Iapetus Outermost of the main SATELLITES of SATURN. It is the third-largest but was the second to be discovered, by G.D. CASSINI in 1671, thanks to its great distance from the glare of the planet. It has the remarkable property of appearing much fainter when lying to the east of Saturn in the sky, when we see its leading hemisphere, than when lying to the west, when we see its trailing hemisphere. This observation can be explained by the fact that most of the leading hemisphere has a sooty coating with an albedo of less than 0.1, whereas the trailing hemisphere is relatively clean ice with an albedo of about 0.5. The best VOYAGER images revealed that the trailing hemisphere is heavily cratered, but they were not capable of showing any features in the opposite hemisphere. It has been suggested that the dark material is dust originating from PHOEBE, a dark red irregular satellite in a retrograde orbit outside that of Iapetus, but the spectroscopic match is rather poor. See data at SATURN



IC Abbreviation of index catalogue

Icarus APOLLO ASTEROID discovered in 1947. Icarus possesses a particularly small PERIHELION distance (0.187 AU), which led to its being named after the myth of the son of Daedalus, who flew too close to the Sun. The early determination of its orbit led to the recognition that Icarus makes repeated close approaches (within 0.04 AU) to the Earth. It gained wide fame as a result of the publication in 1967 of Project Icarus, a student exercise at the Massachusetts Institute of Technology in which the feasibility of deflecting this object, assumed to be heading for our planet, was studied.


Ida MAIN-BELT ASTEROID, number 243, examined by the Jupiter-bound GALILEO spacecraft in 1993 August. Ida was the second asteroid (after GASPRA) for which such imagery was obtained.

Ida is elongated in shape, with a narrow waist, and measures about 56 X 24 X 21 km (35 X 15 X 13 mi). It is heavily cratered from impacts by smaller bodies. Its spectral reflectance indicates it to be an S-class asteroid, composed largely of metal-rich silicates of the type that make up the meteorites known as ordinary chondrites. Galileo images revealed Ida to be accompanied by a small moon, DACTYL, the orbital distance of which is consistent with Ida having an average density of between 2 and 3 g/cm3. Ida's surface is heavily cratered and has linear grooves and a scattering of large blocks or boulders. It is thought that Ida is coated with a thick regolith. Its interior comprises a number of discrete components, making the entire object a type of rubble pile.

IDA Abbreviation of INTERNATIONAL DARK-SKY ASSOCIATION igneous rock Rock formed as a result of cooling of molten material. Formerly, rocks formed from the cooling of impact melts were considered to be igneous, but geologists now only consider rocks to be igneous if they are derived from melts formed in the interiors of planets or satellites, usually as a result of decompression of high temperature solid material from the interior, as, for example, in a hot mantle PLUME. Igneous rocks formed on the surface of a planet or satellite, where the melt cools rather fast, are known as volcanic rocks; they typically contain varying proportions of silicate glass together with mineral crystals. Igneous rocks formed at depth in the interior, where the cooling is much slower, are known as plutonic rocks; they are fully crystalline. The principal minerals in igneous rocks are olivine (Mg,Fe)2SiO4, pyroxene (Mg,Fe,Ca)2Si2O6, feldspars CaAl2Si2O8-(Na,K)AlSi3O8 and quartz SiO2. The most widespread volcanic igneous rock on the surface of TERRESTRIAL PLANETS and non-icy satellites is BASALT. Ices on the surface of satellites of giant planets, if formed from nonimpact melts, should be considered as igneous rocks.

Ikeya-Seki, Comet (C/1965 S1) Spectacular, bright comet, discovered independently by the Japanese observers Kaoru Ikeya (1943-) and Tsutomu Seki (1930- ) on 1965 September 18. The comet, a KREUTZ SUNGRAZER, reached perihelion on October 21, 0.008 AU (1,200,000 km/750,000 mi) from the Sun. At perihelion, Comet Ikeya-Seki had an estimated peak mag. 10, and could be seen in daylight. During perihelion passage, the nucleus broke into three fragments. The tail reached a maximum length of 60 soon after perihelion and was a spectac ular sight in the pre-dawn sky for observers at southerly latitudes. The orbit is elliptical, with a period of 880 years.

IkeyaSeki, Comet Photographed in the pre-dawn skies of 1965 October, this spectacular sungrazing comet had a long, bright tail. As it passed perihelion, the nucleus split into three fragments.

IMAGE Acronym for IMAGER FOR MAGNETOPAUSE TO AURORA GLOBAL EXPLORATION image intensifier Opto-electronic device for amplifying the brightness of an image. The optical image from a telescope is focused on to a photocathode, causing electrons to be liberated, the number of which depends on the intensity of the incoming optical signal and its wavelength. The electrons are accelerated through a potential of around 40 kV and focused by an electric or magnetic field on to a phosphor screen. In addition to being accelerated, the number of electrons may be increased by cascading successive photocathode stages (see also CASCADE IMAGE TUBE). A drawback of the system is the slight distortion of the image caused by the electronic focusing.

In general terms, image processing is the manipulation of information recorded by the pixels (light-sensitive areas) that make up a modern astronomical detector. The most basic form of image processing is linear scaling, where individual pixels on an image are manipulated to enhance brightness and contrast. This type of processing is useful to remove the effects of LIGHT POLLUTION from the sky background or to enhance faint features such as the arms of spiral galaxies. Another method of boosting the level of information is by the co-addition or stacking of multiple images to improve the SIGNAL-TO-NOISE RATIO of faint astronomical objects.

More complex is the use of logarithmic scaling to boost selectively or withhold brightness values on images that contain a wide range of intensity levels. When applied to images of globular clusters, for example, a logarithmic scale extracts information contained in the bright core region, which would normally be overexposed, yet maintains a high level of detail in the outer reaches of the cluster. In a new technique called digital development processing, a dual routine simultaneously compresses the brightness levels contained in an image and applies UNSHARP MASKING to produce unprecedented levels of detail.

Spatial filters are used to sharpen images or to blur as a means of reducing instrument noise or processing artefacts. Other useful routines include positional translations, notably rotation and scaling of image size. Many astronomers use these functions when combining images for the analysis of transient events, for example in supernova and asteroid searches. One of the most powerful of all image-processing procedures is the use of fast Fourier transforms. In principle, an image is broken down into a discrete spread of spatial frequencies, to which mathematical operations are applied before the image is reconstructed. By varying the type of operations applied, many different forms of image manipulation are possible. Perhaps the best known of these is deconvolution. Indeed, this method was used to correct images that were taken by the original faulty mirror of the Hubble Space Telescope. For ground-based astronomy, deconvolution is used to improve resolution that has been degraded by viewing through an unsteady atmosphere.

Most CCD detectors can only take images in shades of grey, so to build a colour picture it is necessary to take multiple exposures of astronomical objects through different coloured FILTERS. A common method is to employ red, green and blue filters and to take an exposure through each of these in turn, although other filter combinations are possible. Once the images are stored on a computer, they can be combined and image processing applied to synthesize a colour image and to extract useful scientific information. If care is taken during the combination and processing of these image sets, it is possible to produce true-colour images, which is very difficult to do using modern photographic films.

Imager for Magnetopause to Aurora Global Exploration (IMAGE) NASA science satellite launched into a polar Earth orbit in 2000 March. IMAGE uses neutral atom, ultraviolet and radio-imaging techniques to study the magnetic phenomena involved in the interaction of the SOLAR WIND with the Earth. Mission objectives are to identify the dominant mechanisms for injecting plasma into the Earth's magnetosphere, to determine the response of the magnetosphere to changes in the solar wind and to discover how and where magnetospheric plasmas are energized and transported.

Imbrium, Mare (Sea of Showers) Lunar lava plain, roughly 1300 km (800 mi) in diameter, located in the north-west quadrant of the Moon. It represents flooding of a multi-ring impact basin, produced by the impacts of an asteroid approximately 100 km (60 mi) in diameter. The lava was generally contained by the outer wall, but it flooded over the inner rings. The highest peaks of these inner rings are still visible above the lava (for example Mons Piton), and the rings are marked by circular mare ridges. Apollo 15 visited Hadley Rille in Mare Imbrium.

immersion Disappearance of a star or planet behind the Moon's leading limb at an occultation. The term may also be used to describe the Moon's entry into Earth's umbra at a lunar eclipse.


impact feature Any feature formed by METEOROID impacts on the surface of planets and satellites. Impact features include impact CRATERS, both primary and secondary, of any size from microcraters to multi-ring BASINS, all varieties of EJECTA from impact craters, as well as impact-generated fractures and faults. The dark spots resulting from the impact of fragments of Comet SHOEMAKER-LEVY 9 on Jupiter in 1994 are examples of transient impact features.

impact feature A satellite image of the circular Manicouagan impact structure in Quebec, Canada. The 70 km (44 mi) diameter feature was formed by an asteroid impact about 212 million years ago.

inclination (of an orbit) Angle between the plane of an ORBIT of a body and a suitable fixed reference plane. In the Solar System the reference plane is chosen on dynamical considerations. PERTURBATIONS on an orbit cause the LINE OF NODES to regress around the plane of action of the dominant perturbing forces while maintaining constant inclination to this plane, and thus this is the most suitable plane to use as the reference plane. For the planets, the main perturbations come from the other planets; as these all lie close to the ECLIPTIC plane this is the chosen reference plane. For artificial satellites of the Earth and most satellites of other planets, the equatorial bulge (oblateness) of the planet is the dominant perturbation, and so the equatorial plane is chosen as the reference plane. The Moon is relatively very distant from the Earth (in units of Earth radii), and solar perturbations are much larger than the oblateness perturbation, so the ecliptic is chosen as the reference plane. For a few satellites the oblateness and solar perturbations are of similar size, and then the appropriate reference plane, named the Laplacian plane, lies somewhere between the equator and the planet's orbit plane. This effect was first noted by Pierre LAPLACE in 1805 for the Saturnian satellite Iapetus. The effect also occurs for the Earth's geostationary satellites, for which the Laplacian plane is inclined at 7 to the equator.

For binary stars the inclination of the orbit is measured relative to the plane at right angles to the line of sight. The angle is zero if the orbit is seen in plan, and 90 if seen in profile.

inclination (of a planet's equator) Angle between the equatorial plane of a planet or satellite and its orbital plane; this is equivalent to the angle between the axis of rotation and the perpendicular to the orbital plane. For the Earth this angle is named the OBLIQUITY OF ECLIPTIC.

Index Catalogues (IC) Two supplements to the new general catalogue (NGC), the Index Catalogue (1895) and Second Index Catalogue (1908), between them adding another 5386 galaxies, nebulae and clusters (in one continuous numbered sequence) to the NGC's 7840. Like the NGC, the Index Catalogues were compiled by J.L.E. DREYER; some objects in them are still referred to by their IC numbers.

Indian astronomy Astronomy as practised in India from ancient times until the 18th century, when Western European astronomy became prevalent. The precise origins of Indian astronomy are unknown, but probably date from the Indus Valley civilization (c.2000 BC). Some practical astronomy existed during the Vedic period, which preceded the foundation of the Buddhist and Jain religions around 500 BC.

The early Indian calendar was luni-solar, based on 12 lunar months with extra months inserted as necessary to keep it in step with the solar year.

The beginning of scientific astronomy in India, based partly on Greek knowledge, dates from about AD 500, during the lifetime of ARYABHATA, who introduced the use of sines and other mathematical techniques to the prediction of solar, lunar and planetary positions. These he extrapolated from observations made during his own lifetime. His and other early astronomical writings were written in verse and are exceedingly cryptic in form, which led to later scholars doubting they contained anything of scientific value (modern opinion has removed this doubt). Aryabhata believed that the Earth rotated on its axis once a day, but this view was not accepted by later writers.

BRAHMAGUPTA was the other great Indian astronomer of this period, and his works and those of Aryabhata remained influential. At this time Indian astronomy began to spread to other parts of Asia. From the start of the Moghul period in the 16th century, Islamic astronomy gained influence. The Moghul ruler Jai Singh II (1688-1743) had a special interest in Islamic astronomical knowledge. The mammoth instruments he constructed at impact feature A satellite image of the circular Manicouagan impact structure in Quebec, Canada. The 70 km (44 mi) diameter feature was formed by an asteroid impact about 212 million years ago.

INDUS (gen. indi, abbr. ind)

Southern constellation, between Grus and Pavo, introduced by Keyser and de Houtman at the end of the 16th century, representing a Native American. Its brightest star is a Ind, mag. 3.11, distance 101 l.y., spectral type K0 III. e Ind, an orange dwarf of visual mag. 4.69 and spectral type K5 V, is among the closest naked-eye stars, distance 11.8 l.y. 0 Ind is a double for small telescopes, mags. 4.5 and 7.0.

Infrared Astronomical Satellite (IRAS) An all-sky map of infrared sources recorded by IRAS in 1983. The bright band running across the middle is the plane of the Milky Way

Infrared Astronomical Satellite (IRAS) An all-sky map of infrared sources recorded by IRAS in 1983. The bright band running across the middle is the plane of the Milky Way.

Jaipur for determining the positions of celestial objects are still standing.

Although Western astronomical techniques accompanied the European domination of India, traditional calendars are still in use for astrological and religious purposes, and large numbers of traditional almanacs are published each year.

Indus See feature article

inequality Departure from uniform orbital motion of a body due to perturbations by other bodies. The most notable is the great inequality that affects Jupiter and Saturn, as a result of the close 5:2 commensurability between their mean motions. This inequality causes perturbations of period about 938 years in their longitudes, of amplitude 0.36 for Jupiter and 0.87 for Saturn.

inertia Tendency of a body to resist acceleration; it is the tendency to remain at rest or to continue moving at a constant velocity in a straight line unless acted on by a force. Inertia is quantified by the mass of the body. See also newton'slawsofmotion

inferior conjunction Alignment of an inferior planet between the Earth and the Sun. If the alignment takes place at a favourable node, then the inferior planet may be seen to transit the Sun's disk. conjunction occurs when two bodies have the same celestial longitude as viewed from the Earth. For an inferior planet, this can also occur when it is on the far side of the Sun to the Earth, at which point it is said to be at superior conjunction. Because of the inclination of the orbits of Mercury and Venus to the ecliptic, they rarely transit the Sun's disk at inferior conjunction, instead passing either north or south. See diagram at conjunction

inferior planet Planet with an orbit that lies closer to the Sun than does the orbit of the Earth; the inferior planets are mercury and venus. Since both planets' orbits are within that of the Earth, they do not appear far from the Sun in the sky and are, therefore, best observed as early morning or early evening objects. Again, because of their orbital positions, both planets display distinctive phases like those of the Moon.

infinite Universe, idea of Concept of a cosmos with no bounds. Since ancient Greek times, astronomers had generally considered the planetary Universe to be enclosed within a sphere of fixed stars. By the 15th century ad, however, some scientific thinkers within the Catholic Church, such as Cardinal Nicholas of Cusa (1401-64), were coming to argue that there was no reason why an infinite, all-powerful God should not have created an infinite Universe, containing a multitude of stars and planets. But it was the implications of the copernican system after 1543 that re-opened the debate about infinity, for if the Earth and planets were revolving around the Sun, why could not other stars also have planets rotating around them? In 1576, indeed, the English Copernican Thomas digges spoke of the starry 'sphere' as extending 'infinitely' into space. Yet all of this speculation about an infinite Universe suddenly acquired a physical ground in 1610, when galileo resolved the Milky way into individual stars with his telescope. Thereafter, every increase in telescopic power revealed yet more dim stars, creating the impression that our Sun was but one star in an infinite three-dimensional Universe. In 1695, moreover, Christiaan and Constantijn huygens even wondered whether intelligent creatures in space were looking at us with powerful telescopes! By the 1780s William herschel realized, from the then known velocity of light, that when we look into deep space we look into 'times past'. By Herschel's time, however, extensive telescopic observation had confirmed that the Universe appeared to be infinite, and after about 1950, that it was also expanding. See also cosmology; olbers' paradox; perfect cosmological principle

inflation Faster-than-light expansion of the universe very early in its evolution. The big bang theory had several problems, specifically the horizon problem and the flatness problem. The horizon problem occurred because measurements of the background temperature from every direction are nearly identical, but these regions of the Universe had to be out of causal contact, and therefore would not be expected to have identical temperatures. The flatness problem arose because the background radiation implied a critical density very close to one, and thus at early times in the universe the density had to be very close to one. The inflationary model of Alan Guth proposed that at approximately 10~35 s after the Big Bang the universe expanded exponentially for about 10 ~24 s and increased in size by a factor of 1050. The energy to drive this expansion came from the condensation higgs field as the temperature of the universe fell below a critical value. This idea solved the horizon problem by assuming that the opposite sides of the universe were in causal contact before inflation, but were propelled out of causal contact by the inflationary expansion. It also solved the flatness problem by decreasing our view of the universe to a small portion so that it would appear to be flat, even if the entire universe were generally not flat. Inflation has been refined and survived a number of observational tests, and is now on a solid theoretical and observational footing.

Infrared Astronomical Satellite (IRAS) International project involving the USA, the Netherlands and the UK. Its main objective was to make the first all-sky infrared survey from space, searching for objects with temperatures between 10 and a few hundred kelvin. Its 60-cm (23-in.) reflecting telescope focused incoming radiation on to an array of 64 semiconductor detectors, which were cooled to 1.8 K by liquid helium.

IRAS was launched by a delta rocket from Vanden-berg Air Force Base, California, on 1983 January 26. It was placed in a 900-km (560-mi) Sun-synchronous, circular orbit inclined at 99 to the equator.

During the main survey in 1983, between early February and the end of August, most of the sky was observed twice. A second sky survey began in September and continued until the helium coolant ran out on 1983 November 22. By then IRAS had scanned 98% of the sky and carried out many thousands of targeted observations. The IRAS catalogue published in 1984 contained 245,000 infrared point sources, more than 100 times the number known before launch.

Many discoveries were made both during and after the mission. Solar System discoveries included: six new comets, notably Comet IRASARAKIALCOCK; huge invisible tails on Comet Tempel 2 and other comets; several asteroids, including the unusual object 3200 PHAETHON; and bands in the ZODIACAL DUST.

Dust shells, possibly related to planetary formation, were discovered around VEGA and several other stars. Star formation regions in DARK NEBULAE were studied in great detail and many PROTOSTARS were discovered. The star BETELGEUSE was found to have ejected three huge dust shells, and clouds of dust named infrared cirrus were discovered all over the sky. IRAS also studied the galactic centre in great detail. Beyond the Milky Way, IRAS observed that many galaxies are powerful emitters of infrared radiation and some of these, the STARBURST GALAXIES, emit much more infrared than visible light. See also INFRARED ASTRONOMY

infrared astronomy Study of astronomical objects at infrared wavelengths. INFRARED RADIATION penetrates dust clouds more easily than that at optical wavelengths, so astronomers using the infrared can study stars forming deep inside dense dust clouds, the centre of the Galaxy, and other galaxies (both normal and peculiar). Astronomical sources of infrared radiation also include the planets and other Solar System bodies, stars and the dusty regions themselves. Wien's law shows that relatively low-temperature objects (less than 3000 K) emit most strongly, and so are easiest to observe, in the infrared.

Astronomers on the ground, using the infrared, must confine their observations to the atmospheric 'windows' where there is very little absorption by water vapour or carbon dioxide, and infrared telescopes are usually sited on mountain tops (above about 3000 m/l0,000 ft) to take advantage of the windows at 0.752.5 jm, 35 jm, 7.514 jm and 1621 jm. Mauna Kea in Hawaii and Ata-cama in Chile are examples of good sites. To avoid the Earth's atmosphere, infrared observations have been made from high-flying aircraft, notably the KUIPER AIRBORNE OBSERVATORY (KAO) and the new STRATOSPHERIC OBSERVATORY FOR INFRARED ASTRONOMY (SOFIA), from unmanned balloons, rockets, and from Earth-orbiting satellites. The INFRARED ASTRONOMICAL SATELLITE (IRAS) surveyed the entire sky at several infrared wavelengths cataloguing almost a quarter of a million objects. More recently the INFRARED SPACE OBSERVATORY (ISO) targeted over 30,000 astronomical objects for detailed study. There were several other early surveys, two of which were particularly noteworthy: one from the ground, the 2| m Sky Survey by Gerry Neugebauer (1932 ) and Robert Leighton (191997), producing the IRC catalogue published in 1969; and one using rockets, the Air Force Geophysics Laboratory survey by Steve Price and Russ Walker, producing the AFGL catalogue published in 1976.

Astronomical sources can emit infrared radiation either throughout the ELECTROMAGNETIC SPECTRUM (according to the PLANCK DISTRIBUTION of BLACK BODY RADIATION) or in one or more spectral lines or bands, depending upon the temperature and composition of the emitting gas (see EMISSION SPECTRUM). The emission in the infrared lines is sometimes the main mechanism for cooling gas/dust clouds (such as the dense clouds where new stars are forming). Among the most important infrared atomic lines are the FORBIDDEN LINES of carbon ([C II] at 157 jm) and oxygen ([O III] at 88.35 jm). There are very important molecular bands found in emission and ABSORPTION, in particular hydrogen (H2), water (H2O), carbon monoxide (CO) and carbon dioxide (CO2). Infrared observations from space, with ISO, have shown that water is very common in the Solar System, in the Galaxy and in other galaxies. The infrared has revealed the existence of another type of molecule, polycyclic aromatic hydrocarbons (PAHs), the emission bands of which were originally known as the Unidentified Infrared Bands

A infrared cirrus Infrared emission from dust grains heated by stellar radiation produces this wispy structure in the Puppis-Vela region of the Milky Way.

The infrared region of the electromagnetic spectrum has many broad features due to the solid material (dust) in interstellar space or around stars. The dust grains range greatly in size, but a typical value is a few tens of micrometres. Very small dust grains (with sizes of a few micrometres) can be transiently heated to about 1000 K and emit MID-INFRARED radiation. Silicate dust has often been found in crystalline form as well as the amorphous (astronomical) silicate form, giving sharp spikes in the normally very smooth broad emission features. Water-ice and carbon dioxide ice have also been detected in the infrared, in star-forming regions. Carbon-rich dust, such as silicon carbide, can also be observed via its infrared emission features. Both old and young astronomical objects show these dust features.

Probably the most studied and most famous molecular cloud is in Orion's sword, where star formation is observed in progress in the densest parts of the ORION MOLECULAR CLOUDS. Temperatures in this cloud reach about 2000 K in regions where shock waves excite molecular hydrogen, but other regions range from a few tens to a few hundred degrees. The infrared region of the spectrum is very sensitive to the slightest heating of the dust cloud as it starts to collapse. IRAS showed that there is a great deal of star formation occurring in Orion, not just in the sword region.

The huge infrared emission output when stars are forming means that these regions can be detected at large distances; the STARBURST GALAXIES are one example. Another type that is very bright in the infrared, and hence can be detected at large distances, is a galaxy with an ACTIVE GALACTIC NUCLEUS (AGN), which is thought to be powered by a BLACK HOLE. The infrared can be used to detect the difference between these two types of galaxy, since the starburst galaxy lacks some of the most energetic forbidden lines found in the AGN spectrum.

Closer to home, Solar System scientists have used the techniques of infrared astronomy to work out the basic chemistry of the planets, their satellites and the asteroids. In the NEAR-INFRARED, molecules in planetary atmospheres exhibit a rich absorption spectrum, the analysis of which permits the composition and temperature of the atmospheres to be established. By careful selection of a precise infrared wavelength, different layers in the atmosphere can be probed in detail because of the variations in temperature. Observations of the outer planets from ISO have shown the presence of water in their upper atmospheres due to icy material falling on to them from interplanetary space.

infrared cirrus Tenuous cold dust in interstellar space which emits faintly in the infrared. When FAR-INFRARED data are displayed as maps, the emission resembles the wispy structure of cirrus clouds in the sky. The infrared cirrus clouds were first mapped with the infrared astronomical satellite (IRAS) and they have been detected everywhere in the Galaxy. The temperature of the dust is typically around 20-30 K.

infrared cirrus Infrared emission from dust grains heated by stellar radiation produces this wispy structure in the PuppisVela region of the Milky Way.

Infrared Imaging Surveyor (IRIS) Japan's Astro F series spacecraft, to be launched in 2003-2004 into a 750-km (460-mi) Sun-synchronous Earth orbit. The satellite will survey the infrared sky with greater sensitivity than any previous Astro mission, using a 70-cm (28-in) telescope cooled with liquid helium. It will investigate the formation and evolution of galaxies, stars and planets.

infrared radiation Portion of the electromagnetic spectrum in the wavelength range 0.75-350 um, lying between optical and radio wavelengths. Historically, wavelengths between 1 Lint and 1 millimetre (mm) were considered to be infrared. Recent developments in detectors mean that the submillimetre region is now considered to start at 350 um. Most objects emit some infrared radiation, but according to wien'slaw those with temperatures less than 3000 K emit most intensely in the infrared. Many molecules (for example molecular hydrogen, H2) have important spectral features in the infrared. See also infrared astronomy

Infrared Space Observatory (ISO) European space agency (ESA) spacecraft launched in 1995 November to observe the sky with enhanced sensitivity and resolution using a 60-cm (24-in.) diameter primary mirror. It had four science instruments - an infrared camera, a photopolarimeter, and two spectrometers, provided by France, Germany, the Netherlands and the UK, cooled by a cryostat of liquid helium. The craft was able to operate fully for an additional eight months until 1998 May when the coolant was depleted. A short-wavelength spectrometer, however, was used until 2001.

initial mass function Distribution of stellar masses at birth, which is taken to be when nuclear fusion begins in the stars' cores. Determined by Edwin salpeter in 1955, and sometimes called the Salpeter mass function, the initial mass function 4>(M represents the number of stars with mass M at birth per unit volume of space. In the solar vicinity, 4>(M) is approximately equal to M~2'35, but there are deviations from this law for massive stars. It is also difficult to estimate how many low-mass stars (less than 0.1 solar mass) exist, even in the solar neighbourhood.

inner planet Term used to describe any planet, the orbit of which lies inside that of the asteroid belt. The inner planets are Mercury, Venus, Earth or Mars. See also terrestrial planets

Innes, Robert Thorburn Ayton (1861-1933) Scottish-born double star observer and discoverer of proxima centauri, director of the Union (later Republic) Observatory in Johannesburg (1903-27). Innes emigrated to Australia in 1884 and again in 1896, this time to the Cape of Good Hope (South Africa), where he became a first-rate observer at the observatory there. Innes observed and catalogued many new southern double stars, culminating in the Southern Double Star Catalogue (1927). He also revised the Cape Photographic Durchmusterung, a massive catalogue of stars visible from the southern hemisphere. Innes was an ardent advocate of southern hemisphere astronomy, urging many older observatories to establish southern stations. He was one of the first to recognize the astronomical usefulness of the blink comparator, which he used in 1915 to discover Proxima Centauri (known for a while as Innes' Star).

insolation Total amount of radiant energy from the Sun falling on to a body per unit area perpendicular to the direction of the Sun, in unit time. For the Earth, the insolation is also called the solar constant. At the top of the Earth's atmosphere it has a value of 1366.2 Wm~2.

instability strip Part of the hertzsprung-russell diagram where pulsating stars are located. It is a narrow strip, extending from the cepheids through the rr lyrae variables, delta scuti variables and dwarf Cepheids, down to the pulsating white dwarf (zz ceti) stars.

Most stars will pass through this region at some time in their lives: they become pulsating variables of some type when they have a small imbalance in the gravitational force and the outward internal pressure so that they are not in hydrostatic equilibrium. The part of the instability strip that a star passes through depends on its mass.

Institut de Radio Astronomie MiNimetrique (IRAM) Multi-national scientific institute that operates two major facilities: a 30-m (98-ft) telescope on Pico Veleta in the Sierra Nevada, southern Spain, and an array of five 15-m (49-ft) telescopes on the Plateau de Bure in the French Alps. Tragedy struck the Plateau de Bure site in 1999 July when a cable car fell to the ground, killing all 20 scientists and engineers on board.

Institute for Astronomy, University of Edinburgh Research institute within the Department of Physics and Astronomy of Edinburgh University, located in the grounds of the royal observatory edinburgh. Areas of specialization include cosmology, the Universe at high redshifts, X-ray surveys, galaxy formation and studies of the intergalactic medium. The Institute's Wide-Field Astronomy Unit supports the united kingdom schmidt telescope and other wide-field telescopes. It is responsible for the operation of the SuperCOSMOS plate measuring machine and the overall management of the 6dF Galaxy Survey.

Institute for Astronomy, University of Hawaii Research institute founded at the University of Hawaii in 1967 to manage the Haleakala Observatory and mauna kea observatory and to pursue its own programme of fundamental astronomical research. Its main base is at Manoa on the island of Oahu, close to the main campus of the university. Its sea-level telescope operations and instrument-development facility for Mauna Kea is at Hilo on the Big Island of Hawaii.

Institute of Astronomy, University of Cambridge (IoA) Department of the University of Cambridge engaged in teaching and research in theoretical and observational astronomy. It came into being in 1972 with the amalgamation of the Cambridge University Observatory (founded 1823), the Solar Physics Observatory (1912) and the Institute of Theoretical Astronomy (1967). Some of the best-known names in modern astronomy have been associated with the IoA, including Fred Hoyle and Martin Rees.

Institute of Space and Astronautical Science (ISAS) Japanese institute for space science research, operating its own launch vehicles, scientific satellites, planetary probes and balloons. ISAS had its origins in the University of Tokyo in the 1950s, but took its present form in 1981. Its work complements that of the national space development agency of japan, which operates applications satellites and their launch vehicles.

Instituto Argentino de Radioastronomia (IAR) Principal institution for radio astronomy in Argentina, created in 1962, located near Buenos Aires. It operates two 30-m (98-ft) radio telescopes.

Instituto de Astroffsica de Canarias (IAC) International research centre in the Canary Islands, comprising the Instituto de Astrofisica, La Laguna, and the Observa-torio del Teide (both situated on the island of Tenerife) and the roquede los muchachos observatory on La Palma. Together they constitute the European Northern Observatory. Both observing sites are noted for their exceptional sky-quality. The IAC is the host organization for the gran telescopio canarias.

INT Abbreviation of isaac newton telescope Integral (acronym for International Gamma Ray Astrophysics Laboratory) european space agency (ESA) Horizon 2000 mission to be launched on a Russian proton booster in 2002 into a high (40,000 km/25,000 mi) Earth orbit. Developed in collaboration with NASA and Russia, Integral is dedicated to fine spectroscopy observation and high-resolution imaging of celestial gamma-ray sources, with concurrent source monitoring in X-ray and visible wavelengths.

integrated magnitude Total brightness of an extended body. Stellar and planetary bodies typically have brighter and darker regions and the integrated magnitude is the sum of all of these.

intensity interferometer Instrument used to study an astronomical object by means of interferometry (see also interference) in order to obtain more detail in the map of the object. The first interferometer was developed by Albert Michelson (1852-1931) in the 1920s, and it worked at optical wavelengths. It consisted of mirrors at either end of a steel beam placed across the aperture of the 100-inch (2.5-m) telescope at Mount Wilson, and with it Michelson measured diameters of a few large stars by examining the interference pattern formed in the eyepiece. Modern systems link two telescopes, either electronically or by laser beams, and use electronic devices, such as photometers, to record the signals. The technique has been used with great success at radio wavelengths (see also radio interferometer). The two telescopes receive the signal at different times because the waves of the electromagnetic radiation have to travel farther to one of the telescopes than to the other. This delay is slightly different for separate, but adjacent, points on the sky, so that for an extended object, such as a large galaxy or nebula, the interference pattern is washed out.

interacting galaxies Pairs or groups of galaxies whose forms are distorted by the gravitational influence between them, sometimes leading to a merger. These interactions can set long tidal tails of stars and gas into motion well away from the original galaxy (some pieces of which can eventually clump together to form dwarf galaxies). Galaxy interactions are though to cause starburst activity, including what appear to be newly formed globular clusters, and perhaps some kinds of active galactic nuclei.

interacting galaxies A Hubble Space Telescope image of the edge-on galaxy ESO 510-G13 in Hydra. The dark, dusty disk shows warping indicative of a recent collision with another galaxy.

Interamnia Sixth-largest main-belt asteroid;number 704. It has a diameter of 316 km (196 mi). Interamnia was not discovered until 1910 due to its low albedo (0.07).

interference Effect observed when two trains of waves of the same wavelength meet. If maxima (crests) of the waves arrive simultaneously at the same place, their maxima add together to produce a wave of larger amplitude. This is constructive interference. If the maxima of one train coincide with the minima (troughs) of the other, this is destructive interference. They cancel totally if the amplitudes of the two wave trains are identical, otherwise they cancel partially. Thus two wave trains crossing each other produce an interference pattern, with alternate lines of constructive interaction and of destructive interaction. This applies to any wave motion - electromagnetic radiation or waves on the surface of liquids.

interferometry Study of point-like (unresolved) astronomical objects using interference to reveal more detail in the object via the interference pattern produced.

There are two types of interferometry, using speckle interferometry or an intensity interferometer, to determine spatial detail, and there are other types to study spectral lines in detail (see fabry-perot interferometer). An interferometer works on the principle that electromagnetic radiation (usually optical or radio) will follow two paths to produce the interference, either because of Earth's atmosphere in the case of speckle interferometry, or through two paths in the same instrument (as in the case of Fabry-Perot), or via two telescopes (intensity interferometer). If the two signals are combined correctly they will either reinforce or cancel, depending on the delay: when the signals are in phase (that is, they either have no delay or a delay corresponding to a whole number of wavelengths) then the maximum combined signal will be obtained. See also stellar interferometer; very long baseline interferometry

intergalactic matter Matter in the space between galaxies. There is no significant amount of dust in intergalactic space, but there is ample evidence for several kinds of gas. Within galaxy clusters, the hot intracluster medium is at temperatures of typically 20 million K and has been chemically enriched by supernovae. Outside these clusters, observations of absorption lines, above all from material in front of distant and luminous quasars, shows an intricate medium tracing the large-scale distribution of ordinary matter (which must be close to that of the dark matter as well, since its gravity will be the dominant force). Hydrogen and helium absorption shows that this intergalactic matter is highly ionized everywhere, and more so in the least dense regions, where particle collisions that could lead to recombination are less frequent. Because it is so highly ionized, and only the tiny neutral fraction is observed, the amount and chemistry of this matter are still very uncertain. Even intergalactic matter in the lowest-density regions, generally most remote from luminous galaxies, has been enriched to some degree with atoms synthesized in stars, as shown by the presence of highly ionized oxygen traced in observations from FUSE and the Hubble Space Telescope. Thus the intergalactic medium is not simply leftover material that never formed galaxies: it has, at least in the early Universe, participated in the stellar recycling inside galaxies.

International Amateur-Professional Photoelectric Photometry (IAPPP) Organization that fosters partnerships among amateur, professional and student astronomers who wish to make precise brightness observations of celestial objects. Formed in 1980, the IAPPP was far ahead of its time in promoting such collaborations, which are becoming more important as the pace of astronomical discovery quickens and the need for continuing follow-up observations grows.

International Astronomical Union (IAU) Principal coordinating body of world astronomy. Its mission is to promote and safeguard the science of astronomy through international cooperation. Founded in 1919, the IAU has 11 scientific divisions and 40 commissions covering all aspects of astronomy. Its membership includes most of the professional community active in astronomical research and education at PhD level and beyond, and amounts to some 8300 individuals in 67 countries.

The IAU is perhaps best known as the sole authority responsible for naming celestial bodies and their surface features. However, its remit extends far beyond that, and ranges from the definition of fundamental astronomical constants to strategic planning of future large-scale facilities. It holds a General Assembly every three years at which its long-term policy is defined, and sponsors about a dozen high-profile symposia and colloquia each year. The IAU also promotes education research in developing countries through its International Schools for Young Astronomers. The organization's headquarters are at the Institute d'Astrophysique in Paris, where a permanent secretariat is based.

International Atomic Time (TAI) Continuous and uniform time scale derived from atomic clocks and used for scientific purposes. TAI is based on the SI second (see ATOMIC TIME) and is formed retrospectively by intercomparing data from around 200 atomic clocks, or frequency standards as they are known, at around 40 laboratories across the globe. Each of these atomic clocks should be accurate to within one second in three million years, but a large number are used to form the time scale in order to reduce the likelihood of the results from any rogue timepieces affecting the overall combined mean. The resultant TAI time scale is then used as a standard against which other clocks can be measured.

TAI has run continuously, without adjustment, since 0h 0m 0s GMT on 1958 January 1 and is co-ordinated by the International Bureau of Weights and Measures (BIPM) in Paris. Because it is both uniform and continuous, it is ideal as a time scale for scientific purposes but not practical for everyday use, since it is not linked to the rotation of the Earth. For the purposes of forming an accurate civil time scale COORDINATED UNIVERSAL TIME (UTC) was introduced. This is still derived from atomic clocks but is kept in step with the Earth's rotation through the periodic introduction of LEAP SECONDS. Because of this, UTC differs from TAI by an integral number of seconds.

Every major industrial nation contributes to International Atomic Time. In the UK, responsibility for maintaining the national time service is held by the National Physical Laboratory (NPL) at Teddington. Prior to 1984 the Time Service was the responsibility of the Royal Greenwich Observatory. See also TIMEKEEPING

International Cometary Explorer (ICE) NASA spacecraft originally launched in 1978 as ISEE-3 (International Sun-Earth Explorer-3); it was renamed when it was diverted by means of a lunar gravitational assist to fly through the tail of Comet GIACOBINI-ZINNER. The comet flyby - the first ever made - took place on 1985 September 11. ICE later passed the sunward side of Halley's Comet in 1986 March at a distance of 28 million km (17 million mi).

International Dark-Sky Association (IDA) International non-profit organization, based in the USA, founded in 1988 to campaign against the adverse impact of LIGHT POLLUTION on optical astronomy. The IDA seeks to raise public awareness about good and bad outdoor-lighting practices, including aesthetic, security and economic issues. The IDA is also building awareness of other threats to the astronomical environment, such as from radio-frequency interference and space debris and from other pollutants such as aircraft contrails.

International Date Line Imaginary, irregular line, close to and sometimes coincident with that of 180 longitude, marking the point on the Earth where the date changes; points east of the line being one day earlier than those west of it.

Located mainly in the Pacific Ocean, therefore avoiding places of habitation, the line avoids crossing land by skirting around Siberia, the Aleutian Islands, the Fiji Islands and New Zealand. It was adopted by international agreement in 1885.

International Geophysical Year (IGY) Period of intensive, multi-nation collaborative research, including studies of solar and auroral process, meteorology and oceanography, that was was carried out between 1957 July 1 and 1958 December 31.

International Meteor Organization (IMO) Organization dedicated to the study of meteors and their parent dust particles, established in 1988, with a worldwide membership. The IMO collects data taken visually, photographically, by video and by radio. It maintains a database of visual observations extending back to 1984 and publishes WGN, a bimonthly journal.

International Occultation Timing Association (IOTA) International organization founded in 1975 to encourage and facilitate the observation of occultations and eclipses. It provides predictions for grazing occulta-tions of stars by the Moon and of stars by asteroids and planets, and acts as a coordinating body for reports of such events. IOTA is based at Topeka, Kansas, and has a European Section.

International Solar Terrestrial Physics (ISTP) Missions Major multi-agency, multi-spacecraft programme of the 1990s aimed at exploring the terrestrial MAGNETOSPHERE and near-Earth space. The overall goal of the programme is to improve our understanding of the solar-terrestrial interaction, particularly the coupling of matter and energy between the SOLAR WIND and the magnetosphere. Central elements include WIND, a NASA satellite instrumented to study the solar wind upstream of the Earth and thus provide information on the likely external conditions affecting the magnetosphere. A sister NASA spacecraft, POLAR, in an eccentric polar orbit with apogee over the northern pole, observes activity of the AURORA and makes plasma measurements within the cusps of the magnetosphere. The Japanese space agency ISAS provided the spacecraft GEOTAIL, which is designed to make measurements of the nightside magnetotail region of the Earth. Data from this spacecraft are particularly relevant to studies of the PLASMA SHEET and magnetospheric substorms. The European element, cluster, was finally launched in the summer of 2000, after the original four-spacecraft mission was lost to launch failure in 1996. This mission is designed to study boundaries, such as the bow shock and magnetopause. A number of other missions and facilities are associated with ISTP and provide supporting and context information. These missions include SOHO (the solar and heliospheric observatory), the Russian Interball missions, Equator-S, and many ground-based radars, all-sky cameras and magnetometer networks located around the northern auroral zones.

International Space Station (ISS) space station, which when completed in 2006 - budgets, schedule and technology permitting - will be a space superlative, measuring 111.32 m (365 ft) from end to end. The space station was originally given the go-ahead in 1984, in response to the Soviet Union permanent presence in orbit. This US programme, first called Freedom, which was to have been operational in 1994, became embroiled in politics and financial problems. Following the collapse of the Soviet Union, Russia had no money to build a new space station. The US government was on the point of cancelling Freedom, but the station was saved when Russia joined the programme. The reconfigured ISS was to have been declared operational in 2001 but that deadline has already slipped to 2006. Still known as the International Space Station, although having an unofficial name, Alpha, the project is subject to further delays due to financial and technical difficulties.

International Space Station Seen from the Space Shuttle Endeavour is the International Space Station under construction in 2001 December.

It is planned that the ISS will eventually be crewed by up to seven people, but that is unlikely before 2008. It will have 1624 m3 (46,000 cubic feet) of pressurized space in several modules - the equivalent of a Boeing 747 Jumbo jet - and will be equipped with four photovoltaic modules, each with two arrays 34.16 m (112 ft) long and 11.89 m (39 ft) wide, generating 23 kW, and with a surface area of about half an acre. The electrical power system will be connected by 12.8 km (8 mi) of wire. A major external part of the ISS will be the Canadian remote manipulator system comprising two robot arms and a mobile transporter travelling along the length of the station.

The ISS will concentrate on five main areas of science research - life sciences, space science, Earth science, engineering research and technology, and space product development. It will consist of six major scientific modules serviced by connecting passageways (called nodes), service and control modules, living quarters, an airlock, the manipulator system, logistics vehicles and crew transfer vehicles and propulsion modules. A major benefit of the ISS will be the cooperative work of so many nations of the world. The ISS is going to be the largest international civil, cooperative programme ever attempted, involving 16 nations - the USA, Russia, Canada, Japan, Brazil, Belgium, Denmark, France, Germany, Italy, the Netherlands, Norway, Spain, Sweden, Switzerland and the UK. The likelihood is that the ISS will not quite resemble what has been designed and will develop more on a step-by-step basis, according to the state of delays and finances, with redesigns and compromises being continually made. It may not be declared operational until 2010.

International Sun-Earth Explorer (ISEE) Series of three NASA-ESA scientific satellites launched in 1977-78 in an international project to study the near-Earth environment (particularly the magnetosphere) and its interaction with the solar wind.

International Ultraviolet Explorer (IUE) Joint project of the national aeronautics and space administration (NASA), the european space agency (ESA) and the UK. It was the longest-lived astronomical spacecraft ever flown, and arguably the most important ultraviolet space observatory so far launched.

International Ultraviolet Explorer (IUE) An artists impression of the IUE. Launched in 1978, this highly successful satellite operated until 1996.

IUE was launched by a delta rocket in 1978 January. It carried a 45-cm (18-in.) telescope and spectrographs equipped with ultraviolet-sensitive cameras to study Solar System objects, stars and galaxies. The observing time on IUE was initially shared out between the three agencies in approximate proportion to their contributions, with NASA getting two-thirds of the time and the UK and ESA one-sixth each. After 1981 the combined European share was assigned on scientific merit alone.

IUE was inserted into a geosynchronous elliptical orbit between 26,000 and 45,000 km (16,000-28,000 mi) high and inclined at 28.4 to the equator. As a result, it was visible from the ESA ground station in Spain for up to 12 hours a day and permanently visible from the NASA ground station in Maryland. Control of IUE was transferred permanently to Europe in 1995 October.

During its lifetime, IUE made 104,000 ultraviolet observations. The data are kept in three archives, one operated by each of the collaborating agencies, easing access for thousands of astronomers around the world.

In the Solar System, IUE observed dozens of comets, including Comet halley, Comet hyakutake and Comet encke, which was studied during several returns. It was also used to monitor asteroids, the atmospheres of the giant planets and the cloud of ions associated with Jupiter's moon Io.

Beyond the Solar System, IUE provided new insights into stellar winds and energy transport in the atmospheres of hot, massive stars. Its flexibility opened the door to studies of unpredictable novae and Supernova 1987A. Ultraviolet emissions from 'normal' galaxies, Seyfert galaxies and quasars helped to unravel the processes taking place in accretion disks around massive black holes. See also ultraviolet astronomy

International Years of the Quiet Sun (IQSY) Period of collaborative research involving scientists from many nations; it was organized as a follow-up to the international geophysical year in order to study the same phenomena under conditions of minimal solar activity during 1964-65.

interplanetary dust (zodiacal dust) Lens-shaped cloud of dust particles centred on the Sun and with its major axis lying in the ecliptic plane. The cloud consists of dust particles a few micrometres in size, and it extends for at least 600 million km (370 million mi). It is extremely tenuous: at the Earth's orbit its average density is equivalent to a single particle, two micrometres across, in a region of space one hundred cubic metres in volume.

Sunlight scattered by the cloud is responsible for the zodiacal light. In the scattering process the dust particles absorb energy from the Sun on their illuminated side but re-radiate it isotropically. This process, known as the

International Ultraviolet Explorer (IUE) An artist's impression of the IUE. Launched in 1978, this highly successful satellite operated until 1996.

The rate of depletion of the dust particles indicates that the cloud must be continually replenished by some mechanism, otherwise it would have completely dispersed by now. Collisions between asteroids had long been suspected as a possible source of replenishment, and in 1983 the infrared astronomical satellite (IRAS) located three bands of dust in the region of the asteroid belt between the orbits of Mars and Jupiter. These bands are believed to be the result of collisions between asteroids. Over a long period of time, the particles within the bands slowly decelerate, moving inwards from the asteroid belt towards the Sun and spreading out into the background cloud of interplanetary dust.

Asteroid collision alone cannot produce sufficient material to replenish the cloud at the required rate, however, and analysis of data from both the IRAS and the cosmic background explorer (COBE) satellites indicates that main-belt asteroids are the source of approximately only 33% of the interplanetary dust.

Comets are another source of interplanetary dust, producing dust each time they enter the inner Solar System. The largest particles produce meteor streams, whilst the smallest are incorporated into the dust cloud. Even this does not account for all the new material required, though, and the remainder is thought to be cosmic in origin. The issue as to the ultimate source of the dust is still open to debate, but satellite observations of the sky brightness and in-situ measurements of the dust particles are advancing our understanding of the relative contributions from comets, asteroids and the interstellar medium.

interplanetary dust particle See micrometeorite

interplanetary magnetic field See solar wind

interplanetary medium Tenuous mixture of interplanetary dust, charged atomic particles and neutral gas that occupies the space between the planets. The dust is believed to have originated from collisions between asteroids and from comets entering the inner Solar System. The charged particles - electrons, protons and helium nuclei (alpha particles) - stream out from the Sun as the solar wind, while the neutral gas exists in the form of hydrogen and helium atoms. As the Sun moves through space it passes through interstellar matter, resulting in a continual stream of neutral atoms through the Solar System. Interstellar matter is quickly ionized by the solar wind. See also zodiacal light

interstellar absorption Absorption or extinction of light by interstellar dust (see interstellar matter). On average, the extinction amounts to one stellar magnitude per kiloparsec of distance.

interstellar dust See interstellar matter

interstellar grain Volumetrically insignificant, but scientifically critical, component within chondrite meteorites. Interstellar and circumstellar grains comprise several populations of nanometre-sized diamond particles and micrometre-sized silicon carbide, graphite and aluminium oxide particles. The presence of the grains was first inferred in the late 1970s to early 1980s on the basis of the isotopic composition of noble gases found in mineralogical analyses of chondrites. The unusual isotopic signatures of the noble gases implied the existence of several different hosts; analyses of acid-resistant residues suggested that the hosts might be carbon-rich. Together with the noble gas results, carbon and nitrogen isotope data imply a variety of extra-solar sources for the grains, including supernovae and red giant stars. Currently, grains from at least 15 different extra-solar sources have been isolated from chondritic meteorites. These grains were presumably introduced into the pre-solar nebula prior to its collapse and the onset of protoplanet formation.

interstellar matter Molecules that exist in interstellar molecular clouds. The main constituents of interstellar matter are everywhere the same, mostly hydrogen with some helium, but the proportions of the minor constituents differ widely between different locations. The total density and other properties of this matter also differ widely from place to place. Mostly, the matter is gaseous but a small proportion is embodied in the form of minute solid dust particles. On average there are about 106 atoms m~3 and one dust grain per 100,000 m3 of space (for comparison, there are about 3 X 1025 molecules mT3 in Earth's atmosphere at sea level). Although the density of interstellar matter is extremely low, the volume of the space in a galaxy is so great that the total quantity of interstellar material is very considerable. Our own Galaxy contains about 1010 solar masses of material between the stars, making up about 10% of its total mass. Most of this matter is distributed in the spiral arms and the disk of our Galaxy and is confined to a layer only a few hundred light-years thick.

The most obvious manifestations of interstellar material are nebulae. Several types have been classified -reflection nebulae, hii regions, planetary nebulae, supernova remnants, globules and dark nebulae. The differences between these stem mainly from the way the material is illuminated or the way light from other sources is obscured; they also vary in density or recent history. Other types of nebulae not apparent on optical photographs are molecular clouds, which can be detected by their emission or absorption of radio, microwave or infrared radiation rather than of visible radiation.

The existence of gas between the stars was discovered in 1904 by Johann hartmann through the observation that a few of the absorption lines recorded in the spectrum of the binary star 8 Orionis did not change in wavelength (by the Doppler effect) as the star moved around its orbit. This followed the observation in 1874 by William hug-gins that certain nebulae had a spectrum characteristic of rarefied gas. Since that time interstellar absorption lines have been recorded in the spectra of many stars. In the optical region these lines are few in number and are usually much narrower than the stellar features themselves. Frequently they have multiple Doppler-shifted components, arising from clouds with different line-of-sight velocities. The strongest optical lines are due to neutral sodium and singly ionized calcium atoms. A large number of atoms in various states of ionization have their absorption lines at ultraviolet wavelengths and have been studied by means of telescopes borne on balloons, rockets or spacecraft. The Lyman-o transition (see lyman series) of neutral hydrogen, falling at 121.6 nm wavelength, is by far the strongest of all absorption lines observed. While most of the hydrogen is neutral, some elements exist in the interstellar medium primarily in an ionized state. The ionization of such elements arises largely from the presence of energetic photons from stars. The low density of the general interstellar medium ensures that the time an atom in the ionized state has to wait before it can recombine with a free electron is quite long. Hydrogen molecules are detected at far-ultraviolet wavelengths from absorption lines in the spectra of hot stars. Some 120 other interstellar molecules have now been identified, many quite complex and mostly to be found in the depths of giant molecular clouds.

The total number of atoms or molecules of each kind between a background star and us can be calculated from the shape and strength of the characteristic absorption features in the star's spectrum. Significant differences in the abundances of the heavier elements relative to hydrogen are found between the interstellar medium and the stars. For example, in some cases the interstellar gas seems to contain only a hundredth of the iron and a thousandth of the calcium that is commonly present in the atmospheres of stars. The missing proportions are in the interstellar dust grains (small grains of matter, typically about 100 nm in diameter, in interstellar space). These dust grains are very effective at absorbing and scattering visible and ultraviolet light, thus making distant stars appear fainter and redder. Because the grains produce polarization of starlight, they are believed to be elongated particles aligned by the galactic magnetic field. Most grains appear to be composed of silicates or graphite; some have icy mantles. The grains may form by condensing from gas flowing out of the atmospheres of cool stars.

Most of our knowledge about the large-scale distribution of the interstellar gas in our Galaxy has come from the study of the emission and absorption of radiation detected at the twenty-one centimetre line of neutral atomic hydrogen. From the Doppler shifts of the lines, clouds with different velocities along the same line of sight can be distinguished, allowing the distribution and motion of the neutral gas in distant parts of the Galaxy to be studied. Such observations show that neutral hydrogen clouds (hi regions) are clustered mostly along the spiral arms.

Even when taken together, molecular clouds and neutral hydrogen clouds very far from fill the volume of interstellar space. Although it is still a subject of speculation, much of the gas between the clouds is thought to be very hot and very tenuous, at a temperature of around a million K and a density of a few thousand particles per cubic metre. It is thought to be the outcome of the expansion of numerous supernova remnants.

interstellar molecules Molecules that occupy the space between the stars. In this harsh environment few molecules are able to survive. The ultraviolet radiation from stars will quickly cause many of them to dissociate. Some molecules, however, such as titanium oxide (TiO), cyanogen (CN) and diatomic carbon (C2), are sufficiently stable to survive in the immediate vicinity of stars, or even in the outer layers of cool stars. These molecules can be observed through the absorption bands that they produce in the optical and infrared spectra of the stars.

Most interstellar molecules, however, require shelter from ultraviolet radiation in order to survive. The molecules themselves provide this shelter, since they can exist in either gaseous or solid form (no liquids are known or to be expected in interstellar space). The solid takes the form of interstellar dust particles (see interstellar matter), which appear to be composed of silicates or graphite and sometimes possess outer layers of frozen gases. The composition of the particles is difficult to establish with certainty, but they are known to be a few hundred nanometres in size and possibly needle-shaped; they absorb starlight very efficiently (see dark nebulae). Concentrations of particles can shelter gaseous interstellar molecules from the stellar ultraviolet radiation. Thus the main sites where molecules can be found are inside cold dense gas and dust clouds (see molecular clouds).

Prior to the advent of radio astronomy, a few molecules were discovered from the absorption lines that they produced in the optical spectra of stars. These molecules were CH, CH+ and CN, and they could only be detected within clouds that were thin enough for stars beyond the cloud to be observed. Such clouds would not be dense enough to provide shelter for more complex interstellar molecules.

Most interstellar molecules have been detected from their emissions or absorptions at radio wavelengths. The reason for this is that molecules can emit or absorb radiation by three separate processes. In the optical region, most spectrum lines from molecules are due to changes in the energy state of electrons within the molecule. Energy can also be stored by molecules in the form of the vibrations of its constituent atoms and by the rotation of the molecule itself. Just as the energies of electrons are quantized, so also can these vibrational and rotational energies only take specific values. Thus when the molecule changes its vibrational or rotational energy it does so in discrete steps and produces emission or absorption lines. The energy stored in the form of molecular vibrations or rotations is much less than that stored by electrons, and so the resulting spectrum lines are at long wavelengths. Vibrational lines mostly appear in the infrared, and rotational lines in the microwave and radio regions. Inside molecular clouds the temperature is low, and so the molecules typically emit or absorb only via the lowest energy rotational transitions.

The most abundant molecule in molecular clouds is expected to be that of hydrogen (H2). However, the hydrogen molecule is symmetrical, like a miniature barbell, which means that the rotational transitions are forbidden and so molecular hydrogen does not produce spectrum lines within the molecular clouds. It can, however, sometimes be observed weakly when temperatures reach 500 K or more, for example in shock fronts between colliding gas clouds. In 1963 the first interstellar molecule detected by radio astronomy was hydroxyl (OH). Detection of the second most abundant molecule, carbon monoxide (CO), soon followed, from its lines at 115, 230 and 340 GHz (wavelengths of 2.6, 1.3 and 0.9 mm). The carbon monoxide lines are now widely used as an easily observed tracer for molecular clouds.

Detection of polyatomic molecules such as water (H2O) and ammonia (NH3) came in 1968. It was soon followed by the discovery of some of the hydrocarbons (compounds of hydrogen and carbon such as methane, CH4). More recently, much more complex atoms including, possibly, the amino acid glycine (NH2CH2COOH), which forms one of the building blocks for our form of life, have been found. Currently a thirteen-atom molecule, HC11N, is the most complex found. About 120 interstellar molecules are now known and more are being added at the rate of a few every year.

interstellar planet Planet that wanders in interstellar space, not gravitationally bound to any star. In recent years candidate objects have been discovered in deep space, for example in the Orion Nebula. There is debate as to whether these objects should properly be classed as planets or whether they should be categorized as stars, even though many of them have insufficient mass ever to initiate hydrogen or even deuterium fusion in their cores, making them sub-brown dwarfs. The terms 'planetar' or 'grey dwarf' have been coined to label them. Considering bodies of a quite different size and nature much closer to home, there are several trans-neptunian objects with aphelion distances so large (some hundreds of astronomical units) that they may be on the verge of escaping into interstellar space. An example is 2000 CR105, which is about 250 km (155 mi) in size and has perihelion at 44 AU, aphelion at 413 AU.

interstellar reddening Reddening of starlight passing through interstellar dust (see interstellar matter). It arises because the dust is less effective at attenuating longwave (red) light than short-wave (blue) light.

interstellar scintillation Analogue of the twinkling of stars seen with the naked eye that occurs for unresolved radio sources. The source of the scintillation is irregularities in the electron and ion density of the interstellar matter. Movements within the medium, or more usually of the Earth and the radio source, cause the radio source to fluctuate in brightness on a time scale of a few minutes to a few hours as the scattering and delay within the interstellar medium alters.

intraterrestrial asteroid Asteroid that has an orbit entirely interior to that of the Earth; it thus has an aphelion distance of less than 0.9833 AU. By analogy, intravenusian and intramercurian asteroids might also be defined. No such bodies are known, although they have been suggested in the past (see vulcan). The discovery of such bodies would be difficult using ground-based telescopes because they are always on the sunward side of our planet.

intrinsic variable Variable star in which the variations in brightness arise from processes that cause an actual change in the amount of radiation emitted, rather than modulate a fixed output. Typical processes are pulsation (both radial and non-radial) and eruptions (both from the accretion of material, as in a cataclysmic variable, and from internal mechanisms as in a flare star). See also extrinsic variable

invariable plane Plane of reference that is at right angles to the total angular momentum vector of the Solar System; it is unaffected in orientation by any of the perturbations between the bodies in the Solar System. It is inclined at 1.577 to the ecliptic of epoch J2000. As most of the angular momentum of the Solar System is contained in the orbit of Jupiter, the invariable plane has an inclination of just 0.324 to Jupiter's orbital plane. The angular momentum of a body depends on its position, velocity and mass. The positions and velocities of the planets at any instant relative to the ecliptic are well known, but the masses of some planets are not so well known, and thus the calculation of the location of the invariable plane is a little uncertain and it is not used much as a reference plane. It is, however, used in the definition of the north and south poles of planets and satellites, which are defined as north if they are above the invariable plane.

inverse-square law Frequently encountered relationship whereby the magnitude of a physical quantity diminishes in proportion to the square of distance. Common examples of the inverse square law include

invisible astronomy Astronomical research at wavelengths of electromagnetic radiation invisible to the human eye. This covers gamma-ray astronomy, x-ray astronomy, ultraviolet astronomy, infrared astronomy, radar astronomy, radio astronomy, and the study of particles such as cosmic rays and neutrinos. By studying celestial bodies across the whole of the electromagnetic spectrum, and not just at visible wavelengths, astronomers are able to build up a more complete picture of the Universe.

Io Innermost of the galilean satellites of jupiter. Its size, mass and density are all only a little greater than those of the Moon, yet strong tidal heating by Jupiter makes it by far the most volcanically active body in the Solar System. Unlike the other Galilean satellites, Io is ice-free, and it has a sulphur-stained rocky surface.

Io A composite of images from the Galileo orbiter, showing the volcanically active surface of Jupiters moon Io. Intense volcanism driven by tidal stresses leads to rapid resurfacing of Io.

Of the many revelations from the voyager tours of the outer Solar System, the discovery of active volcanoes on Io probably ranks among the most important. Before Voyager, most people had assumed that bodies of Io's size, whether rocky or icy, would be geologically dead like the Moon. It is now realised that the orbital resonance between the three innermost Galilean satellites results in tidal heating. The effect is greatest for Io because it is closest to Jupiter and hence experiences the strongest tidal forces.

There are often more than a dozen volcanoes erupting on Io at any one time. These are identified either by a visible 'eruption plume' powered by the explosive escape of sulphur dioxide, and rising 100100 km (60-250 mi) above the surface, or by infrared detection of a hot spot. The record for the highest local temperature is at least 1700 K, whereas the normal daytime surface temperature on Io is only 120 K.

Io's density indicates that it is predominantly a silicate body, like the terrestrial planets. Gravity and magnetic observations by the galileo orbiter confirm that it has a dense, presumably iron-rich core below its rocky mantle. Spectroscopic data show that Io's surface is covered by sulphur, sulphur dioxide frost and other sulphur compounds. However, these are no more than thin, volatile veneers resulting from volcanic activity, and the crust as a whole is some kind of silicate rock.

Io Material ejected from Ios volcanoes forms a torus around Jupiter. A magnetic flux tube links Io and Jupiter: particles ejected during the satellites volcanic eruptions can lead to enhancements of the Jovian aurorae.

Io's tenuous atmosphere contains sulphur dioxide, as well as atoms of oxygen, sodium and potassium. The surface pressure is less than a millionth of the Earth's but nearly a billion times greater than the atmospheric pressure at the surface of the Moon or Mercury. Io's atmosphere continually leaks away into space, contributing to a 'cloud' of sodium and potassium atoms falling inwards towards Jupiter and into a magnetically confined belt of ionized sulphur that stretches right round Jupiter, concentrated around Io's orbit (the Io torus). The atmosphere is replenished by a combination of volcanic activity and collisions on to Io's surface by high-speed ions channelled by Jupiter's magnetic field. When Io passes into the shadow of Jupiter its atmosphere can be seen faintly glowing in an auroral display caused by these same magnetospheric ions impinging on the atmosphere.

Io's surface is totally dominated by the results of volcanic activity. There are lava flows up to several hundred kilometres in length and vast swathes of mostly flat terrain covered by fallout from eruption plumes. Most of the lava flows are now believed to have formed from molten silicate rock, which is often discoloured by a sulphurous surface coating, but there could also be some flows that formed from molten sulphur. The theory that prevailed for several years after the initial Voyager observations - that all or most of Io's lava flows are formed from sulphur - has been disproved by temperature measurements of eruption sites by the Galileo spacecraft and by infrared telescopes operating from Earth. Indeed, some of the temperatures detected are not only too high to represent molten sulphur, which would boil away at less than 700 K, but also too high to be characteristic of most types of molten rock, including basalt, which is the most common lava on the Earth and Moon. The exceptionally hot sites on Io might possibly be where a silica-poor, magnesium-rich relative of basalt known as komatiite is able to reach the surface.

In some places volcanoes rise above the general level of Io's plains. Their summits are occupied by volcanic calderas, up to 200 km (120 mi) across, which are formed by subsidence of the roof of the volcano after magma has been erupted from within. No impact craters are visible, even on the most detailed images, because the volcanic eruptions deposit fresh materials across the globe at an average rate of approximately a centimetre thickness per year.

More than 500 volcanoes have been identified on Io, and about 100 of these have been seen to erupt. The long duration of the Galileo mission enabled many changes on Io's surface to be documented, including deposits left by fallout from eruption plumes and new lava flows. Io's volcanoes appear to be randomly distributed, and Io certainly lacks the kind of well-defined global pattern displayed by the Earth. Unlike Earth, which releases internal heat by plate TECTONICS, and Venus, where heat escapes by conduction, probably punctuated by bouts of resurfacing every half billion years or so, Io's heat escapes through a multitude of volcanoes. One factor that probably influences the difference between the Earth and Io is that, to maintain a steady temperature, Io has to lose heat at a rate of about 2.5 W/m2 compared with only 0.08 W/m2 for the Earth. Possibly, the tidal heating experienced by Io is sufficient to keep a large fraction of its MANTLE partially molten. See date at JUPITER

ion An ATOM that has lost (or gained) one or more electrons compared with the normal, or 'neutral', atom. A positive ion has fewer electrons and a negative ion has more electrons than a neutral atom. See also IONIZATION numeral that is one larger than the number of lost electrons. For example, neutral iron is Fe I, singly ionized iron (one electron missing) is Fe II, doubly ionized iron is Fe III, and so on. An alternative notation is to indicate the net charge of the ion by superscript ' + ' and ' ' signs. Using this system gives Fe+ for singly ionized iron, Fe+ + for doubly ionized iron, and so on. This latter system has the advantage of incorporating the negative ions, as in H, but it becomes cumbersome for high levels of ionization. Ionization normally occurs through the absorption of radiation or through collisions with other atoms and ions.

The material that forms stars, planetary nebulae and HII REGIONS is almost completely ionized. In other regions some atoms may be ionized while others remain as neutral atoms; for example, in the Earth's IONOSPHERE only about one atom in one million is actually ionized.

ionosphere Region in the Earth's ATMOSPHERE that extends from a height of about 60 to 500 km (40-310 mi) above the surface. Within this layer most of the atoms and molecules exist as electrically charged ions. This high degree of ionization is maintained by the continual absorption of ultraviolet and X-ray radiation from the Sun. These free electrons and ions can disturb the transmission of radio waves through the ionosphere. There are several distinct ionized layers, which are known as the D, E, F1, F2 and G layers. The layers are rather variable: at night the D layer disappears, the E layer weakens or disappears, and the F1 and F2 layers merge. The D layer is situated at a height of between 60 and 90 km (40-60 mi), the E layer at 90-150 km (about 60-90 mi), the F1 layer at 200 km (120 mi) and the F2 layer at 300-400 km (190-250 mi). The free electrons in the E and F layers strongly reflect some radio waves: they enable long-distance radio communications by successively reflecting the waves between the layer and the ground. In RADIO ASTRONOMY, the presence of the E and F layers makes ground-based observations almost impossible below 10 MHz. The D layer, where collisions between the molecules and ions are more frequent, tends to absorb radio waves rather than reflect them.

Io Material ejected from Io's volcanoes forms a torus around Jupiter. A magnetic flux tube links Io and Jupiter: particles ejected during the satellite's volcanic eruptions can lead to enhancements of the Jovian aurorae.

ionization Name given to any process by which normally electrically neutral atoms or molecules are converted into IONS, through the removal or addition of one or more electrons. This gives them a positive or negative electrical charge. An ion can itself be ionized, by losing or gaining a second electron. The minimum energy required to remove an electron from an atom, ion or molecule is called its ionization potential. For negative ions, the degree of ionization is denoted by a Roman


Iota Aquarids Minor METEOR SHOWER active during July and August, with peak around August 6. It has a zenithal hourly rate (ZHR) no greater than 10. The meteors are generally swift and faint.

IPCS Abbreviation of IMAGE PHOTON COUNTING SYSTEM IRAS Abbreviation of infrared astronomical satellite

IRAS-Araki-Alcock, Comet (C/1983 H1) Bright long-period comet that passed remarkably close (0.031 AU) to Earth on 1983 May 11. The comet was discovered by Japanese amateur astronomer Genichi Araki (1954- ) and, independently, English amateur astronomer George Alcock (1912-2000) on May 3, having earlier been detected by the Infrared Astronomical Satellite (IRAS). At closest approach, IRAS-Araki-Alcock reached mag. + 2.0, showing a diffuse 2 diameter coma. Due to its proximity, the comet moved rapidly across the northern sky. Perihelion, 0.99 AU from the Sun, was reached on 1983 May 21. The orbital period is roughly 1000 years.

Iridum, Sinus (Bay of Rainbows) Lunar lava plain (45N 31W), 255 km (160 mi) in diameter. Sinus Iridum was formed by a large impact in a ring of the imbrium multi-ring impact basin. Later, when lunar basalts poured into Mare Imbrium, they flooded this crater as well, covering the eastern side of the rim so that it is no longer visible. Multiple mare (wrinkle) ridges are visible in Sinus Iridum, including the circular mare ridge of an inner Imbrium basin ring.

IRIS Acronym for infrared imaging surveyor

iron meteorite An etched cross-section through an iron meteorite, showing the Widmanstatten pattern. This pattern indicates that iron meteorites crystallized slowly in the pre-solar nebula.

Iris Large main-belt asteroid discovered in 1847; number 7. Iris is c.200 km (c.124 mi) in diameter. Because of its orbit, near the inner edge of the main belt, this asteroid appears particularly bright, with only vesta, ceres and pallas exceeding it.

iron (symbol Fe) Seventh most abundant element by numbers of atoms, and the fifth most abundant in terms of the mass content of the Universe. It is a metal. Iron's properties include: atomic number 26; atomic mass of the natural element 55.847 amu; melting point 1808 K; boiling point 3023 K. It has 10 isotopes, with iron-54 (5.8%), iron-56 (91.7%), iron-57 (2.2%) and iron-58 (0.3%) being stable and occurring naturally on Earth.

Iron is the major constituent of the cores of the planets Mercury, Venus and Earth, and of iron meteorites. Iron grains are believed to make up a proportion of the interstellar dust population.

Elements up to and including iron can be built up by energy-releasing fusion reactions, which act as energy sources in stars. Iron, however, has the highest binding energy per nucleon, and so these nucleosynthesis reactions halt when it has been produced. See also metals

iron meteorite Meteorite composed of iron metal, generally with between 5 and 20% by weight nickel. Iron meteorites account for approximately 5% of all observed meteorite falls. The mineralogy of iron meteorites is dominantly an intergrowth of the two iron-nickel alloys kamacite and taenite. Kamacite (aFe,Ni) has a body-centred cubic structure and a nickel content less than 7% by weight. Taenite (yFe,Ni) is face-centred cubic and c.20-50% by weight nickel.

Iron meteorites are highly differentiated materials, the products of extensive melting processes on their parent bodies. They can be divided into magmatic irons and non-magmatic irons. Magmatic irons are those that have solidified by fractional crystallization from a melt. Non-magmatic irons are those that seem not to have completely melted; they may have formed during impact processes.

The iron meteorites are subdivided into 13 different groups on the basis of nickel and trace element chemistries (gallium, germanium and iridum contents). Each separate group of magmatic irons has a fairly restricted range of nickel contents but a wide range of trace element abundances; these trends are consistent with fractional crystallization from a melt. In contrast, the non-magmatic irons show a wide range in nickel contents but less variation in trace element composition; these trends can be better explained by formation by partial melting. Many irons defy chemical classification and simply remain 'ungrouped'. It is thought that each chemical group derives from its own parent asteroid.

Prior to classification on the basis of trace-element chemistry, iron meteorites were classified in terms of their metallographic structure. Laths of kamacite intergrown with nickel-rich phases form the 'Widmanstatten pattern' revealed in polished and etched iron meteorites. The width of the kamacite lamellae allows classification of iron meteorites into five structural groups: the coarsest, coarse, medium, fine and finest octahedrites. Plessitic octa-hedrites are transitional between octahedrites and ataxites. Ataxites are nickel-rich, with more than 20% by weight nickel, and are mainly taenite. Hexahedrites have nickel less than 6% by weight and comprise kamacite only. Neither hexahedrites nor ataxites display a classic Wid-manstatten pattern. Meteorites from an individual chemical group can display a range of structural types.

irradiation Process by which a region of space or material is subjected to radiation, whether by light, radio, infrared or other forms of electromagnetic radiation, or by energetic particles such as protons and electrons.

irregular galaxy Galaxy that shows no symmetry. Some irregular galaxies are smaller than spiral and elliptical galaxies, contain much gas, and are undergoing star formation. Some are classified as irregular for the only reason that they do not fit into other categories of galaxy. Many irregular galaxies have overall rotation and a relatively thin, gas-rich disk, sometimes including a bar (as in the large magellanic cloud), forming a continuation of the hubble classification beyond the spirals of types Sc and SBc.

irregular variable Variable star that displays no periodicity in its light changes; there are two broad types.

One type (I) includes many poorly understood variables as well as the various forms of NEBULAR VARIABLE. The second type (L) consists of slowly varying pulsating stars that are otherwise very similar to various types of LON-PERIOD VARIABLE and SEMIREGULAR VARIABLE. Although many types of variable star (see CATACLYSMIC VARIABLE and FLARE STAR) exhibit fluctuations at random, unpredictable intervals, these types are not defined as 'irregular' under the classification scheme.

Isaac Newton Group of Telescopes (ING) Group of instruments consisting of the 4.2-m (165-in.) WILLIAM HERSCHEL TELESCOPE, the 2.54-m (100-in.) ISAAC NEWTON TELESCOPE and the 1.0-m (39-in.) JACOBUS KAPTEYN TELESCOPE at the ROQUEDE LOS MUCHACHOS OBSERVATORY on the island of La Palma. There is a sea-level base at Santa Cruz de La Palma. The ING is funded by the UK's PARTICLE PHYSICS AND ASTRONOMY RESEARCH COUNCIL, the NWO (Nederlanse Organisatie voor Wetenschappelijk Onderzoek) in the Netherlands, and Spain's INSTITUTO DE ASTROFtSICA DE CANARIAS.

Isaac Newton Telescope (INT) Optical 2.54-m (100-in.) telescope, part of the ISAAC NEWTON GROUP on La Palma, which can be used either for wide-field imaging or spectroscopy. It was installed at the ROYAL GREENWICH OBSERVATORY, Herstmonceux, in 1967 and was originally fitted with a 2.50-m (98-in.) mirror made from a Pyrex blank cast in 1936 for Michigan University Observatory. However, atmospheric conditions at Herst-monceux were poor, and when the UK's participation in ROQUEDELOS MUCHACHOS OBSERVATORY was proposed in the 1970s, it was decided to move the INT there. Equipped with a new mirror, the telescope began operation on La Palma in 1984.



Ishtar Terra Continent-sized highland block of VENUS. It covers an area comparable to Australia and drops steeply to the surrounding plains, especially on its southwestern flank. It is unique among Venus' upland regions inasmuch as its perimeter rises several kilometres above the interior. Western Ishtar comprises Lakshmi Planum (a vast plateau encircled by a series of mountain belts), Freya Montes to the north, Akna Montes in the west, and the Danu Montes, which extend some 1200 km (750 mi) to the south and south-east. Immediately east are the Maxwell Montes, rising to 17 km (11 mi) above the mean planetary radius, the highest point on Venus. Like the mountains around Lakshmi Planum, the Maxwell Montes have a complex banded structure clearly seen on radar images. Eastern Ishtar takes the form of a rather hummocky plateau extending outwards between about 100 and 1000 km (60-600 mi). These highly deformed zones are below the level of the mountain chains; they slope down towards the exterior plains or terminate in steep scarps.

Isidis Planitia Impact basin on MARS (13.0N 273.0W). It is situated between SYRTIS MAJOR PLANITIA and the Elysium Rise and is approximately 1100 km (680 mi) across. The basin is poorly defined on its eastern side, where it merges on to the plains associated with the Elysium volcanoes.

Islamic astronomy Astronomy as practised in the Middle East, North Africa and Moorish Spain during the flowering of the Islamic Empire, from around the 8th to the 14th century. Although it is sometimes referred to as Arab or Arabic astronomy, some of its practitioners were from other ethnic or linguistic groups, and the unifying cultural force in this region and during this period was Islam. Two circumstances fostered the growth of astronomy under Islamic rule. The first was that the seats of ancient learning lay within orjust outside the bounds of the empire, and Islam was tolerant of scholars from other creeds. A second impetus came from Islamic religious observances, which gave rise to many problems in mathematical astronomy, mostly related to timekeeping. In solving these problems, Islamic astronomers went far beyond the Greek mathematical methods and provided essential tools for the creation of Western RENAISSANCE ASTRONOMY.

Following the foundation of Baghdad, the new capital of the Abbasid dynasty, in AD 762, there began a massive effort to translate into Arabic all the major scientific texts of antiquity. The most vigorous patron was the Caliph al-

Isaac Newton Group of Telescopes High above the clouds on the Roque de Los Muchachos peak on La Palma, the ING telescopes enjoy some of the best observing conditions in Europe. Largest of these instruments is the Isaac Newton Telescope itself (large dome at left), a 2.5 m (98 in.) reflector.

Ma'mun. Shortly after he came to power in 813 he founded the Bayt al-Hikma (House of Wisdom) in Baghdad. There, scholars of all creeds worked to translate manuscripts acquired from the ancient libraries that now lay within the empire, which stretched from Spain to India, to stock what was to become one of the world's great academies.

The chief scholar of this great enterprise was Abu'l-Hasan Thabit ibn Qurra (c.835-901), who wrote over a hundred scientific treatises, including a commentary on Ptolemy's Almagest. Another astronomer (and geographer) in 9th-century Baghdad was Abu'l-Abbas al-Farghanl (c.825-61), whose Elements helped to spread the more elementary and non-mathematical parts of Ptolemy's geocentric astronomy to the West. By 900 the stage was set for the spread of scientific knowledge throughout the Empire, with a single language, Arabic, as its vehicle. This knowledge later diffused into Christian Europe via Spain, where there was considerable scholarly interaction with visiting European translators until the defeat of the Moors in the 12th century (see MEDIEVAL EUROPEAN ASTRONOMY).

The times and dates of Islamic religious activities are regulated according to a lunar calendar, and the first appearance of the new moon is of great importance. Predictions of this and the preparation of almanacs regulating the hours of prayer led to a considerable interest in spherical trigonometry, and the development of the modern trigonometric functions (although some originated in India) and the identities between them.

Islamic astronomers did not make exhaustive observations of the sky. They restricted their sightings, or at least those they chose to record, primarily to measurements that could be used for re-deriving key parameters of solar or planetary orbits. An impressive example of an Islamic astronomer working strictly within a Ptolemaic framework but establishing new values for Ptolemy's parameters was Muhammad AL-BATTANT, a younger contemporary of Thabit ibn Qurra. Al-BattanT's ZTj ('[Astronomical] Tables') was one of the most important works of astronomy between the time of Ptolemy and the Renaissance -Nicholas COPERNICUS cites his 9th-century predecessor no fewer than 23 times.

By contrast, one of the greatest astronomers of medieval Islam, 'Ali ibn 'Abd al-Rahrrian ibn Yunus (950-1009), remained virtually unknown to European astronomers until around 1800. Working in Cairo a century after al-BattaanaT, Ibn Yunus wrote a major astronomical handbook called the Hakimi zTj. Unlike other Islamic astronomers, he prefaced his zTj with a series of more than a hundred observations, mostly of eclipses and planetary conjunctions.

Ptolemy's Almagest had contained a catalogue of over a thousand stars. The first critical revision of this catalogue was carried out in the 10th century by Abu'l-Husain AL-SOFT. His KtTTb suwar al-kawakib al-thabita ('Book on the Constellations of the Fixed Stars') followed Ptolemy's often faulty list, but it did give improved magnitudes. The book's splendid pictorial representations became known in the Latin West; it also contains the first known representation of the Andromeda Galaxy.

Although most Islamic astronomers remained securely within the geocentric framework of Ptolemy and Aristotle, some criticized particular technical details of the PTOLEMAIC SYSTEM which seemed to violate the ancient belief that only uniform circular motion can explain the movements of celestial bodies. One of the first critics was the physicist Ibn al-Haytham (965?-c.1041), known in the West as Alhazen, who held the planetary models of Ptolemy's Almagest to be false. Later, Muhammad ibn Rushd (1126-98), known in the West as Averroes, declared that Ptolemy's eccentrics and epicycles were 'contrary to nature'.

A fresh attack on the Ptolemaic system was undertaken in the 13th century by NasTr al-DTn AL-TUST. A prolific writer with 150 known titles to his credit, al-TuasaT constructed a major observatory at MaraTgha (in present-day Iran). Other astronomers at the MaraTgha observatory also offered new arrangements of circles, but a fully acceptable alternative (from a philosophical point of view) did not come until the work of Ibn al-Shatir (1304-75/6) at Damascus around 1350. Although al-ShaTtir's solution, as well as the work of the MaraTgha observatory, remained generally unknown in the West, this and other Islamic criticisms of Ptolemy may have had an influence on Copernicus.

Conspicuous examples of modern astronomy's Islamic heritage are found in its vocabulary in terms such as 'nadir' and 'zenith', but in particular in the names of stars: Betel-geuse, Rigel, Aldebaran and Altair are just a few of the star names that are Arabic in origin or are Arabic translations of Ptolemy's Greek descriptions. (Some of the names as we have them today bear little resemblance to their original forms, having been corrupted in transliteration and by centuries of transcription.) Many of these star names begin with 'Al-' because al is Arabic for 'the'. The Arabic star nomenclature entered the West by another route - the making of ASTROLABES, on which star names were inscribed. The earliest dated astrolabe in Arabic dates from 927-928. It is primarily from Spain that astrolabe-making, together with Arabic names for the stars, reached the West via England in the late 13th and 14th centuries.

isostasy Principle that recognizes there to be a state of balance between topographic masses and the underlying materials that support them. Higher elevations tend to be supported by material of lower density; for example, continental crust is less dense than oceanic crust and is also thicker, and mountain ranges have 'roots' of crustal material extending downwards towards the mantle. The depth of such features can be deduced from precise measurements of local variations in the gravity field. The concept owes much to the work of Sir George AIRY.


isotherm Line drawn on a weather map, joining all places that are, at a given moment of time, experiencing the same temperature. Isotherms can also be drawn on plots of radio or infrared emission from astronomical bodies.

isothermal process Process in thermodynamics in which a change in a system occurs with transfer of heat to or from the environment so that the system remains at a constant temperature. The collapse of a PROTOSTAR (see HAYASHI TRACK) and the collapse of a star to form a WHITE DWARF are both isothermal processes.

isotopes Atomic nuclei that have the same atomic number but different atomic masses; that is, they contain the same number of protons but different numbers of neutrons. See also ATOM

isotropy Idea that the UNIVERSE looks the same in all directions.


Istituto di Radioastronomia, Bologna Institute of the Italian National Research Council (CNR) which operates two stations, respectively in Medicina (Bologna) and Noto (Siracusa). Its three radio telescopes, the 600 X 600-m (2000 X 2000-ft) Northern Cross array and two single-dish 32-m (105-ft) antennae, are used mainly for VERY LONG BASELINE INITERFEROMETRY.


Izar (Pulcherrima) The star e Bootis, visual magnitude 2.35, distance 210 l.y. It is a striking double star of mags. 2.5 and 4.6, difficult to resolve in the smallest telescopes because of the closeness of the components, less than 3" apart. Their spectral types are K0 II or III and A0 V, producing a beautiful colour contrast of orange and blue. The name Izar comes from the Arabic meaning 'girdle' or 'loincloth'. Its alternative title, Pulcherrima, is Latin for 'most beautiful', from its telescopic appearance.