID is rather vague; rocky bodies of, say, 10 m (30 ft) in diameter could fit into either category. Millimetre-sized dusty cometary meteoroids entering Earth's atmosphere are completely destroyed by ABLATION, producing short-lived streaks of light seen in the night sky as METEORS. Cometary meteoroids have typical densities of 0.2 g/cm3.

meteor shower Enhancement of METEOR activity produced when Earth runs through a METEOR STREAM. About 25 readily recognisable meteor showers occur each year, the most prominent being the QUADRANTIDS, PERSEIDS and GEMINIDS. Meteor showers occur at the same time each year, reflecting Earth's orbital position, and its intersection with the orbit of the particular meteor stream whose meteoroids are being swept up. Shower meteors appear to emerge from a single part of the sky, known as the RADIANT. Some showers are active only periodically as a result of uneven distribution of stream meteoroids, the GIACOBINIDS perhaps being the best example. The accompanying table lists some of the more important meteor showers; the zenithal hourly rate (ZHR) can vary, and values given here are intended only as a rough guide.

meteor stream Trail of debris, usually from a COMET, comprised of METEOROIDS that share a common orbit around the Sun. Passage of Earth through such a stream gives rise to a METEOR SHOWER. Meteor streams may also be associated with some asteroids - most notably 3200 PHAETHON, the debris from which produces the GEMINIDS.

Meteor streams undergo considerable evolution over their lifetimes, which are probably measured in tens of thousands of years. Initially, when the parent comet has only recently been captured into a short-period orbit, debris (released only at perihelion) is found relatively close to the comet NUCLEUS. Since the nucleus rotates, meteoroids are ejected both behind and ahead of the comet. Over successive returns, more material is added to the near-comet meteoroid cloud, which begins to spread around the orbit.

Eventually, over an interval of only a few tens of years for a very short-period comet, up to thousands of years for those with longer periods, meteoroids will spread right around the orbit, completing loop formation. Many meteor streams are believed to have a filamentary structure, with interwoven strands of meteoroids, released at separate perihelion returns of the parent comet, running through them.

Gravitational perturbations by the planets, and the POYNTINGROBERTSON EFFECT, serve to increase the spread of material in a stream. Stream meteoroids come to perihelion at very similar orbital positions (since this is where they are released from the parent nucleus) but show considerable spread in aphelion distances: consequently, meteor streams are much narrower and more concentrated around the perihelion point.

As a stream ages further, and its parent comet ceases to add new material, it begins to disperse, eventually merging into the general background of the zodiacal dust cloud permeating the inner Solar System.

methane (CH4) First of the alkanes or saturated acyclic hydrocarbons. Its melting point is 90.7 K and its boiling point 109 K. It was discovered on Earth in 1778 by the Italian chemist Count Alessandro Volta (17451827). In 1935 it was detected in the atmosphere of Jupiter by Rupert WILDT from its spectrum lines. In 1977 it was identified in interstellar space through its emissions at a wavelength of 3.9 mm.

Methane is principally significant as a greenhouse gas. On the Earth, it presently makes a smaller contribution to global warming than does carbon dioxide. Potentially though, methane could become much more important, even closing the 8 to 12 um 'window' if the vast quantities of the gas currently locked in permafrost regions are released as the Earth's temperature rises. Methane is also found in significant quantities on TITAN, TRITON and PLUTO. On Titan, methane is the second most abundant atmospheric gas after nitrogen, though it still forms only a few per cent of the atmosphere. It may, however, also be present in liquid and/or solid form on the surface. On Triton and Pluto methane is present in small amounts, along with other frozen gases, as a thin solid surface coating.

Metis One of the inner moons of JUPITER, discovered in 197980 by Stephen Synnott in images obtained by the VOYAGER project. It is irregular in shape, measuring about 60 X 34 km (37 X 21 mi). With ADRASTEA, Metis orbits within Jupiter's main ring, about 128,000 km (80,000 mi) from the centre of the planet, taking 0.295 days to complete one of its near-circular equatorial orbits.

Metis is the also name of a large MAIN-BELT ASTEROID, number 9, discovered in 1849. It has a diameter of about 190 km (120 mi).

Metonic cycle Interval of19 years after which the phases of the Moon recur on the same days of the year. This occurs because 19 years (6939.60 days) is almost exactly equal to 235 lunar months (6939.69 days). Its discovery is often attributed to the Greek astronomer Meton around 433 BC and the cycle forms the basis of the Greek and Jewish calendars.

Meudon Observatory French observatory in a southern suburb of Paris, dating from 1876. It originated when Meudon's ancient royal estate was placed at the disposal of astronomer Jules JANSSEN to pursue his research away from urban pollution. A large refractor of 0.83 m (32 in.) aperture and a 1-m (39-in.) reflector were commissioned in 1893, and other instruments followed, including solar telescopes. In 1925 Meudon became part of PARIS OBSERVATORY, and saw further expansion. Solar physics flourished at the observatory in the 1960s, and a 36-m (118-ft) high solar tower was erected in 1969. Astronomers from Meudon have traditionally used French facilities such as the PIC DU MIDI OBSERVATORY, and today use EUROPEAN SOUTHERN OBSERVATORY and other international telescopes.



Miaplacidus The star p Carinae, visual mag. 1.67, distance 111 l.y., spectral type A1 III. The origin of its name is unknown.

Mice (NGC 4676 A and B) Pair of interacting galaxies a spiral and a lenticular in the constellation Coma Berenices (RA 12h 46m.2, dec. +3044'); they are also catalogued as IC 819 and IC 820. Tails of stars, gas and dust extend away from the pair, which have overall magnitude +14 and total angular size 4'. The Mice lie at a distance of 350 million l.y.

Michell, John (172493) English physicist and one of the first to suggest the possibility of black holes. In 1784 he made complex mathematical calculations describing the physical characteristics of a star with the same density as the Sun but having a much larger diameter, showing that the escape velocity for such a hypothetical object was the velocity of light. Michell concluded that light itself could therefore not escape from such a supermassive object.

Michelson-Morley experiment Experiment carried out by German-American physicist Albert Michelson (18521931) in 1881 and, with greater precision, by Michelson and the American chemist Edward Morley (1838-1923) in 1887. The experiment attempted to detect the motion of the Earth through the luminiferous ether (see light). Their apparatus split a beam from a common source into two parts, one travelling at right angles to the other. Both beams were reflected and recombined to produce an interference pattern. If one beam were to travel out and back along the direction of the Earth's supposed motion through the ether then, if light moved through the ether at a constant speed, it should take a marginally longer time to cover the same distance compared with a beam travelling at right angles to the Earth's motion. Rotation of the two beams should have produced a readily detectable change in the observed interference pattern. However, no change was detected. The null result of this experiment was explained by the theory of special relativity. See also fitzgerald contraction

MichelsonMorley experiment Light from a source is split into two beams, which travel at right angles to each other before being recombined in an interference pattern. The table is rotating, and if the Earth were moving through an ether, as was believed by some astronomers in the 19th century, the light in one path would take longer to reach the mirror and the interference patterns would be seen to change as the table rotated.

Michelson stellar interferometer See stellar interferometer

microdensitometer Computer-controlled measuring machine primarily used for retrieving data from astronomical photographic plates. The microdensitometer provides a measure of both plate position and photographic density, lending itself to applications in stellar astrometry and photometry. It operates by shining a beam of light through the plate and measuring the transmitted light. The plate is scanned in small strips, a massive x-y table being used to move it relative to the beam. A complete scan is built up by moving the table in the y direction with the x coordinate fixed; once a strip is complete, the table is stepped by a few millimetres in the x direction and the next strip measured. See also plate-measuring machine

micrometeorite (interplanetary dust particle (IDP), cosmic dust) Natural object from space that is sufficiently small (typically less than c.200 urn in diameter) to escape destruction during atmospheric flight. Micrometeorites arise from comets or from collisions between asteroids. The particles are dominantly silicates, with additional carbon, sulphides and metal. They are collected routinely from the atmosphere by research aircraft flying at altitudes between 18 and 22 km (11-14 mi). Micrometeorites may also be collected from localities on the Earth's surface where the terrestrial dust component is in low abundance. One of the most successful recovery programmes involves the melting of large volumes of Antarctic ice and the subsequent filtering of the water; the Antarctic micrometeorites so recovered are little altered by terrestrial processes.

micrometer Instrument attached to a telescope for measuring small angular separations of celestial objects. Several different types exist.

Both the binocular micrometer and the comparison image micrometer produce an artificial image of a pair of stars that can be viewed simultaneously with the real star images. The separation of the artificial pair can be adjusted until they match the real binary pair and the angle between them read on a scale.

The cross wire micrometer contains two wires, crossed at 90 in the focal plane of the telescope. By measuring the transit times of stars, including those of known position, the right ascension and declination of an object can be determined.

A double image micrometer uses a split lens or prism to produce a doubling of the image. The orientation of the field may be adjusted to bring the line of the separated lenses into line with a pair of double stars, so determining their position angle relative to the north point or zero.

The objective-grating micrometer uses a coarse grating or bars to produce a diffraction pattern from which the separation of double stars can be measured, while the reticulated micrometer consists of a grid of parallel lines which cross each other at 90. Any two stars in the field of view can be so lined up on the grid so that their relative positions can be estimated.

The ring micrometer consists of a ring of opaque material on glass placed in the focal plane of the telescope's objective or primary mirror. The ring, whose internal dimensions against the sky have been determined, is used to time any two stars close enough in the field of view to cross the ring so that their relevant angular separations can be calculated. See also filar micrometer

Microscopium See feature article

microwaves Part of the electromagnetic spectrum between 1 mm and 30 cm (300 GHz and 1 GHz) wavelength, contained in the millimetre and radio region of the spectrum. Originally the equipment used to detect astronomical microwaves had been built to measure directly the microwave's varying electric field, but the term is not often used to describe instruments or telescopes, for example the arecibo radio telescope works at microwave frequencies. The cosmic microwave background peaks at around 1 mm, although it was

MICROSCOPIUM (gen. microscopii, abbr. mic) originally detected at 7 cm using a microwave receiver system. Molecules such as neutral hydrogen (21 cm) and the hydroxyl radical (18 cm) were first detected in the microwave region of the spectrum.

mid-infrared Region of the electromagnetic spectrum between around 5 and 25 ujii, part of which is observable from ground-based telescopes at very dry sites. Important spectral features from molecules and dust are found in this range. Warm objects (with temperatures of a few hundred kelvins), in particular many asteroids, emit strongly in the mid-infrared. See also infrared astronomy

midnight Sun Visibility of the Sun above the horizon at midnight, from inside the Arctic Circle (6633'N latitude) or Antarctic Circle (6633'S latitude) for a period centred on the summer solstice in the northern and southern hemispheres respectively. Visibility of the midnight Sun is restricted to the day of the solstice at the Arctic or Antarctic Circle. At either pole, however, the Sun is always above the horizon between the spring and autumn equinoxes.

Milky Way Faint luminous band that encircles the sky at an angle of about 63 to the celestial equator. It is irregular and patchy and varies from about 3 to 30 in width. It is easily visible to the naked eye from a good site on a clear moonless night, especially the southern section around Sagittarius. Through even a small telescope it can be seen to be composed of millions of faint stars, and it is in fact the part of our own galaxy immediately surrounding the Sun (see orion arm). The distribution of stars is essentially uniform, and the patchiness results from absorption of starlight in some regions by dark nebulae such as the coalsack, pipe nebula and cygnus rift. The term 'galaxy' derives from the Greek word for milk, yaka (gala).

Miller, Stanley See urey, harold millimetre-wave astronomy Study of extraterrestrial objects that emit in the region of the electromagnetic spectrum between 1 mm and 10 mm (300 GHz and 30 GHz) wavelength, originally part of the radio and microwave region. It was defined as a separate region of the spectrum because of the special equipment and techniques needed to observe astronomical objects at these wavelengths. The science undertaken using the millimetre wavelength range focuses on studying the places where new stars form, molecules, and very cold material, but many types of astronomical object can be detected at millimetre wavelengths.

Millimetre-wave telescopes are built at high, dry sites, since millimetre waves are absorbed by the water vapour in Earth's atmosphere. The detectors are cooled to lower the background noise and to increase sensitivity in a manner similar to the techniques used for infrared astronomy. Many radio techniques can be applied at millimetre wavelengths, such as those used for radio interferometers and aperture synthesis. Since the wavelengths are smaller, the telescopes can be smaller and the distances between them less, to achieve the same resolution. Several large millimetre arrays have been built, such as institut de radio astronomie millimetrique (IRAM) in Europe, Owens Valley Radio Observatory (OVRO) and berkeley illinois maryland association (BIMA) in the United States, and nobeyama in Japan; there is an international project to build an array of 64 telescopes in Chile called atacama large millimetre array (ALMA), due for completion around 2010 (first science observations in 2006). The arrays are used to map small structures such as cold dust disks around protostars. The energy distribution of the cosmic microwave background peaks at 1 mm.

Several important molecules are observed in the millimetre region, the most important being CO (carbon monoxide) at 2.6 mm and 1.3 mm, the spectral emission

Small, inconspicuous southern constellation, representing a microscope, between Sagittarius and Piscis Austrinus/Grus. Microscopium was introduced by Lacaille in the 18th century. Its brightest stars, y and e Mic, are both mag. 4.7. lines from the lowest energy levels in the molecule. CO can be used to trace molecular hydrogen and thus CO surveys have mapped the distribution of the coldest gas in the Galaxy. The CO line is easily saturated because of the large number of molecules in the lines of sight in the Galaxy, but several isotopes (13CO and C18O) can also be detected and these have lower densities and so are not saturated. Whilst CO traces the low-density gas, CS and HCN can trace higher densities, so that regions of star formation can be probed using different molecules to map different temperature and density regimes. See also sub-millimetre astronomy

millisecond pulsar pulsar with a period of a few milliseconds (ms). The first millisecond pulsar to be discovered had a period of 1.6 ms or 0.0016 s (as compared to the crab pulsar, which has a period of 0.033 s). It is believed to be over a hundred million years old, and it has a weak magnetic field, which is why it has not slowed its rotation as expected. There are now more than forty millisecond pulsars known, with periods up to 300 ms; roughly half are in binary systems.

Mills, Bernard Yarnton (1920- ) Australian engineer and radio astronomer, and inventor of the Mills Cross radio telescope. He designed and built many different types of radio telescope. Mills Cross, named after its designer, was built in 1954 and featured two 457-m (1500-ft) arrays. The culmination of Mills' career came with the design and construction of the Molonglo Cross, a huge 408-MHz radio telescope with an aperture of 1600 m (1 mi), completed in 1967.

Mills Cross One of the earliest radio arrays, designed by Bernard Mills of the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Sydney, Australia. The antennae were arranged along two lines 1500 ft long (457 m) at right angles. It was used to map the Magellanic clouds. See also radio astronomy

Milne, (Edward) Arthur (1896-1950) English theoretical astrophysicist and cosmologist. At Cambridge's Solar Physics Observatory (1920-32) he solved fundamental problems of stellar atmospheres and structure. His most valuable work, involving the mathematical modelling of radiative equilibrium and thermal ionization in stellar atmospheres, enabled astronomers to predict a star's temperature and atmospheric pressures from measurements of the widths and intensities of its spectral lines, and by relating the optical depths, or opacities, of stellar atmospheres to spectral features. Milne was the first astrophysicist to correctly explain that instabilities in the balance between radiation pressure and gravity in the chromosphere of the Sun and similar stars could eject atoms at velocities of 1000 km/s (600 mi/s).

Mimas Innermost and smallest of Saturn's spherical satellites. It was discovered by William herschel in 1789. Unlike its neighbour, the slightly larger enceladus, Mimas appears to have been a totally passive world, with no sign of the effects of tidal heating. It has an ancient, heavily cratered, icy surface. There is one giant crater, named Herschel, which is 130 km (80 mi) across. Relative to the satellite's size, Herschel is almost as large as the crater Odysseus on tethys, but unlike Odysseus it has retained its original topography, including an impressive central peak. The impact event that created supernovae and sources of radio waves. He moved to the USA, joining the staff of Mount Wilson Observatory (1935-60), and later worked at the Radio Astronomical Laboratory in Berkeley, California (1960-65). Minkowski was an expert on the spectra of non-stellar objects and made an important study of the Orion Nebula (M42). In 1939 he classified supernovae into Types I and II based upon their spectral lines. He also studied supernova remnants such as the Crab Nebula (M1), which was found to contain a pulsar. He made extensive studies of planetary nebulae and set up his own survey which more than doubled the number known. With Walter BAADE, Minkowski was able to identify the optical counterparts of many of the strongest radio sources in the sky, including Cygnus A (1951). In 1960 he used the 200-inch (5-m) Hale Telescope to discover the galaxy that corresponded to the radio source 3C 295; for 15 years, the redshift he found for this object (z = 0.48) remained the highest known. Minkowski supervised the palomar observatory sky survey (POSS).

Minnaert, Marcel Gilles Josef (1893-1970) Flemish astrophysicist who was an expert on the behaviour of light. Minnaert joined the University of Utrecht's (Belgium) solar physics group after World War I and later became director of Utrecht Observatory (1937-63). He compiled the Utrecht Atlas of the solar spectrum (1940) and devised a new way of diagnosing stellar atmospheres by constructing a 'curve of growth'.

Herschel was almost large enough to break Mimas into fragments; indeed, it may be that Mimas re-accreted from fragments of a previous object that was shattered by a catastrophic collision. See data at SATURN

Mimosa Alternative name for the star p Crucis. See BECRUX

Minkowski, Hermann (1864-1909) German mathematician and professor of physics at Zurich and Gottingen who, in 1908, suggested that non-Euclidean space and time were intimately related and that events should be considered in the context of a four-dimensional spacetime continuum. Before Minkowski's theoretical work, space and time were held to be separate entities. Einstein incorporated the concept of the spacetime continuum into his theory of general relativity.

Minkowski, Rudolph Leo (1895-1976) German-American astrophysicist (born in Strasbourg, now in France) who greatly advanced the understanding of

minor planet Synonym for ASTEROID. 'Minor planet' is the term preferred by the INTERNATIONAL ASTRONOMICAL UNION.

Minor Planet Center (MPC) Organization based at the Smithsonian Astrophysical Observatory, responsible for collecting, computing, checking and disseminating astro-metric observations and orbits for asteroids and comets. It operates under the auspices of the International Astronomical Union and publishes the Minor Planet Circulars, monthly in printed form and 'on demand' electronically.

Mintaka The star 8 Orionis, visual mag. 2.25, distance about 2000 l.y., spectral type O9.5 II. The northernmost of the three stars that form the Belt of Orion, Mintaka is an eclipsing binary, an ALGOL STAR with a period of 5.37 days and a total range of just over 0.1 mag. It has a wide companion of mag. 6.9, visible through small telescopes or even good binoculars. The name comes from the Arabic mintaqa, meaning 'belt' or 'girdle'.

minute of arc (symbol ') Unit of angular measure also known as an arcminute; it is equivalent to 1/60 of a degree. A minute of arc is subdivided into SECONDS OF ARC, or arc-seconds, being 1/60 of a minute of arc and 1/360 of a degree. ANGULAR MEASURE is used widely in astronomy to determine the diameter or separation of celestial objects.

Mir ('Peace') Successor to the Soviet Union's SALYUT SPACE STATIONS. The core module (also known as the base block), which was launched on 1986 February 20, was the backbone of the Mir SPACE STATION. Its most notable innovation was the provision of six docking ports, which enabled additional modules to be attached to the station in 1987-96 as well as the temporary docking of manned and unmanned resupply ships.

Mir The space shuttle Atlantis is seen here leaving Mir after mission STS-71. Despite many setbacks, Mir was a notable success for the Soviet/Russian space agency.

The Mir core module was the principal control centre for the station, and it was equipped with its own orbital manoeuvring engines. Although these could not be used after the arrival of Kvant (the first station module), the base block still provided the principal propellant storage tanks and primary attitude control for the entire space station. The core section also provided 90 cubic metres (3200 cubic feet) of habitable space, and contained the main computers, communications equipment, kitchen and hygiene facilities, and primary living quarters.

The base block was divided into four compartments, designated as the working, transfer, intermediate and assembly compartments. All except the assembly compartment were pressurized. A small airlock was also available for experiments or for the release of small satellites or refuse. Exercise equipment was located in the conical portion of the working compartment. Spatial orientation was aided by dark-green carpet on the 'floor', light-green walls and a white 'ceiling' with fluorescent lamps. The Vozdukh electrolytic system was used to recycle station atmosphere, with a back-up chemical scrubbing system.

The first addition to the core section came in 1987: the 11-tonne Kvant (Quantum) astrophysical module contained a number of X-ray, gamma-ray and ultraviolet astronomy experiments. Next to arrive, in 1989, was the 19.6-tonne Kvant-2, which was designed to improve living conditions on the station as well as carry out scientific research. The 19.6-tonne Kristall followed in 1990, with a number of furnaces intended for materials processing. In 1995 the Spektr (Spectrum) module delivered many US scientific experiments to the station. This module was abandoned after a collision with a Progress spacecraft on 1997 June 25. The final Russian-made module to be attached to Mir was Priroda (Nature), which was devoted largely to Earth observation. A US interface module was added to Kristall in 1995, opening the way for the Space Shuttle to dock with Mir on nine occasions.

Cosmonauts on Mir carried out hundreds of experiments involving such diverse research as production of new alloys, protein crystal growth, production of semiconductors, Earth observation and astronomy. However, the space station was plagued by problems, including equipment failures, lack of power, decompression after a collision and at least one serious fire.

Mir was almost permanently occupied from 1986 February until 1999 August. During its lifetime it hosted 28 long-term crews and was visited by 104 cosmonauts/astronauts. Mir re-entered Earth's atmosphere in 2001 March.

Mira (Omicron Ceti) First star to be discovered to vary in a periodic manner. David FABRICIUS first noticed Mira as of third magnitude on 1596 August 13. He could not find it in star catalogues, atlases or globes. A few months later it was invisible, but he saw it again on 1609 February 15 at third magnitude. Johann BAYER, in 1603, lettered it Omicron and noted it as of fourth magnitude. It was observed from 1659 to 1682 and thought to be a new star, catalogued 68 of sixth magnitude. Johann Holwarda (1618-51), in 1638, noted that Mira became visible to the naked eye from time to time and invisible in between these times.

Mira The light-curve of Mira ( Ceti) shows a slow fall in brightness followed by a long minimum and then a steep climb up to a short, sharp maximum.

In 1660 the star was shown to vary in an approximate period of 11 months. Its period is 331.96 days, but subject to irregularities. Mean range in brightness is 3.0 to 9.9 mags. Maxima have been observed as bright as mag. 1.7 and as faint as mag. 4.9. Minima show a similar variation of from mag. 8.6 to 11.1. The magnitude of a future maximum or minimum is not predictable. Usually Mira has a protracted minimum phase followed by a steep rise and a slow decline.

The spectrum varies M5e to M9e, which is that of a cool star. There are strong bands of titanium oxide and also lines of neutral metals. An EMISSION SPECTRUM becomes superimposed on the ABSORPTION spectrum when Mira has reached seventh magnitude on the rise, with the intensities of the lines increasing to well after maximum light; they then disappear. The emission spectrum is mainly hydrogen.

Mira lies at a distance of 196 l.y. Robert AITKEN discovered a faint companion to Mira in 1923. The separation is 0".61; the companion is now known as VZ Ceti, a variable with a range of mag. 9.5 to 12. It shows small variations in times of several hours on which are superimposed 10- to 15-minute variations. It also has very occasional flares, which last for about two minutes. It is possible that VZ Ceti is a white dwarf with an accretion disk. It orbits Mira in 1800 years. See also MIRA STAR

Mirach The star p Andromedae, visual mag. 2.06, distance 199 l.y., spectral type M0 III. Its name is a corruption of the Arabic word mi'zar, meaning 'loin cloth'.

Miranda Smallest and innermost of the five SATELLITES of URANUS that were known before VOYAGER 2's flyby in 1986. It was discovered in 1948 by Gerard KUIPER. Miranda's dimensions of 480 X 468 X 466 km (298 X 291 X 290 mi) are just large enough for it to be pulled into a near-spherical shape by its own gravity. Miranda was expected to be a passive ice-ball, like Saturn's satellite MIMAS, and therefore not especially interesting. However, at the time of the Voyager 2 encounter, the orientation of Uranus and its satellite orbits was such that it was convenient to direct the probe closer to Miranda than to any of the other satellites, and Miranda turned out to be one of the most fascinating bodies in the Solar System.

Miranda Uranus satellite Miranda was discovered by Gerard Kuiper in 1948. Its surface is unlike anything else in the Solar System.

Like all Uranus' satellites seen by Voyager 2, only the southern hemisphere of Miranda was sunlit at the time, but that was enough to show a world unlike any other known planetary body. Just over half the area seen is heavily cratered, but most of the craters are blurred, as if blanketed with dust or 'snow', perhaps dispersed from explosive cryovolcanic eruptions (see CRYOVOLCANISM). The rest of the visible hemisphere is occupied by three remarkable tracts of terrain, referred to as 'coronae'. They do not closely resemble one another and, apart from their vaguely concentric pattern, they have little in common with the features called coronae on Venus. Miranda's coronae are named Arden, Inverness and Elsinore after the settings of some of Shakespeare's plays. Inverness Corona was informally referred to as 'the Chevron', for its V shape. Craters within these coronae are fresh and sharp in appearance, as are the younger craters elsewhere, so evidently the global mantling of the older terrain took place before the coronae were created.

It is unclear how Miranda's coronae originated. Arden and Elsinore Coronae were only partly visible, so their complete shape is unknown. When the images were first received it was suggested that Miranda is a body that re-accreted following collisional break-up. Each corona was thought to represent a discrete fragment, the tonal banding apparent in Arden and Inverness Coronae representing cross-sections through the interior of the original body. This explanation now seems unlikely, if only because of the much greater age of the terrain between the coronae, as judged by comparing the relative numbers of impact craters. The edges of Inverness Corona appear to be related to faults. An alternative explanation, however, is that this and other coronae overlie internal density anomalies inherited from a re-accretion event. If so, each corona could have been created by a past episode of TIDAL HEATING through orbital interaction with ARIEL or UMBRIEL. The bright patches on Arden and Inverness Coronae could perhaps be fresh powder erupted from explosive vents. On the other hand, Elsinore Corona lacks tonal variations and is more likely to be a product of multiple eruptions of cryovolcanic lava. In contrast with Saturn's satellites, where the presence of ammonia is possible but not required by models of Solar System formation, it is extremely likely that Uranus' icy satellites contain a significant proportion of ammonia mixed with the water ice. The lavas this mixture is likely to produce would be highly viscous, which could explain Elsinore Corona's bulbous ridges. See data at URANUS

Mira star (M) Class of VARIABLE STAR named after MIRA, the first star found to vary with an approximate period of several months. They are also called long-period variables and red variables, although the latter term is ambiguous, given that there are several other types of variable with late spectral classes. Mira stars are GIANTS or SUPERGIANTS belonging to the disk population and with periods ranging from 100 to 1000 days. The difference in their periods stems from whether they are Population I or II. The former generally have periods longer than 200 days, while the periods of Population II stars tend to be shorter and less than 200 days.

The visual light variations of Mira variables range from mag. 2.5 to 11, but in the infrared their amplitudes are much smaller, below mag. 2.5 and less than one magnitude for most of them. Each star has a mean cycle - that is, an average period during which it goes through a complete cycle from maximum and back to maximum again or from minimum to minimum. These periods for any one star may vary by about 15%. Thus their periods are to some extent irregular.

Amplitudes may also vary widely from one cycle to the next. Their light-curves show a wide diversity, but fall into three main divisions. There are those stars that have rises steeper than the fall; these tend to have wide minima and sharp, short maxima. As the asymmetry becomes greater, the period lengthens. Stars with symmetrical curves have the shortest periods. A third group shows humps on their curves or have double maxima and have long and short periods.

Mira variables have late-type emission spectra. The longer the period, the later the spectral type. They show a spectral (or temperature) relationship that is a continuation of the same relationship found in CEPHEID VARIABLES. Both Mira variables and Cepheids are hotter at maximum, Mira variables having the smaller range in temperature. Pulsation is thus the underlying cause of their variability. Pulsations send running waves through the surface layers.

Mirphak (Algenib) The star a Persei, visual mag. 1.79, distance 592 l.y., spectral type F5 Ib. Its name, which is also spelled Mirfak, comes from the Arabic mirfaq, meaning 'elbow'. Its alternative name, Algenib, comes from the Arabic al-janib, meaning 'the side'.

mirror Reflecting surface used to shape or steer light beams. Mirrors have been used to collect and focus starlight since Sir Isaac NEWTON constructed the first successful reflecting telescope in 1668. His design is still in use today and the NEWTONIAN TELESCOPE remains one of the most popular types amongst amateur astronomers. It consists of a concave mirror that reflects the incoming light from a star towards a focal point immediately in front of the mirror. A second small flat mirror is used to intercept the light before it reaches the FOCUS and to steer it out to one side where the observer can use an EYEPIECE to magnify and examine the image.

Newton's first telescope used mirrors made of polished metal that had low reflectivity and tarnished easily. Nevertheless, it overcame the inherent problem of CHROMATIC ABERRATION that had affected early refracting telescopes. Improvements in materials made metal reflectors more useful, in particular the development of speculum metal, an alloy of 71% copper and 29% tin. This metal could be polished to provide fairly high reflectivity, but it still tarnished and needed periodic re-polishing.

The largest successful reflector using speculum metal was the 72-inch (1.8 m) Newtonian telescope built by the Earl of Rosse at BIRR CASTLE in Ireland in 1842.

Optical mirror technology took another big step forward with the development of coating techniques to place a highly reflective thin metal coating on to glass. Initially silver was used but aluminium is more common today as it produces a more durable surface. Almost all mirrors used in astronomy are coated in aluminium, although gold, silver and other materials are used where their special properties are needed. Amateur telescope mirrors often have a protective overcoat of silicon dioxide to increase the life of the aluminium coating. Professional telescopes usually dispense with the overcoat but are re-coated periodically, often every one or two years.

The largest single (monolithic) mirrors used in astronomy at present are just over 8 m (310 in.) in diameter. They are quite thin relative to their diameter (to keep weight down) and require elaborate support mechanisms to maintain their very accurate shapes as they point to different parts of the sky. Segmented mirrors of 10-m (393-in.) diameter are in use and even larger telescopes are planned using this technique.

The largest mirror in space at present is the 2.4-m (94-in.) diameter mirror of the HUBBLE SPACE TELESCOPE; the NEXT GENERATION SPACE TELESCOPE is planned to have a segmented mirror around 4 m (160 in.) in diameter.

Mirzam The star p Canis Majoris, visual mag. 1.98, distance 499 l.y., spectral type B1 II or III. It is a BETA CEPHEI STAR, a pulsating variable with a period of 6 hours and a total range of less than 0.1 mag. The name comes from the Arabic murzim, meaning 'announcer', thought to refer to the fact that it rises before the much brighter Sirius.

missing mass problem Quandary whereby most of the matter in the Universe seems to be either subluminous or else invisible. This dark matter is thought to surround individual galaxies as haloes and to pervade clusters of galaxies.

missing mass problem The dotted curve shows the rate at which bodies in our Galaxy should be circling the centre, according to Keplers third law, if it consisted wholly of the visible matter we see. The white line shows that objects farther from the centre of the Galaxy are, in fact, moving faster, and this can only be explained if there is a substantial amount of missing dark matter surrounding the Milky Way.

The first evidence of dark matter came from the work of Jan OORT, who, in 1932, determined the thickness of our Galaxy in the solar neighbourhood from the locations of red giants in the sky. These bright stars are sufficiently numerous that a map of their distribution in space reveals that the galactic disk has a thickness of about 2000 l.y. In addition, however, Oort used the distribution and speeds of the red giants in his study to calculate the vertical gravitational field of the Galaxy as well as the mass in our vicinity needed to produce it. A dilemma arose when Oort compared this calculated mass with the observed masses of all the stars and gas clouds in the solar neighbourhood. The observed mass is only about half that needed to confine the red giants to a disk 2000 l.y. thick. Thus half of the matter in our part of the Galaxy seems to be invisible.

Observations of the rotation of the Galaxy confirm the existence of this invisible matter. The rotation rate of the inner regions of our Galaxy is determined from bright stars and emission nebulae. In the dim outer regions, radio observations of hydrogen and carbon monoxide in giant gas clouds provide the required data. In all cases, Doppler shift measurements (see DOPPLER EFFECT) are combined with information about the position of a source to deduce its circular velocity around the galactic centre. The results are best displayed on a plot of circular velocity against distance from the galactic centre.

Beyond the confines of most of the Galaxy's matter, the orbital velocity of outlying stars should decrease according to Kepler's third law, just as the velocities of the planets decrease with increasing distance from the Sun. The dashed line in the accompanying diagram indicates such a Keplerian decline. However, the rotation curve of the Galaxy is nearly flat, even to a distance of 60,000 l.y. from the galactic centre. This means that astronomers have still not detected the edge of the Galaxy and thus a substantial quantity of invisible matter must exist beyond the observable stars, nebulae and gas clouds.

The conventional picture of our Galaxy is a flattened disk 100,000 l.y. in diameter surrounded by a spherical halo of old stars and globular clusters roughly 130,000 l.y. across. Recent analyses of rotation curve data strongly suggest that the galactic halo is embedded in an enormous 'corona' that is roughly 600,000 l.y. in diameter and contains at least 1012 solar masses of subluminous matter. Thus the corona is at least five times more massive than the disk and halo combined. Most other spiral galaxies also exhibit flat rotation curves and thus they too must be surrounded by large, massive coronae.

The distribution of matter in such a galaxy can be compared with its luminosity by constructing the mass-to-light ratio, M/L. In essence, M/L tells us how much matter there is in a given region compared with the radiation that it generates. By definition, M/L for the Sun is 1.0 (that is, 1 solar mass produces 1 solar luminosity), and 0.8 for a typical globular cluster.

From a rotation curve, astronomers calculate the rate at which the matter density declines with distance from a galaxy's centre. However, a galaxy's surface brightness decreases much more rapidly with distance from its centre. Since the brightness falls much more rapidly than the matter density, M/L climbs to 50 or more in the outer regions of such a galaxy. This dramatic increase in the mass-to-light ratio is the crux of the missing mass problem: an increasing proportion of non-luminous matter is found as one moves outwards from a galaxy's centre.

A partial solution of this aspect of the missing mass problem came with the detection of several faint BROWN DWARF stars in the halo of our Galaxy. If the brown dwarfs are distributed as evenly as they appear to be, then they might account for some of the missing mass in our Galaxy.

Apparently, the space between galaxies in a cluster is also dominated by non-luminous matter. In the 1930s, Fritz Zwicky and Sinclair Smith pointed out that the Virgo Cluster must contain a substantial amount of non-luminous matter, otherwise there would not be enough gravity to hold the cluster together.

The mass of a cluster of galaxies can be determined from the VIRIAL THEOREM, which relates the average speed of the galaxies to the size of the cluster. For example, the Coma Cluster contains roughly 1000 bright galaxies spread over a volume 10 million l.y. in diameter. Doppler shift measurements are available for 800 Coma galaxies and the average velocity relative to the cluster's centre is about 860 km/s (530 mi/s). These data along with the vir-ial theorem give a total mass of 5 X 1015 solar masses for the Coma Cluster. If each of the 1000 brightest galaxies has a mass of 1012 solar masses then we are able to account for only one-fifth of the cluster's mass.

The dotted curve shows the rate at which bodies in our Galaxy should be circling the centre, according to Kepler's third law, if it consisted wholly of the visible matter we see. The white line shows that objects farther from the centre of the Galaxy are, in fact, moving faster, and this can only be explained if there is a substantial amount of 'missing' dark matter surrounding the Milky Way.

Similar results are obtained for other rich clusters. Such clusters typically have mass-to-light ratios in the range of 200 to 350, clearly indicating a substantial presence of dark matter. This missing mass problem was also recently solved when X-rays from the centres of large galaxy clusters were seen. These X-rays indicated the presence of large amounts of hot gas that radiated primarily in the X-ray region of the spectrum, and thus were invisible in the optical region. The amount of this gas is generally estimated to be enough to bind the clusters together as required by the virial theorem.

The issue of missing mass also arises on the largest cosmological scales. According to the general theory of relativity, the expansion rate of the Universe must be slowing down because of the mutual gravitational attraction of all the matter in the Universe. Astronomers detect this cosmic deceleration by measuring the recessional velocities and distances of extremely remote galaxies. The purpose of such observations is to determine the so-called DECELERATION PARAMETER (q0), which is directly related to the average density throughout space. Thus, by measuring the rate at which the cosmic expansion is slowing down, astronomers can deduce the average density of matter in the Universe.

Observations indicate that the average density of matter in the Universe is near the CRITICAL DENSITY of 5 X l0~30 g/cm3, which is equivalent to about three hydrogen atoms per cubic metre of space. This density is called 'critical' because it ensures that the Universe will just barely continue expanding for ever, without ever collapsing back upon itself. However, the average density of matter that astronomers actually observe in space is about 3 X l0~31 g/cm3. Thus the observed matter is less than a tenth that needed to account for the Universe's behaviour.

With the inflationary model (see INFLATION), and observations indicating that the Universe is actually accelerating, the need for non-luminous matter is eliminated.

Mitchell, Maria (1818-89) First American woman astronomer of note. Her father, William Mitchell, was a respected amateur astronomer, and from him she got her love of astronomy and her observing skills. She assisted him in observations he made, for example timing the contacts of an annular eclipse in 1831. In 1836 she became a librarian, and in the same year the US Coast Survey equipped the Mitchell family home as an outstation, installing an observatory. She began making regular observations, and achieved world fame with her discovery of a comet, C/1847T1.

From 1849 to 1868 she was employed by the US Government Almanac Office to compute ephemerides for Venus. From 1865 until her death she worked at Vassar Female College at Poughkeepsie, NY (of which she had been a founding member in 1861), as both professor of astronomy and director of the observatory there.

Mitchell, known as 'the female astronomer', was widely honoured, and was elected the first woman member of the Inconspicuous equatorial constellation, representing a unicorn, between Orion and Hydra. Monoceros was probably introduced by the Dutch theologian and geographer Petrus Plancius in 1613. Its brightest star, p Mon, is a fine triple system with bluish-white components, mags. 4.5, 5.2 and 5.6 (combined mag. 3.8), separations 7".2 and 10".1. The constellation also contains plaskett's star (HD 47129), a 6th-magnitude spectroscopic binary whose components are among the most massive stars known; and the variables U Mon (an rv tauri star, range 6.1-8.8) and RMon (a ttauri star, range 11.0-13.8), which illuminates hubble'svariable nebula. Deep-sky objects include the star clusters NGC 2264, with over 100 stars, including 15 Mon (also known as S Mon), mag. 4.7 (variable); and NGC 2244, which is embedded in the rosette nebula (NGC 2237-9).

American Academy of Arts and Sciences. She was active in many campaigns for the advancement of women. The maria mitchell observatory is named after her.

Mizar The star Ursae Majoris, visual mag. 2.23, distance 78 l.y., spectral type A2 V. As well as being a spectroscopic binary with a period of 20.53 days, it has a binary companion of mag. 4.0 visible through a small telescope. Mizar also forms a much wider naked-eye double with alcor.

MK system Abbreviation of morgan-keenan classification

MMT Observatory (MMTO) Major US optical astronomy facility operating the MMT Telescope at fred l. whipple observatory. It takes its name from the Multiple Mirror Telescope, which occupied the same housing between 1979 and 1998. The original MMT had six 1.8-m (72-in.) primary mirrors mounted together and feeding a common focus, with a combined light-gathering power equivalent to that of a single 4.5-m (177-in.) mirror. The new MMT has a 6.5-m (256-in.) mirror made at the steward observatory Mirror Laboratory. The refurbished instrument was dedicated in 2000. The MMTO is a joint facility of the Smithsonian Institution and the University of Arizona.

mock Sun Popular name for a parhelion

modified Julian date (MJD) See juliandate

molecular cloud Cloud in which the gas is primarily in the form of molecules. Such clouds abound within the spiral arms of galaxies, including our own. The clouds' temperatures may be as low as 10 K, and their number densities as much as 1012 m~3, which is a million times the average density of interstellar matter. The clouds' sizes range from less than a light-year to several hundred light-years, and their masses range from a few solar masses to several million times the mass of the Sun. Clouds with masses that exceed 10,000 solar masses are called giant molecular clouds.

The clouds are difficult to observe because of their low temperatures. In the visible region, they may sometimes be seen silhouetted against bright backgrounds, since the dust particles within them absorb light. In the radio region the most abundant molecule, hydrogen, does not emit, so the clouds have to be found from the emissions of their less abundant molecules such as carbon monoxide (see interstellar molecules).

Molonglo Observatory Synthesis Telescope (MOST) Radio astronomy facility near Canberra, Australia, developed in the late 1970s from the earlier One-Mile Mills Cross Telescope. It consists of two cylindrical paraboloids measuring 780 X 12 m (2560 X 39 ft), separated by 15 m (49 ft) and aligned east-west. The instrument is steered by rotating the cylinders about their long axis and 'phasing' the feed elements along the two arms. MOST is operated by the University of Sydney and is used for analysing complex structure in radio objects, and for major surveys of the radio sky.

monocentric eyepiece Solid eyepiece design, now largely obsolete, comprising three elements cemented together, and therefore free from ghosting. The limited field of view restricts the usefulness of the monocentric eyepiece to planetary or deep-sky observing.

Monoceros See feature article

Montanari, Geminiano (1633-87) Italian astronomer and mathematician who discovered the variability of Algol (1669). Using a specially made reticule, he drew one of the earliest maps of the Moon (1662). He made some of the earliest systematic magnitude estimates of variable stars. Montanari studied the effects of atmospheric refraction, taking these into account in his own observations, which formed the basis for ephemerides he published in 1665.

month Unit of time based on the period of revolution of the Moon around the Earth. This can be measured with respect to a number of different reference points, but the most commonly used is the synodic month, the period between successive new or full moons (29.53059 days of mean solar time). The calendar month is a man-made method of dividing the year into twelve, roughly equal, parts.

Because the total of complete lunar cycles in a year is not a whole number, synodic months cannot be simply reconciled with the common calendar. However, 235 lunar months is almost exactly equal to 19 years (see metonic cycle) and while the modern civil calendar is based on the year, the dates of religious festivals (such as Easter) are still set by reference to the lunar month. See also anomalistic month; draconic month; sidereal month; tropical month

Moon Earth's only natural satellite. Because of its proximity, it is the brightest object in the sky, apart from the Sun, being at a mean distance of only 384,000 km (239,000 mi). Like the Sun, its apparent diameter is about half a degree. With an actual diameter of 3476 km (2160 mi) and a density of 3.34 g/cm3, the Moon has 0.0123 of the Earth's mass and 0.0203 of its volume. The Earth and Moon revolve around their common centre of gravity, the BARYCENTRE. Although it is so bright (the full Moon has apparent visual magnitude of 12.7) its surface rocks are dark, and the Moon's albedo is only 0.07. It is a cratered world, in many ways a typical Solar System satellite. It is the only other world whose surface features can easily be seen from the Earth, and the only other world on which humans have walked.

Moon When it is at its brightest, at full moon, the differences between the bright lunar highlands and the dark, lava-filled basins become very apparent.

As the Moon orbits the Earth, it is seen to go through a sequence of PHASES as the proportion of the illuminated hemisphere visible to us changes. A complete sequence, from one new moon to the next, is called a lunation. At new moon or full moon, eclipses can occur. An observer on the Earth always sees the same side of the Moon because the Moon has SYNCHRONOUS ROTATION: its orbital period (the SIDEREAL MONTH) around the Earth is the same as its axial rotation period. The visible side is called the nearside, and the side invisible from the Earth is the farside. In fact, the face the Moon presents to us does vary slightly because of a number of effects known collectively as LIBRATION.

The Moon has been studied by many space probes, including the LUNA and ZOND series launched by the former Soviet Union, and the US series RANGER, SURVEYOR and LUNAR ORBITER. The manned APOLLO missions and some of the later Luna probes returned samples of lunar material to the Earth for study. In the 1990s, the CLEMENTINE and LUNAR PROSPECTOR probes gathered much valuable new information.

Moon Much of the Moons surface is covered in a fine dust, called the lunar regolith. Because there is no wind or weathering, this material remains just as it settles, and the marks of small impacts, as well as astronauts with lunar carts, remain more or less permanently.

The surface features may be broadly divided into the darker maria (see MARE), which are low-lying volcanic plains, and the brighter HIGHLAND regions, which occur predominantly in the southern part of the Moon's nearside and over the entire farside. There are impact features of all sizes. The largest are called BASINS, produced during the early history of the Moon when bombardment by impacting objects was at its heaviest. On the nearside, where the crust is thinner, some basins were subsequently filled with upwelling lava to produce the maria, which in turn became cratered. In the regions of CRATERS at the north and south poles, the interiors of which are in permanent shadow, water ice exists just below the surface. The smaller basins are similar to the largest craters, formerly called walled plains, which have flat floors and are surrounded by a ring of mountains. Other features are mountain peaks and ranges, valleys, elongated depressions known as RILLES, wrinkle ridges, low hills called domes, and EJECTA and bright RAYS radiating from the sites of the more recent cratering impacts.

Early lunar cartographers assumed that the darker lunar features were expanses of water, and named them after fanciful oceans (oceanus), seas (mare), bays (sinus), lakes (lacus) and swamps (palus). Other features are named after famous people, principally astronomers and other scientists. The old names are still in use on lunar maps.

At present, the most generally accepted theory of the Moon's origin is the GIANT IMPACTOR THEORY, according to which a Mars-sized body collided with the newly formed Earth, and debris from the impact accreted to form the Moon. Impacts during the accretion process melted the lunar surface to a depth of 300 km (190 mi), forming a magma ocean. As accretion slowed, the magma ocean began to cool, allowing minerals to crystallize. DIFFERENTIATION led to formation of a crust.

Many of the most prominent lunar features are scars from a final stage of accretion (LATE HEAVY BOMBARDMENT). Massive impacts produced basins such as Mare IMBRIUM and Mare ORIENTALE. Impact ejecta covered the lunar surface in a deep blanket of fragmented material, the mega-REGOLITH. Flooding of the basins by lava from the lunar mantle (kept molten by radioactive decay) led to formation of the darker maria.

Smaller impacts gave rise to the craters. A volcanic origin for the craters has long been dismissed by most astronomers, but Clementine photographed a volcanic crater near the large farside crater Schrodinger. Mild moonquakes occur at depths of roughly 700 km (450 mi), and LUNAR TRANSIENT PHENOMENA appear to be associated with them; otherwise, the Moon is now geologically inactive.

Seismic measurements at Apollo landing sites provided some information on the Moon's internal structure. The lunar crust is about 100 km (60 mi) thick in the highland regions, but only a few tens of kilometres thick under the mare basins. Under the largest basins the underlying mantle has bulged upwards to form MASCONS. An iron-rich core a few hundred kilometres across might exist, with a temperature at the centre of about 1500 K. There is a small iron core at the Moon's centre, accounting for about 4% of its mass. Although there is now no significant overall magnetic field, there are distinct 'magnetic areas' extending for hundreds of kilometres.

Knowledge of lunar rocks and their compositions is based largely on laboratory analyses of the 382 kg of material returned to Earth by the Apollo missions. Planetary geologists have identified several types of lunar rocks. Lunar BASALTS are fine-grained igneous rocks that formed from cooled lavas; they contain two characteristic minerals - pyroxene and plagioclase. Ending about 3 billion years ago, the volcanic activity that produced these rocks consisted of very gradual, thin flows of lava with much lower viscosity than terrestrial lavas; these spread out in thin sheets covering millions of square kilometres instead of building up shield volcanoes of the kind seen on Earth and Mars.

Another important class of lunar rocks are the BRECCIAS found in the highlands. Breccias consist of a matrix of stony fragments (mostly granite), with finer mineral components gluing the fragments together. The breccias are rich in whitish-coloured calcium, magnesium and aluminium, which explains why the lunar highlands appear much brighter than the maria.

The Apollo missions also discovered two minerals unique to the Moon. ANORTHOSITE is a coarse calcium feldspar that is a major constituent of the lunar crust. The other, KREEP, consists of potassium (chemical symbol K), rare earth elements and phosphorous.

Moon Much of the Moon's surface is covered in a fine dust, called the lunar regolith. Because there is no wind or weathering, this material remains just as it settles, and the marks of small impacts, as well as astronauts with lunar carts, remain more or less permanently.

The Moon has only the most tenuous of atmospheres. Apollo instruments detected traces of gases such as helium, neon and argon. The atmosphere is probably made up of solar wind particles retained temporarily, and atoms sputtered (knocked off) from the surface by the solar wind. Consequently the surface temperature variation is extreme, ranging from 100 to 400 K.

moon Natural satellite of one of the larger bodies in the Solar System. Mars, for example, possesses two moons, phobos and deimos. Each of the giant planets is accompanied by a large retinue of moons. Smaller moons may be termed moonlets, and the distinction between a moon/moonlet and a body too small to be so classified (for example, the billions of individual items comprising planetary ring systems) is arbitrary.

Moore, Patrick Alfred Caldwell (1923- ) English author, television presenter and popularizer of astronomy, an accomplished lunar and planetary observer who has directed the Lunar Section of the British Astronomical

Association. After serving in the Royal Air Force during World War II, he wrote the first of over 60 books on astronomy aimed at a popular audience. In the 1950s, with Hugh Percy Wilkins (1896-1960), he used Meudon Observatory's 33-inch (0.84-m) refractor to make a thorough survey of the Moon's topography, discovering many new features. Since 1957 Moore has presented 'The Sky at Night', the longest-running BBC television programme in history. The caldwell catalogue is his extension of the Messier Catalogue.

Mopra Telescope See Australia telescope national facility

Moreton waves Wave-like disturbances in the chromosphere and lower corona by flares. They are named after Gail E. Moreton, who first observed them in 1960 as a circular hydrogen-alpha brightening that expanded away from a flare and across the solar disk. Moreton waves travel to distances of about a million kilometres at velocities of about 1000 km/s (600 mi/s).

Morgan, William Wilson (1906-94) American astronomer whose work led to the Morgan-Keenan system of spectral classification and the UBV system of photometry. He spent his entire career at Yerkes Observatory (1926-94), briefly serving as its director (1960-63).

Assisted by Philip Childs Keenan (1908-2000) and Edith Marie Kellman (1911- ), Morgan classified huge numbers of stars into six different LUMINOSITY CLASSES. This luminosity classification was combined with a new two-dimensional spectral classification in An Atlas of Stellar Spectra With an Outline of Spectral Classification (1943), still the standard work in its field. The MORGAN-KEENAN CLASSIFICATION (abbreviated to MK or MKK system) allowed 'normal' stars to be distinguished from peculiar A-type and metallic-line A stars. It also made possible vastly more accurate calibrations of stellar distances, especially for the nearer of the O and B stars.

In the late 1940s and early 1950s, Morgan and Harold JOHNSON developed a new system for measuring stellar magnitudes in three basic colours or wavebands: U (ultraviolet), B (blue) and V (visual, or yellow). They combined their UBV SYSTEM (which Johnson later extended) with the MKK system to improve distance estimates for stars and star clusters. In 1957 Morgan used these tools to devise the first system for classifying the integrated spectra of globular clusters. He went on to classify galaxies by comparing the galaxy's integrated spectrum with its bulge-to-disk ratio, in what was called the Yerkes system.

Morgan-Keenan classification (MK system, MKK system) Categorization of a stellar SPECTRUM from the visual appearance of its ABSORPTION LINES. In the early 1940s, William MORGAN, Philip Keenan (1908-2000) and Edith Kellman (1911- ) extended the one-dimensional temperature-dependent HARVARD CLASSIFICATION (OBAFGKM) to two dimensions by adding Roman numerals to indicate luminosity - I for supergiant, II for bright giant, III for giant, IV for subgiant and V for dwarf. Brighter supergiants are Ia, lesser ones Ib. (The Roman letters are also commonly used to separate giants of different luminosities, with IIIa being brighter than IIIb.) R, N, C and S stars also use MOON (continued)

MK luminosity classes. Keenan later added Arabic '0' for extra-luminous 'hypergiants'.

The MK system defines each spectral class by the spectrum of a representative star (B0 V by t Scorpii, B1 V by t) Orionis, and so on.). The essence of the system is a set of MK blue-violet photographic spectra of standard stars against which others stars are compared and which form the basis for the extension of the system to other wavelength domains.

Though commonly applied to stars of Population II, MK classification is formally appropriate only for solar-metallicity Population I. Outside the system, subdwarfs and white dwarfs are sometimes referred to as VI and VII.

morning star Popular name for the planet venus when it appears, prior to superior conjunction, as a bright object in the eastern sky before sunrise. The term is occasionally applied to mercury. See also evening star

Moulton, Forest Ray (1872-1952) American mathematician and astronomer who specialized in celestial mechanics. He was a long-time professor at the University of Chicago (1898-1926). With Thomas chamberlin, he conceived the planetesimal hypothesis - that the Solar System formed when a passing star caused the Sun to eject filaments of matter that condensed into protoplanetary bodies, which later accreted to form the planets.

Mount Graham International Observatory Major US optical observatory on Mount Graham in the Pinaleno Mountains, some 110 km (70 mi) north-east of Tucson, Arizona, at an altitude of 3260 m (10,700 ft), operated by the University of Arizona. The site is in the Coronado National Forest, and in 1988 controversy followed Congressional approval of the observatory's construction because the forest is home to the endangered Mount Graham red squirrel (the population of which has since increased). The observatory hosts the vatican advanced technology telescope, the heinrich hertz telescope of the Submillimeter Telescope Observatory and, when it is completed, the large binocular telescope.

mounting Means of supporting the weight of a telescope and aiming it at different points in the sky. Of necessity, all large professional instruments are on permanent mounts; many amateur telescopes and their mounts are portable. Whether a mounting is permanent or portable, rigidity is a prime requirement. See altazimuth mounting; equatorial mounting

Mount Palomar Observatory See palomar observatory

Mount Pleasant Radio Observatory Observatory run by the Physics Department of the University of Tasmania, Australia, located 20 km (12 mi) east of Hobart. It is equipped with two antennae: a 26-m (85-ft) donated to the University by NASA in 1985, and a 14-m (46-ft).

Mount Stromlo Observatory One of the oldest institutions in the Australian Capital Territory, at an altitude of 760 m (2500 ft) in the south-western suburbs of Canberra. It was founded in 1924 as the Commonwealth Solar Observatory, and in the 1920s and 1930s research was conducted there on solar and atmospheric physics. In 1957 the Observatory became part of the Australian National University. During the same decade, four new instruments came to the mountain, including the 1.88-m (74-in.) reflector (for twenty years the joint-largest telescope in the southern hemisphere) and a 1.27-m (50-in.) reflector that had started life in 1868 as the Great Melbourne Telescope. This instrument, now refurbished, is used for the massive compact halo object dark-matter project. During the 1960s, under the directorship of Bart bok, Mount Stromlo founded siding spring observatory to escape the light pollution of Canberra, and the institution became the Mount Stromlo and Siding Spring Observatories (MSSSO). In 1998 the name was changed again to the Research School of Astronomy and Astrophysics (RSAA).

Mount Wilson Observatory One of the most famous observatories in the world, located in the San Gabriel range near Pasadena, California, at an elevation of 1740 m (5700 ft). It was founded in 1904 as Mount Wilson Solar Observatory by George Ellery hale, using funds from the Carnegie Institution of Washington, and the snow telescope for solar spectroscopy was installed the following year. Mount Wilson was developed as a site for stellar astronomy with the completion of a 1.5-m (60-in.) reflector by George W. ritchey in 1907. It was followed in 1917 by the 2.54-m (100-in.) hooker telescope, which dominated US astronomy until the completion of the hale telescope at Palomar Observatory in 1948. This was the instrument Edwin hubble used in 1919 to identify cepheid variable stars in the Andromeda Galaxy, and for subsequent work that led to the realization that the Universe is expanding. In 1917 the word 'Solar' was dropped from the observatory's name.

Today, Mount Wilson remains part of the carnegie observatories. Although seriously affected by the lights of Los Angeles, 32 km (20 mi) to the south-west, the observatory has entered a new era with the installation of Georgia State University's CHARA Telescope Array. CHARA, the Center for High Angular Resolution Astronomy, consists of a Y-shaped arrangement of five 1-m (39-in.) telescopes within a 400-m (quarter-mile) radius linked interferometrically. It is possible that the Hooker and 60-inch telescopes will be incorporated into the array.

moving cluster moving groups of stars close enough to have their distance determined by the moving cluster parallax method, as follows.

moving cluster The velocity of stars in an open cluster can be determined if they are close enough for their parallax to be measurable over a short period. Their parallax is combined with their line-ofsight rate of motion towards or away from Earth to obtain their speed of movement at right angles to the line of sight. Combined with the measurement of the angle of movement, this allows astronomers to judge their distances.

If the moving cluster is close to Earth, then projecting the spatial motions of the stars backwards will produce a set of paths that converge at one point. The direction of this convergent point is parallel to the space motion of the cluster. Observing individual stars within the cluster will give a value for the parallax and hence the distance of the system. This is known as the moving cluster parallax method of distance determination. Measuring the radial velocity of individual stars and the angle 6 to the convergent point gives a value for the space velocity and hence the tangential velocity. The proper motion of the individual stars then gives an idea of the parallax of the cluster.

The moving cluster parallax method has been used to determine the distances of several nearby open clusters. The proximity of these clusters allows distances to be calculated with a high precision. The hyades in particular is often used as the main calibrator of the distance scale of the Universe.

moving group Group of stars that share the same motion through space, and have the same age and similar chemical compositions. Moving groups can be in many forms, including open clusters, globular clusters, ob associations and t associations. If they lie close enough to Earth for the spatial motions of the individual stars to be determined, they are termed moving clusters and the cluster distance can be determined by the moving cluster parallax method. The nearest moving group to Earth is the ursa major moving cluster.

MPC Abbreviation of minor planet center

Mrkos, Comet (C/1957 P1) long-period comet discovered as a naked-eye object at dawn on 1957 August 2; it became the second prominent comet of that year. When discovered by Antonin Mrkos (1918-96) from Czechoslovakia, the comet was one day past its perihelion (distance 0.35 AU). Closest approach to Earth came on August 13, and Comet Mrkos reached a peak magnitude + 1.0 early in the month, with a dust tail extending for about 5. The ion tail showed considerable activity, the sunspot cycle at this time being close to maximum. The orbit is elliptical, with a period of 13,200 years.

M star Any member of a class of orange-red stars, the spectra of which are defined by molecular absorption bands of metallic oxides, particularly titanium oxide. Vanadium oxide is present in addition to some hydrides and water. Hydrogen is very weak and disappears towards cooler subclasses. Neutral calcium is strong; its weakening with increasing luminosity helps to determine the MK class. Only classes L and T are cooler than M stars. Main-sequence subclasses range from 2000 K at M9.5 to 3900 K at M0; zero-age masses range from the hydrogen-fusing limit near 0.08 solar mass to 0.5; zero-age luminosities range from 2 3 1024 to half solar (most of the radiation emerging in the infrared). Lifetimes far exceed that of the Galaxy. Long lifetimes and high rates of low-mass star formation make M stars numerous. Ignoring classes L and T (whose populations are not yet assessed), class M contains 70% of all dwarf stars (none are visible to the naked eye).

The convective envelopes of cool dwarfs extend more deeply as mass decreases, and at 0.3 solar mass (class M5) they reach the stellar centres, forcing cool M stars below about 3000 K to be completely mixed. Despite slow rotation, the deep convective zones somehow produce stellar magnetic fields and Sun-like activity. A good fraction of M dwarfs (dMe stars) display chromospheric emission and powerful flares that can brighten the stars by one or more magnitudes and that radiate across the spectrum.

No giant or supergiant is cooler than class M. Having evolved from dwarfs of classes G to O, such stars are much more massive than M dwarfs. As a result of lower densities, giant surface temperatures are somewhat cooler than their dwarf counterparts. M giants divide into those with quiet helium cores that are climbing the giant branch for the first time, and asymptotic giant branch (AGB) stars with quiet carbon-oxygen cores that are climbing it for the second time. First-ascent M giants have early subtypes and can reach luminosities of 1000 times solar and radii of 100 times solar. Second-ascent giants can reach into later subtypes, and become brighter (over 10,000 solar) and larger, with the more massive rivalling the diameter of Mars' orbit.

The brighter AGB stars pulsate as Mira variables, changing by five or more visual magnitudes over periods that range from 100 to 1000 days. Much of the visual variation is caused by small temperature variations that send the radiation into the infrared and create more powerful obscuring TiO bands. Bolometric (true luminosity) variation is much less, at about a magnitude. The pulsations create running shock waves, which generate emission lines, rendering Miras a class of me star. When M giants dredge the by-products of nuclear fusion (including carbon) to their surfaces, they become s stars and then carbon stars. Pulsations, plus the large luminosities and radii, promote powerful winds and mass-loss rates of 1025 solar masses (or more) per year. Silicate dust condenses in the winds of M class Miras; carbon dust condenses in the winds of carbon stars. The winds of class M Miras create vast envelopes that can radiate powerful OH maser emission, whence they are known as OH/IR (infrared) stars.

Among lower luminosity giants are found 'semi-regular' (SR) variables. SRa stars (S Aquilae, R Ursae Minoris) are similar to Miras but exhibit smaller light variations and irregularities in their periods. SRb stars (R Lyrae, W Orionis) have periods that are less well defined. 'Lb' (irregular) M giants have no periods at all.

The most luminous M supergiants develop from main-sequence O stars of up to 60 solar masses, most fusing helium in their cores. Bolometric luminosities can reach 750,000 times that of the Sun. Radii can approach 2000 times solar, rivalling the diameter of Saturn's orbit (nearly 10 AU). Many M supergiants are irregular (Lc) variables. betelgeuse (M2 Iab), for example, varies with a range of about half a magnitude on a timescale of years. Some of the lesser M supergiants (SRc stars) display some semi-regular periodicity.

Prominent examples of M stars include Proxima Cen-tauri M5 V, Betelgeuse M2 Iab, Antares M1.5 Ib, Mu Cephei M2 Ia, VV Cephei and M2 Iaep 1 O8 Ve.

Mu Cephei See garnet star

Mullard Radio Astronomy Observatory (MRAO) Radio astronomy observatory of the University of Cambridge, and part of the Cavendish Laboratory (the university's Department of Physics). Radio astronomy at Cambridge dates back to the earliest days of the science just after World War II, and flourished under the direction of Martin ryle from 1945 to 1982. The first Cambridge radio telescopes were built on the western outskirts of the city and specialized in the study of 'radio stars', sources now known to range from relatively nearby supernova remnants to the most distant galaxies. Whereas early work at jodrell bank observatory led to the development of large single dishes, the emphasis at Cambridge was on the design and construction of smaller, widely spaced aerials used as an interferometer to make better positional measurements.

In 1957, through the generosity of Mullard Ltd and with support from the Science Research Council, the MRAO was built on its present site at Lords Bridge, 8 km (5 mi) south-west of Cambridge. Today its instruments include the ryle telescope (formerly the Five-Kilometre Telescope, dating from 1972), the cambridge optical aperture synthesis telescope and the cambridge low-frequency synthesis telescope. In the 1990s the observatory built the Cosmic Anisotropy Telescope (CAT) at Cambridge as a prototype instrument for ground-based mapping of the cosmic microwave background radiation.

moving cluster The velocity of stars in an open cluster can be determined if they are close enough for their parallax to be measurable over a short period. Their parallax is combined with their line-of-sight rate of motion towards or away from Earth to obtain their speed of movement at right angles to the line of sight. Combined with the measurement of the angle of movement, this allows astronomers to judge their distances.

MUSCA (gen. muscae, abbr. mus) Small but distinctive southern constellation, representing a fly, between Carina and Circinus. It was introduced by Keyser and de Houtman at the end of the 16th century. Its brightest star, a Mus, is mag. 2.7. p Mus is a close binary with bluish-white components, mags. 3.5 and 4.0, separation 1".2. Deep-sky objects include the 7th-magnitude globular clusters NGC 4372 and NGC 4833.

In 2000 its successor, the very small array, was installed on the island of Tenerife in the Canaries.

Muller, Johannes See regiomontanus

Multi-Element Radio-Linked Interferometer Network (MERLIN) Group of six radio telescopes linked to jodrell bank Observatory, spread from Cambridge to Wales, UK. MERLIN acts as a radio interferometer with a maximum baseline of around 220-230 km (137-143 mi). At a wavelength of 6 cm MERLIN has a resolution of 0".05. One of the main tasks of MERLIN has been to study the structure of radio jets from quasars.

Multi-Element Radio- Linked Interferometer Network (MERLIN) The longbaseline interferometry of the six radio telescopes that make up MERLIN give the network similar resolving power to that of the Hubble Space Telescope. The network can also be joined to other radio telescopes around the world for even greater accuracy.

multiple star Gravitationally connected group of stars with a minimum of three components. About one in five binary star systems is gravitationally bound to one or more other stars. The additional stars are often far enough from the binary not to affect its evolution significantly.

The similarity between multiple stars and star clusters suggests that they form in similar ways, that is, by condensation from interstellar clouds. A good example of this is the trapezium in the orion nebula. The Trapezium consists of four very young stars enmeshed in nebulosity, with a further five possible members nearby. The Trapezium is the prototype of a special type of quadruple star in which very little relative motion is observed. It may be that the stars will spend most of their lifetimes in this configuration. Many other examples of Trapezium-type systems are known. In hierarchical quadruple systems, the most common situation is two pairs revolving about each other, as, for example, in epsilon lyrae.

The point at which highly multiple stars and star clusters overlap is not clear, but systems such as castor and Alpha2 Capricorni, with six components each, are still regarded as multiple stars. In these systems there is a recognized hierarchy or order. In Castor, for instance, the two main pairs rotate about a common centre of gravity, whilst a pair of cool red dwarfs rotates about the same centre, but at a much larger distance.

mural quadrant Early instrument used for measuring declination. It consists of a graduated circle fixed to a wall (in Latin, murus) orientated north-south. A telescope (or, in early versions, a simple sighting tube) is mounted centrally on a pivot so that its position can be read off the circle. The eyepiece of the telescope contains a horizontal graticule along which the image of the star must move as the Earth turns. Setting of the telescope must be done quickly, as the star passes the meridian. It was superseded in the mid-19th century by the transit circle.

Murchison meteorite that fell as a shower of stones in the state of Victoria, Australia, on 1969 September 28; more than 100 kg of material was collected. Murchison is classified as a CM2 carbonaceous chondrite. Rich in organic compounds, it was the first meteorite in which extraterrestrial amino acids were identified. It also contains abundant interstellar grains.

Musca See feature article

Muses Name of a series of Japanese science missions. When launched, they are given specific names.

Muses B Japanese spacecraft, named Haruka, which was launched in 1997 February and is the first astronomical satellite dedicated to very long baseline interferometry. The 8-m (26-ft) diameter antenna, made of Kevlar wire, with a pointing accuracy of 0.01, is combined with ground-based radio telescopes to provide an extremely high resolution, particularly useful for observing active galactic nuclei and quasars.

Muses C Japanese spacecraft to be launched in 2002 June at the earliest to land on an asteroid in 2003 April, take a sample and return it to Earth in 2006 June. If successful, Muses C will be the first time a sample of an asteroid has been brought to Earth. The target for Muses C is the small asteroid 10302 1989ML, which is in an orbit between the Earth and Mars, so not situated in the large asteroid belt between Mars and Jupiter. The asteroid is thought to be 400 m (1300 ft) across.

MV Japanese solid propellant spacecraft and satellite launcher; it first flew in 1997. The MV launches Japan's science and planetary spacecraft. It can carry payloads weighing 2 tonnes to low Earth orbit and payloads weighing 0.5 tonne to planetary targets.