z Symbol for redshift

Zach, Franz Xaver von (1754-1832) Hungarian astronomer known for his part in the discovery of Ceres. In 1786 he began building the Seeberg Observatory near Gotha, Germany; three years later he began searching for the hypothetical planet between Mars and Jupiter predicted by bode'slaw. Concluding that a more organized search by the world's most skilled observers would be necessary to find the 'missing' planet, in 1800 he convened a meeting at the private observatory of Johann schroter, one of the celestial police. On 1801 January 1, Giuseppe piazzi discovered ceres, orbiting between Mars and Jupiter. The asteroid was subsequently lost, but Zach recovered it using orbital calculations by Carl Friedrich gauss.

ZAMS Abbreviation of zero-age main sequence

Z Andromedae star (ZAND) Type of cataclysmic variable, included in the larger symbiotic star grouping. Z Andromedae star systems resemble dwarf novae, but instead of a main-sequence secondary, they contain a red giant or supergiant together with a hot white dwarf. The components are close together, and mass transfer appears to occur either in the form of a stream of material or as an enhanced stellar wind, depending on the system. The material is accreted by the white dwarf, either directly or through an accretion disk,giving rise to occasional outbursts. Z Andromedae itself has a magnitude range of 8.3 to 12.4.

zap crater Most commonly, a very small crater created by micrometeorite impact. It is a usually lined with glasses and surrounded by fractures.

Zarya Russian Functional Energy module for the international space station (ISS). It was the first ISS component to be launched, in 1988.

Z Camelopardalis star (UGZ) eruptive variable star, a subtype of dwarf nova. Z Camelopardalis stars differ from the common dwarf nova class (see u geminorum star) in that they experience occasional 'standstills', remaining more or less constant in brightness for a long period. These standstills always seem to begin during a decline from maximum. When the standstill ends, the star drops to minimum brightness, and then resumes its 'normal' behaviour. Both the occurrence of the standstills and their duration, ranging from a few days to many months, are quite unpredictable. Z Camelopardalis itself is the brightest member of the class. It may reach magnitude 10.2 at brightest and falls to about 14.5 at minimum. The usual interval between outbursts is roughly 22 days.

Zeeman effect Splitting of a spectral line into several components by the presence of a magnetic field. Where the components are unresolved, a broadened line is seen. The effect allows the measurement of the strengths of magnetic fields on the Sun, the stars and even in the interstellar medium.

Zel'dovich, Yakov Borisovich (1914-87) Russian astrophysicist and cosmologist, born in Minsk, modern Belorussia, who originated the 'pancake model' of large-scale structure in the Universe. With Rashid Alievich Sunyaev (1943- ) he described the Sunyaev-Zel'dovich effect, an apparent reduction in the temperature of the cosmic background radiation as it passes through hot ionized gas between members of galaxy clusters. In the early 1970s, Zel'dovich and others developed a model of the early Universe in which huge discrete lumps of primordial matter collapsed asymmetrically under their own weight as they cooled, forming thin 'pancakes'. The model correctly predicts the arrangement of galaxies in sheets and voids revealed by redshift surveys.

Zelenchukskaya Astrophysical Observatory See special astrophysical observatory

Zenit Heavyweight former Soviet Union satellite launcher, first flown in 1985, which can place payloads weighing 13 tonnes into low Earth orbit. The two-stage Zenit, built in the Ukraine, had 28 successful and 9 failed launches to 2001. It mainly carried Soviet electronic intelligence and Earth observation satellites. The booster also provides the basis of the sea launch commercial satellite launcher.

zenith Point on the celestial sphere directly overhead an observer and 90° from the horizon. This is known as the astronomical zenith. Because the Earth is not a sphere, the geocentric zenith is defined as a line joining the centre of the Earth to the observer. The point 180° away from the zenith, directly beneath an observer, is the nadir.

zenith distance Angular distance from the zenith to a celestial body, measured along a great circle. It is usually expressed as a topocentric measure, from the observer's position on the Earth's surface, but sometimes geocentric, as measured by a hypothetical observer at the centre of the Earth. The zenith distance is equal to 90° minus the altitude of the body above the horizon.

zenithal hourly rate (ZHR) Useful index of meteor shower activity, allowing comparison of observations made at different times and under different sky conditions. Zenithal hourly rate is determined by allowing for the altitude (a) of the shower radiant during observations, the stellar limiting magnitude (LM), and the population index (r, indicative of the proportions of faint meteors that might be lost to background skyglow in the shower under study). The derived ZHR corresponds to the expected number of meteors seen in a perfectly transparent sky (LM = +6.5) with the radiant overhead; it is calculated by multiplying the observed hourly count by 1/sin a X r6.5'LM.

zero-age main sequence (ZAMS) main sequence as defined by stars of zero age, which is the point when they have achieved a stable state, with core temperatures sufficiently high for nuclear fusion to begin. As the star evolves and changes hydrogen into helium, its chemical composition changes and it shifts to the right of its zero-age position on the hertzsprung-russell diagram.

zero gravity Apparent absence of gravitational forces within a free-falling system. A body in a 'zero gravity' or free-fall state experiences no sensation of weight, hence the 'weightlessness' of astronauts, since the spacecraft is continuously free falling towards the Earth while its transverse motion ensures that it gets no closer. The term 'zero gravity' does not imply a total absence of gravity, rather it refers to an absence of any detectable gravitational forces. Although gravity becomes very weak at large distances from massive bodies, it nowhere declines absolutely to zero. See also acceleration of free fall

Zeta Aurigae eclipsing binary with a period of 972 days; the visual range is from magnitude 3.7 to 4.2. It is the faintest of the three 'Kids' near Capella. The Zeta Aurigae system consists of a hot B-type star and a supergiant companion of type K; the distance is about 790 l.y. During the partial phase of the eclipse of the hot star, its light shines through the outer, rarefied layers of the supergiant, and there are complicated and informative spectroscopic effects. The eclipse of the B-type star is total for 38 days; this is preceded and followed by partial stages lasting for 32 days each.

Zhang Heng (ad 78-139) Chinese scientist, the first in China to build a rotating celestial globe and an armillary sphere with horizon and meridian rings. With these and other simple instruments, he observed and catalogued 320 bright stars, and estimated the total number of naked-eye stars as 11,520. He concluded that 'the sky is large and the Earth small' - a radical concept at the time. Zhang understood that the Earth and Moon are spherical, lunar eclipses are caused by the Earth's shadow falling upon the Moon, and the Moon shines by reflected sunlight.

ZHR Abbreviation for zenithal hourly rate

Zijin Shan Observatory See purple mountain observatory

zodiac Belt of constellations, roughly 8° on either side of the ecliptic, through which the Sun, Moon and planets (except Pluto) appear to pass. The zodiac includes the 12 familiar constellations - Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius and Pisces, which are of varying sizes. In astrology, however, the zodiac is divided into 12 equal signs, each 30° long, but because of the effects of precession and re-definitions of constellation boundaries they no longer coincide with the constellations of the same name. Precession has also caused the ecliptic to now pass through the constellation Ophiuchus, while the zodiac now also includes parts of Cetus, Orion and Sextans.

zodiacal band Very faint band of light that extends along the ecliptic and joins the zodiacal light to the gegenschein. Its brightness is variable and it can only be observed under conditions of extreme clarity when no Moon is present. Like the zodiacal light, it is caused by the scattering of sunlight towards the Earth by a cloud of dust particles surrounding the Sun (see interplanetary dust). The band is faintest at around 135° from the Sun and brightens towards the cone of the zodiacal light and towards the gegenschein.

zodiacal catalogue Catalogue of stars in a narrow zone straddling the ecliptic (the zodiac), through which most of the planets and asteroids move. occultations of stars in this region by the Moon, planets or smaller members of the Solar System permit highly accurate position measurements of the occulting body, and may also reveal the binary nature of an occulted star, or even allow an estimation of its radius. Zodiacal catalogues therefore give highly accurate positions of stars. James Robertson's Catalogue of 3539 Zodiacal Stars brighter than 9th magnitude was published in 1940; the largest zodiacal catalogue (USNO-SA2.0) lists about 50 million stars.

zodiacal dust See interplanetary dust

zodiacal light Faint, diffuse conical skyglow seen extending along the ecliptic soon after twilight ends at sunset, or before dawn begins to brighten the sky ahead of sunrise. The zodiacal light is comparable in brightness to the Milky Way, and it is best seen from temperate latitudes in the spring evening sky about 90 minutes after sunset, or in the autumn morning sky about 90 minutes before sunrise. At these times, the ecliptic is steeply inclined relative to the western or eastern horizon respectively. From lower latitudes - between the tropics and the equator - viewing conditions for the zodiacal light are favourable throughout the year. Transparent skies and the absence of moonlight (even a crescent moon can swamp it) are essential for successful observation of the zodiacal light.

Broadest at its base, the zodiacal light extends some 60-90° along the ecliptic from the Sun, and it is produced by the scattering of sunlight from myriad small (1-300 jm diameter) particles lying in the plane of the planets' orbits. This material, originating from emissions by comets close to perihelion and collisions in the asteroid belt, forms a vast zodiacal dust complex, which permeates the inner Solar System out to the orbit of Jupiter, 5 AU from the Sun. Spacecraft measurements show that the dust is very much less abundant beyond Jupiter.

The zodiacal light is joined around the ecliptic to the gegenschein by narrow faint extensions known as the zodiacal bands. Variations in the intensity of the zodiacal light are thought to occur, with maximum brightness being found at sunspot minimum when interplanetary space is pervaded by fast-flowing particle streams from coronal holes.

The zodiacal dust complex contains a mass of material estimated to be equivalent to that of a typical comet nucleus; without continual replenishment from active comets, the complex and its associated zodiacal light would probably disappear within about 10,000 years.

Zollner, (Johann Karl) Friedrich (1834-82) German inventor of astronomical instruments and pioneer astrophysicist. In the late 1850s and early 1860s, he perfected the astronomical photometer, with which the relative brightnesses of stars are measured accurately by comparing them with an artificial star produced by a petroleum lamp. The Potsdam Observatory used this instrument to compile the first photometric star catalogue, the Photometrische Durchmusterung des nordlichen Himmels. He also invented the 'reversion spectroscope' - based on the same principles as the heliometer - which Hermann Carl vogel used to calculate the Sun's rotation period. In theoretical astrophysics, Zollner introduced the idea that a star's temperature determines its spectral characteristics, and that these attributes are both related to the star's evolutionary stage.

Zollner photometer Visual photometer that uses a fixed and a rotating polarizing element to vary the apparent brightness of an artificial star until it is the same as a real star seen in the same field of view. The amount of rotation of the polarizer can be calibrated to give the apparent magnitude of the real star.

zodiacal light Produced by scattering of sunlight from interplanetary dust, the zodiacal light appears as a conical glow extending along the ecliptic in the late-evening or pre-dawn sky. In this photograph, the planet Venus is visible at lower right.

Zond Eight unmanned Soviet spacecraft launched in 1964-70. Zond 1, 2 and 4 returned no data. Zond 3 photographed the farside of the Moon in 1965. Zond 5, 6, 7 and 8 went around the Moon and returned to Earth as part of preparations for a crewed circumlunar mission.

zone of avoidance Region of the sky near the plane of the Milky Way, where dust absorption and the high concentration of stars make it difficult to locate other galaxies optically. It typically spans 10° on either side of the galactic plane. Some kinds of galaxies in the zone of avoidance can now be detected by their radio, infrared or X-ray emissions, which are less vulnerable to dust and gas absorption. Surveys of such galaxies are important for tracing large-scale structure, since some important nearby superclusters and the great attractor either cross the zone of avoidance or lie mostly within it.

Zurich number See relative sunspot number

Zvezda Russian Service Module for the international space station (ISS).

Zwicky, Fritz (1898-1974) Swiss physicist and astrophysicist, born in Bulgaria, known chiefly for his observational and theoretical work on supernovae and his cataloguing of clusters of galaxies. He left Switzerland for the United States in 1925,joining the california institute of technology in 1927, where he was appointed professor of astrophysics in 1942. Although Zwicky lived in America for nearly fifty years, he retained his Swiss citizenship.

In 1934 he and Walter baade coined the term 'supernova'. They had noted that ordinary novae in the Andromeda Galaxy (M31) reached a maximum average apparent magnitude of 17. However, the 'nova' observed in 1885 in that galaxy, designated S Andromedae, had reached 7th magnitude, and now that the great distance to M31 was appreciated, it was clear that this was a phenomenon of a different order from ordinary novae. Zwicky and Baade suggested that a supernova is a cataclysmic stellar explosion that leaves behind a neutron star, and that the crab nebula was a supernova remnant (the latter confirmed in 1968 with the discovery of the crab pulsar). From 1936, using the newly developed Schmidt camera, Zwicky discovered and examined many supernovae in other galaxies.

In 1933 Zwicky inferred the presence of dark matter by observing that outlying members of the coma cluster were moving more rapidly than could be explained by the calculated mass of the cluster, and four years later he suggested that dark matter could be investigated via gravitational lensing by intervening galaxies. His extensive studies of galaxies culminated in the six-volume Catalogue of Galaxies and Clusters of Galaxies (1961-68), listing 30,000 galaxies and 10,000 clusters, compiled with colleagues and completed just before his death.

ZZ Ceti star (ZZ) White dwarf variable star, with low amplitude (0.001-0.2 mag.). The variations arise from non-radial pulsations, and generally multiple periods are present simultaneously in each star. Periods range from about 30 seconds to 25 minutes. There are three recognized subtypes based upon the presence of hydrogen, helium, or helium and carbon absorption lines in the spectra. Approximately 50 examples are currently known.

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