¨ Opik (Opik), Ernst Julius (1893–1985) E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S ă Opik (Opik), Ernst Julius (18931985) ă Born in Estonia, Opik studied at Moscow University, and helped establish Turkestan University in Tashkent, becoming the Astronomer (director) at Tartu Observatory in Estonia He fled the Red Army by horse cart during the Second World War and went to Armagh Observatory (Northern Ireland) in 1948 His wide-ranging interests are reflected in his discoveries and theories These include the discovery of degenerate stars, e.g white dwarfs, in his calculation of the density of o2 Eridani (1915) He calculated the distance of M31 as 450 000 parsecs from the Sun (1922) He computed by hand evolutionary models of main-sequence stars into giants (1938) over a decade earlier than the computer computations of HOYLE and SCHWARZSCHILD He predicted the density of craters on the surface of Mars, which was confirmed 15 years later by planetary probes He put forward an unproven theory of the Ice Ages based on a calculation of changes in the convection in the internal structure of the Sun rather than MILANKOVITCH cycles Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK ă ngstrom, Anders Jonas (181474) E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S ă ngstrom, Anders Jonas (181474) Physicist, born in Lăodgo, ă Sweden He was keeper of the observatory and professor at Uppsala where he studied heat, magnetism and optics and examined the spectra of the Sun and auroras His name is commemorated with the angstrom unit, 10−10 m, which is used for measuring wavelengths of light and x-rays, and the separation of atoms in molecules and crystals Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 61 Cygni E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S 61 Cygni The star 61 Cygni is important because of its large proper motion, first measured at the Palermo Observatory by Giuseppe Piazzi (1746–1826) The star became popularly known as ‘Piazzi’s flying star’ Its measured annual proper motion of 5.23 is still the seventh largest known, and is the largest for a naked-eye star (apparent magnitude 4.8) This star was also the first to have its annual parallax reliably determined, by Friedrich Bessel (1784–1846) Bessel’s measurement of 0.314 , made at Konigsberg ă with Fraunhofers 6.25 inch heliometer in 1838, compares quite well with the modern value of 0.286 Determinations of stellar parallax made in the same year by other observers were far less accurate It is the eleventh closest star, at a distance of 11.4 light-years 61 Cygni is also a well-known binary system, with a period of 653.3 years Its components are currently separated by 30.3 at position angle 150◦ The primary star 61 Cyg A is an orange dwarf, spectral type K5V, of apparent magnitude 5.20 and absolute magnitude 7.5 Its companion 61 Cyg B is also an orange dwarf, of spectral type K7V, with an apparent magnitude of 6.05 and absolute magnitude 8.3 The system is reputed to be the most extensively observed double star, some thousands of visual observations being supplemented by more than 34 000 photographic plates Precise astrometric measurements of these plates have indicated that the system has at least one invisible component, believed to be a planet of similar mass to Jupiter, and perhaps as many as three, with orbital periods of between and 12 years Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Abastumani Astrophysical Observatory E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Abastumani Astrophysical Observatory The Abastumani Astrophysical Observatory (AbAO)— longitude, 42.83; latitude, 41.81 degrees—was founded in 1932 It is located in Abastumani, in the south-west part of the Republic of Georgia, 250 km west of the capital city Tbilisi, on the top of Mount Kanobili at 1700 m AbAO’s primary mission is to enable astronomers of the former Soviet Union to carry out high-quality observations The average number of clear nights is 130 per year with 25% of seeings smaller than one arcsec At present, about 100 staff members work at the Observatory in six departments and four laboratories The Observatory’s main facilities are 125 cm Ritchey–Chr´etien and 70 cm meniscus telescopes The major research areas are accretion disks and pulsars astrophysics, solar system cosmogony, objective prism spectroscopy, low-amplitude short-period variables, AGNs variability, solar physics, solar–terrestrial phenomena and atmospheric physics For further information see http://gamma.bu.edu/webt/abastumani Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Abbe, Ernst (1840–1905) E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Abbe, Ernst (1840–1905) Born in Eisenach, Grand Duchy of Saxe-Weimar-Eisenach (now Germany), Abbe became director of the observatory at Jena and research director of the CARL ZEISS optical works in Jena He discovered the Abbe sine condition, which describes a lens that will form an image, without defects of coma and spherical aberration His mathematical treatment founded the present-day science of optics Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Abbott, Francis (1799–1883) E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Abbott, Francis (1799–1883) Watchmaker, born in Derby, England, convict, transported to Tasmania in 1845 He made astronomical observations at the Rossbank Observatory after the end of his sentence until its closure in 1854 and in his private laboratory in Hobart Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Abell Clusters Abell Clusters Abell clusters are the most conspicuous groupings of galaxies identified by George Abell on the plates of the first photographic survey made with the SCHMIDT TELESCOPE at Mount Palomar in the 1950s Sometimes, the term Abell clusters is used as a synonym of nearby, optically selected galaxy clusters George Abell constructed a catalogue containing 2712 of the richest such groupings in the northern sky, which was later extended to the southern sky It is no exaggeration to say that the total sample of 4076 cluster candidates over the whole sky has revolutionized the study of the large-scale structure in the universe The Abell catalogue has formed the basis for the first quantitative studies of the densest component of the largescale structure in the local universe In recent years, the definition of samples of candidate clusters from wide-field survey plates has been repeated with automatic platescanning machines This showed objectively that the subjective factor in Abell’s visual selection is quite small, at least for the richer and more nearby clusters The reality of the cluster candidates in Abell’s catalogue has been the subject of some debate, until spectroscopic observations of large numbers of galaxies in the directions of the Abell clusters showed convincingly that only a small fraction of the rich clusters are the result of chance superpositions That is, a very large fraction of the rich cluster candidates in the catalogue made by Abell (or, including the southern clusters, by Abell, Corwin and Olowin) represent compact, localized peaks in the spatial distribution of galaxies, mostly with redshifts less than 0.2, and held together by gravity Already in the 1930s, Fritz ZWICKY had concluded that the luminous matter (i.e the galaxies) in clusters represents only about 10% of the total cluster mass, most of which can therefore be detected only through its gravitation This has led to estimates of the total mass (both visible and dark matter) by various means The most common of those are the velocities of the galaxies in the cluster, the amount and temperature of the hot (x-ray-emitting) gas and the distortion of the images of galaxies at distances well beyond that of the cluster by GRAVITATIONAL LENSING For a long time, several of the better-known Abell clusters, like those in the COMA BERENICES and PERSEUS constellations, have shaped our vision of the class of rich, populous, clusters In this schematic view, rich clusters are smooth, round and virialized structures This idealized picture coexisted with the knowledge that there are significant variations in the various properties of the Abell clusters This has led to many studies of those properties, and of correlations between them, as well as to several attempts to describe the formation and evolution of rich clusters It is now realized that clusters are still forming and evolving at the present epoch Among the cluster properties that can be studied, and for which theoretical predictions have been made are the 3D shape (or rather, the axial ratios of the galaxy E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S distribution), the composition of the galaxy population (i.e the fractions of galaxies of different morphological types), the distributions of the luminosities of the galaxies, the detailed dynamics of the various galaxy classes, dynamical substructure and segregation and the fraction of the total mass consisting of baryonic (i.e ‘ordinary’ nucleonic) matter An important recent development is the search for, and study of, galaxy clusters at very large distances (i.e at high redshifts), which are the ‘forebears’ of the local rich clusters in the Abell catalogue For those younger clusters at high redshifts, the Abell clusters serve as a local, present-day, reference population Abell clusters as a subset of the total cluster population When searching for cluster candidates on the Palomar Sky Survey plates, Abell had no information about distances (or redshifts) of the galaxies Therefore he used the distribution of the galaxies in apparent magnitude to select those peaks in the projected galaxy distribution that are most likely to correspond to a spatially compact structure Taking the magnitude of the 10th brightest galaxy as an approximate ‘standard candle’, a redshift was estimated; this yields the angle subtended by a fixed linear size of 1.5h−1 Mpc at the distance of the cluster (where h is the value of the present Hubble parameter, expressed in units of 100 km s−1 Mpc−1 ) In a circular aperture with radius equal to that angle, the number of galaxies with a magnitude not more than two magnitudes fainter than the third-brightest galaxy was counted Finally, the number of unrelated galaxies in the aperture (and down to the same magnitude limit) was estimated from the galaxy density in background fields without obvious cluster candidates The corrected number of galaxies (the richness count, i.e the estimated number of members in the aperture above the magnitude limit) was found to have an uncertainty of about 17 Therefore, only clusters with a corrected galaxy count of at least 50 were considered by Abell to have been sampled in an unbiased fashion out to redshifts of 0.1–0.2 In Abell’s original (‘northern’) catalogue, 1682 of the 2712 cluster candidates have a count of at least 50 The lower limit in richness count must be applied if one uses the Abell catalogue for statistical purposes Clearly, many less rich clusters exist but at larger distances–redshifts their contrast with respect to the field is too low to allow a robust definition of a statistically reliable sample In recent years, an extensive redshift survey (the ESO Nearby Abell Cluster Survey) has been made of close to 6000 galaxies in about 100 cluster candidates (mostly from the southern part of the Abell, Corwin and Olowin catalogue) with a richness count of at least 50 and estimated redshifts less than 0.1 (see GALAXY REDSHIFT SURVEYS) The contamination in these redshift surveys by galaxies that not belong to the main cluster is far from negligible, i.e about 25% However, the majority of the redshift surveys contains a spatially compact cluster to which at least 50% of the galaxies with measured redshifts Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Abell Clusters belong Only about 10% of the candidate clusters appear to be a superposition of two almost equally rich (but relatively poor) systems at different redshifts along the same line of sight For the spatially compact systems, the velocity dispersion shows a global correlation with richness count (clusters with higher richness counts on average have larger velocity dispersions), but the correlation is very broad (at least a factor of in both quantities) The uncertainty in the visually estimated richness counts might be thought to be responsible for this, but the width of the relation does not decrease if one uses richness counts based on machine scanning instead of the original ones For a sample of about 150 Abell clusters with redshifts less than 0.15, cluster masses were calculated from the relative velocities and positions of the galaxies, assuming that the virial theorem holds in the central regions of the clusters The cluster masses correlate fairly well with the velocity dispersions, but the mass distributions in the various intervals of richness counts appear to have considerable overlap Therefore, application of a limit in richness count to a sample of Abell clusters (which is necessary for practical reasons) induces quite a diffuse limit in mass The clusters, or rather cluster candidates, in Abell’s catalogue with richness counts of at least 50 are therefore a subset of all clusters in the mass range from about × 1013 to × 1015 M However, for clusters with a velocity dispersion of at least 800 km s−1 , essentially all richness counts are larger than 50 In other words, all clusters with a velocity dispersion of at least 800 km s−1 are contained in the sample with a limiting count of 50, and the estimate of their space density is unbiased Clusters with apparent velocity dispersions greater than about 1200 km s−1 turn out either to be superpositions or to have lots of dynamical substructure With the advent of all-sky x-ray surveys like those from the EINSTEIN (HEAO-2) and ROSAT missions, it has become possible to construct complete samples of clusters for which the x-ray flux from the hot gas in the potential well of the cluster is larger than a threshold value This produces cluster catalogues that are fundamentally different from, and thus complementary to, the Abell catalogue, although there is quite some overlap The mass of the x-ray gas is generally at least as large as the mass of the cluster galaxies, but the combined mass of these two baryonic components is typically only 10–15% of the total mass When the total mass of a cluster can be estimated both from the kinematics of the galaxies and from the x-ray temperature and brightness, the two estimates in general agree reasonably well Properties of the galaxy population in Abell clusters In the past, several schemes have been proposed for the classification of Abell clusters All of them summarize in one way or another the distribution of the cluster galaxies in position, magnitude or morphological type, E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S or any combination of those The projected distribution of the galaxies has many forms and ranges between the following extremes There may be a central concentration of bright galaxies, generally of early type, i.e ellipticals, and frequently one of them is a cD galaxy, i.e a giant elliptical surrounded by an extended envelope (see ELLIPTICAL GALAXIES) At the other extreme there are clusters that not have a clear central concentration In some clusters, the galaxy distribution is quite smooth, and in general those clusters contain relatively few spiral galaxies When the fraction of spiral galaxies is large, the galaxy distribution is in general less regular The relative fractions of early- and late-type galaxies are correlated with richness count, and this is a manifestation of the morphology–density relation The latter shows a clear correlation between the relative fractions of ellipticals, lenticulars (S0s) and spiral galaxies, and the (local) projected galaxy density (and therefore radial distance) The S0s may contribute up to 50% in the center, with ellipticals not far behind and spiral galaxies about 10% In the outer parts, ellipticals are almost absent while spiral galaxies may represent up to 60% Note that these are global values: individual clusters show a considerable spread around these Even though in a sizeable fraction of the Abell clusters the galaxy distribution is not very regular or circularly symmetric, one can always derive the azimuthally averaged projected number density profile (R), in which R is the projected distance from the cluster center, i.e the shortest distance between the line of sight through a galaxy and the cluster center Several expressions have been proposed for the mathematical description of (R), all of which have three parameters Those are the central value of (R), i.e (R = 0), a characteristic length Rc (the distance at which (R) has decreased by a given factor, say 2) and a measure of the decrease of (R) in the outer parts (generally the logarithmic slope α of (R)) Recently, (R) has been derived for galaxies of different morphological types in about 70 rich Abell clusters In individual clusters, the number of galaxies of a particular type is generally not sufficient to allow an accurate estimate of the three parameters of (R) By properly combining data for many clusters one can compare the representations of (R) for ellipticals, S0s, spirals and galaxies with emission lines (mostly very ‘late’ spirals, such as Sc and Sd, with ionized gas in their interstellar medium) In other words, by sacrificing the detailed properties of individual clusters, one obtains a picture of an average rich Abell cluster There appears to be a clear correlation between galaxy type and (R): the characteristic length Rc increases markedly from early to late galaxy type (from about 0.1 to 0.5 Mpc) This shows that ellipticals are indeed much more centrally concentrated than spirals, while the emissionline galaxies form the most extended population These differences must be accompanied by differences in the kinematics of galaxies of the various types, because all Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Abell Clusters galaxy classes move in the same cluster potential, which is mostly determined by the dark matter Such kinematical differences are indeed observed: the ellipticals and S0s show the smallest dispersion of the line-of-sight component of their velocities, and this dispersion varies little with projected distance from the center Spirals, and in particular emission-line galaxies, have a larger velocity dispersion (by as much as 20– 30%) which decreases markedly towards larger projected distances Actually, the kinematics of the emission-line galaxies indicates that they have not yet traversed the dense central cores, which is probably the reason why they have not yet lost their line-emitting gas in encounters with other galaxies Combining the projected galaxy distributions with the kinematics one may estimate the distribution of the total (visible plus dark) mass via the Jeans equation of stellar dynamics By comparing the distribution of the dark matter with that of the luminosity of the galaxies, one can in principle study the variation of the so-called mass-to-light ratio with distance from the cluster center This may give clues about details of the formation process, such as the effects of galaxy encounters, the role of the dark matter haloes of the galaxies, etc Abell clusters as cosmological probes Several observational properties of Abell clusters have been used to constrain the theories of formation of largescale structure in the universe and the parameters in those theories (see also UNIVERSE: SIMULATIONS OF STRUCTURE AND GALAXY FORMATION) These properties include the spatial distribution of Abell clusters, their shapes and their masses In different ways, these all carry information on the way in which the largest well-developed structures in the universe have formed through the growth of the initial fluctuations in the matter density The spatial distribution of Abell clusters has been analyzed through the two-point correlation function ξ(r), i.e the fraction of cluster pairs with a certain separation, in excess of the expected number of pairs for a random distribution, which has been derived for clusters of various richness counts In general, the correlation function is found to have a power-law form: ξ(r) = (r/r0 )−γ ; the exponent γ (about 2) does not appear to depend on the limiting richness count, but the value of the correlation length r0 does, and is larger for the richer clusters (with a characteristic value of about 20 Mpc) In principle, these data allow one to derive the value of the cosmological density as well as the amplitude of the fluctuation spectrum Another aspect of the distribution of rich Abell clusters is that they are generally located in the vertices where the sheets and filaments in the general galaxy distribution come together Therefore, the distribution of rich clusters has sometimes been compared with the distribution of the vertices in so-called Voronoi tesselations, which are geometric partitionings of space E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S The shapes of Abell clusters have been derived from the projected distributions of galaxies Using the galaxy positions irrespective of galaxy type, one can calculate the apparent ellipticity of a cluster In general, the richer clusters are less elongated than the less rich ones The apparent ellipticities for a cluster sample of about 100 northern Abell clusters suggest that the elongated clusters are prolate (cigar like) rather than oblate Comparison of these data with the results of numerical N-body calculations can constrain the theories of structure formation The full distribution of the masses of a volume-limited sample of Abell clusters (i.e its shape and normalization) can also give information for cosmological structure formation theory As the sample of Abell clusters with a limiting richness count of 50 has a rather badly defined completeness at smaller masses, one must restrict the comparison between observations and predictions to the most massive clusters for which the Abell catalog is complete It is far from trivial to derive independent information for the several parameters in the formation theories that influence the properties of the most massive structures Yet, there seems to be general agreement that the latter are more naturally understood in a universe in which the matter density is considerably smaller than the critical density Bibliography Book: Giurin G and Mezzetti M (ed) 1999 Observational Cosmology: The Development of Galaxy Systems (Astron Society of the Pacific Conf Ser 176) Journal articles: Abell G O 1958 The distribution of rich clusters of galaxies Astrophys J Suppl 211–88 Abell G O, Corwin H G and Olowin R P 1989 A catalog of rich clusters of galaxies Astrophys J Suppl 70 1–138 Reviews: Bahcall N A 1977 Clusters of galaxies Ann Rev Astron Astrophys 15 505 Bahcall N A 1988 Large-scale structure in the Universe indicated by galaxy clusters Ann Rev Astron Astrophys 26 631 Sarazin C L 1986 X-ray emission from clusters of galaxies Rev Mod Phys 58 Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Peter Katgert Aberration E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Aberration (1) The apparent displacement of a star from its mean position on the celestial sphere due to the velocity of the Earth in its orbit around the Sun The phenomenon was discovered in 1729 by James Bradley (1693–1762) who was, in fact, trying to measure stellar parallax The displacement is caused by the combination of the velocity of the Earth and the velocity of light approaching from the source If the Earth were stationary, light from a star would arrive from the true direction of this source, but the motion of the Earth causes the light to appear to be approaching from a point which is slightly displaced in the direction of the Earth’s motion In the course of a year, as the Earth travels round the Sun, a star will trace out a small ellipse in the sky about its mean position The maximum radius of this ellipse (in radians) is equal to the ratio of the speed of the Earth to the speed of light (30 km s−1 :300 000 km s−1 ), that is about 20.5 seconds of arc The eccentricity of the ellipse depends on the celestial latitude of the star (the figure becomes a circle at the pole of the ecliptic and a straight line on the ecliptic) The displacement due to aberration is much greater than that due to parallax (the annual parallax of the nearest star is 0.76 seconds of arc) and this must be allowed for before the parallax can be determined for a star A similar, though smaller, aberration effect occurs due to the speed of rotation of the Earth on its axis This is known as diurnal aberration (2) In optical systems, such as lenses and curved mirrors, aberration refers to the inability of the system to produce a perfect image Unlike a plane mirror, which does not create aberrations, a lens or curved mirror is an imperfect image producer, becoming ideal only for rays passing through (or reflecting from) its center parallel to the optical axis (a line through the center, perpendicular to the lens surfaces) The main aberrations are chromatic, spherical, coma and astigmatism See also: astigmatism, atmospheric refraction, chromatic aberration, coma, scintillation, spherical aberration Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Airglow Airglow The airglow consists of the non-thermal radiation emitted by EARTH’S ATMOSPHERE The thermal emission of Earth, heated by sunlight and subsequently radiating to space, takes place in the infrared portion of the spectrum, at wavelengths longer than µm In its simplest form, the airglow arises from de-excitation of molecules, atoms or ions excited by solar UV photons or energetic particles, and the resulting emitted photons are generally, although not exclusively, located in the visible and UV ranges of the spectrum The existence of a night sky glow, long after sunset, shows that indirect mechanisms play an important role in producing airglow In this case, the initial energy, brought by solar photons, is stored in the high atmosphere under the form of radicals or molecules able to recombine through exothermic reactions during the night and produce atoms or molecules in their excited states, with subsequent photonic emissions The airglow does not include the radiation produced by lightning or meteor trains Although the distinction is more difficult to establish, auroral phenomena, occurring at high latitudes and resulting from localized, intense precipitation of particles whose origins are outside the atmosphere, are not considered as being airglow In contrast to AURORAS, and in spite of the fact that the general mechanism of emission (excitation–de-excitation) is of the same nature, the airglow occurs at any time and any latitude, in a much less sporadic way Historically, the existence of a terrestrial component of the light in the night sky was recognized in the years around 1900 Yntenna, who first established by photometric methods the existence of airglow, remarked on the large variability of this phenomenon from night to night and showed that the star light scattered by atmospheric molecules was insufficient to explain the night sky light Spectroscopic techniques showed that the green line of atomic oxygen at 558 nm was present over the whole sky at all times and confirmed the existence of a ‘permanent aurora’ Rayleigh (JOHN WILLIAM STRUTT) showed in the 1920s that the geographical pattern of the oxygen green line differed from that of the aurora, and that the N+2 bands seen in auroras were absent from the night sky Rayleigh first expressed the brightness of the green line in terms of the number of atomic transitions per second in a column of unit cross-sectional area along the line of sight He gave his name to the photometric unit universally used today for airglow studies: one rayleigh (or R) represents an omnidirectional emission rate of 106 photons per square centimeter per second The oxygen red lines at 630–636 nm were predicted and observed around 1930, as well as the D line of sodium Although ground-based spectrophotometric instruments provide a convenient way to monitor the spatial and temporal variations of the airglow, they are not appropriate for the retrieval of the emission vertical profiles One possibility is to use rocket-borne zenith photometers flown through the airglow layers Differentiation of the signal E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S with respect to altitude allows the retrieval of the local volume emission rate as a function of altitude Another way to proceed is to perform airglow measurements from an orbital platform in limb viewing geometry This technique has several advantages Because the signal is integrated along a tangential path, the intensity of the measured signal is greatly enhanced It makes weak features, like the airglow continuum emission, easier to detect and characterize Second, the tangential view allows the separation of the background galactic signal from the atmospheric glow because the galactic signal is directly provided by measurements performed at very high altitudes, where the atmospheric density is virtually zero The vertical profiles of several identified night airglow lines, as observed from the space shuttle STS-37 mission orbiting at 267 km altitude, are plotted in figure The well known green line of oxygen at 557.7 nm, produced by the O(1 S)→O(1 D) transition, covers a relatively narrow altitude region and strongly peaks around 100 km altitude Some emission is also recorded in the red line of oxygen at 630 nm, due to the O(1 D)→O(3 P) transition The signal at 762 nm results from radiative deexcitation of an excited state of O2 The case of atomic and molecular oxygen lines will be treated in a more detailed way in the following section The D line of Na at 589 nm, due to Na(2 P)→Na(2 S), peaks around 90 km, similarly to the OH (6–0) Meinel band at 527 nm The OH emission is produced by the reaction cycle H + O3 → OH∗ + O2∗ where OH∗ is a rotationally excited level of OH, with a vibrational quantum number ν less than (no bands originating from ν > are detected) and a recycling mechanism of the type OH + O → H + O2 The 527 nm band is produced by the ν = → deexcitation of OH* Note that the strong peak at low altitude in the 630 nm emission is due to the (9–3) band of OH Because the chemistry of sodium also involves ozone (O3 ) through the cycle Na + O3 → NaO + O2 NaO + O → Na + O2 a coupling between the airglow OH intensities and the Na density (derived either from lidar measurements or from airglow measurements) may be expected, and is definitely exhibited by measurements There is also a bright emission that extends from 400 to 600 nm It is due to O2 in the near ultraviolet and blue ranges, up to 480 nm, and NO2 at longer wavelengths The O2 band emission mainly consists of Herzberg I, Herzberg II and Chamberlain bands, as detailed in the next section The NO2 diffuse feature may be attributed to the radiative recombination of NO and O, and is a true continuum Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Airglow E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Figure Altitude profiles of Earth’s airglow in six wavelength ranges as seen from the space shuttle on the STS-37 mission (From Mende et al 1993 J Geophys Res 98 19 117.) A profile taken at 527 nm is shown in figure The marked airglow peak between 95 and 105 km altitude is in good agreement with microwave measurements of thermospheric NO made during the same solar cycle, suggesting a maximum concentration region between 100 and 110 km The study of upper atmospheric airglow (dayglow and nightglow) emissions, either from ground-based spectrometry or by using spectrometers as well as imagers on board satellites like the Dynamics Explorer and Space Shuttle, has seen very substantial progress in the last 20 years These measurements have allowed the characterization of thermospheric and ionospheric processes, in fields of both aeronomy and dynamics, with the help of laboratory and theoretical advances The interpretation of airglow measurements is extremely complicated, involving the detailed characteristics of the excitation fluxes (solar UV/EUV, particles), interaction mechanisms (direct excitation of molecules, atoms or ions, indirect excitation through dissociation and ionization processes, as well as chemical reactions), chemical cycles, radiative transfer of airglow light Oxygen emission lines in the nightglow are now detailed Excitation of oxygen emissions in the nightglow of Earth The red doublet of oxygen at 630 nm and 636.4 nm, resulting from the O(1 D)→O(3 P) transition, that is from the first excited level to the ground state, with transition probabilities of 0.0069 and 0.0022 s−1 respectively, is observed during several hours after sunset Since the average emission time of O(1 D) is equal to the inverse of the transition probability, that is a few minutes, direct excitation of oxygen atoms by solar UV photons followed by de-excitation cannot explain the persistence of the red glow for a long time after sunset The source of O(1 D) during night time is dissociative recombination of O+2 ions O2+ + e− → O + O(1 D) The O+2 ions result from the night time decay of ionization which is controlled by charge exchange reactions that converts O+ ions into O+2 ions The vertical distribution of the red line emission is complex and may be shown to evolve with time in a different way at different altitudes Below 200 km, the red line emission is weak because O∗ is deactivated (or ‘quenched’) by collisions with air molecules in a time smaller than the emission time Above 200 km, the combined effects of the collisional quenching rate, which decreases with altitude, and the vertical variation of the electronic density, which increases with altitude, results in a strong increase, for increasing altitudes, of the red line glow An important application of the measurement of thermospheric nightglow emissions, like the oxygen red line, is the inference of winds and temperatures from high-resolution spectroscopy By using Fabry–Perot Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Airglow E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S and Michelson interferometers, the thermospheric neutral wind at the 250 km altitude level has been retrieved from the O(1 D) 630 nm emission line by measuring the associated Doppler shift, which yields the line-of-sight wind vector Neutral temperatures may be inferred from the width of the line Doppler spectroscopy of airglow lines is a powerful means to study the dynamics of the thermosphere, strongly controlled by thermal tides and general wave activity The green line of oxygen at 558 nm, which results from the O(1 S)→O(1 D) transition between the second and the first excited states, is known to arise from the 100 km neighborhood, that is definitely lower atmospheric levels than the red lines Although a part of the green line may arise from dissociative recombination of O+2 in the F region O2+ + e− → O∗ + O∗ another mechanism must be invoked to explain the low emission altitude of this line Sydney Chapman proposed a mechanism of ‘photochemical excitation’ O + O + O → O2 + O(1 S) where one of the O atoms is excited by some of the energy liberated in the association of two O atoms This process results in a maximum O concentration around 100 km, which is consistent with the location of the green line emission De-excitation of O(1 S) into O(1 D) occurs through quenching by ambient air molecules (mainly molecular nitrogen) The so-called Chapman process, although it may participate in the green line emission, is now thought to be of secondary importance At the beginning of the 1960s, Barth and Hildebrandt found in the laboratory that the emission produced by the previous process is much too small to account for the observed glow This result stimulated Barth to put forward, in 1964, the following two-step mechanism: O + O + M → O2∗ + M where M is an air molecule and O∗2 an excited state of the O2 molecule, followed by the excitation transfer process O2∗ + O(3 P) → O2 + O(1 S) The net effect of these two reactions, obtained by adding them, is similar to the effect of the single Chapman reaction The other loss processes are deactivation through quenching by collision with an air molecule O2∗ + M → O2 + M or radiation of a photon through radiative de-excitation O2∗ → O2 + hν Therefore, the Barth process may give rise simultaneously to both O(1 S) (green line) and O2 (762 nm line) emissions Comparison of the two corresponding profiles in figure may be shown to be consistent with the two-step scheme, despite the 10 km difference between the peak altitudes Figure Nadir viewed UV dayglow spectrum between 350 and 950 Å acquired by the STP78-1 satellite from 600 km between 50◦ N and 50◦ S latitudes (From Chakrabarti et al 1983 J Geophys Res 88 4898.) Dayglow and twilight emissions Airglow emissions are also observed during the day, and they mainly reflect the diurnal variation of the concentration of emitting species Twilight spectra may be observed at the transition between the dark and fully illuminated atmosphere, as the shadow height moves vertically over the full range of emissive layers In the day time, several lines and bands are very bright, mainly due to higher production rates Indeed, dayglow and twilight emissions mainly result from direct excitation by solar ultraviolet photons, photoelectrons produced through ionization processes in the ionosphere, and by resonance scattering or fluorescence of solar radiation Such energetic phenomena result in a complex ultraviolet dayglow spectrum, as shown in figure These measurements were made from an Earth-orbiting satellite (STP78-1), since ultraviolet photons are prevented from penetrating down to the ground due to the screening effect of the atmosphere Even at near-UV or visible wavelengths, dayglow are difficult to acquire with ground-based instruments because of the high level of the background scattered solar light The short-wavelength ultraviolet spectrum is dominated by singly ionized and neutral lines of oxygen (O I, O II) and nitrogen (N I, N II) These lines are produced by ultraviolet photons and photoelectron impact Shortwavelength lines are produced by de-excitation from highenergy upper states, which cannot be excited by chemical or recombination reactions Several atomic and ionic lines in the dayglow spectrum are also lines that appear in the solar spectrum, like the He I line at 584 Å In the case of helium, this phenomenon is due to resonant scattering of Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Airglow E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S the He solar emission line Similar features are observed at 834 Å (O II triplet), 1216 Å and 1025 Å (H I, Lyα and Lyβ), 1302–1306 Å (O I triplet) and others wavelengths They are due to resonant scattering or fluorescence Molecular oxygen emissions in the airglows of Earth, Venus and Mars Whereas the case of the O(1 S) state is well understood, the detailed process of O2 emissions is still not well known In addition to the discrete 762 nm O2 band, there is a near-UV–blue extended emission attributed to Herzberg and Chamberlain bands In previous reactions, the excited bound state O∗2 of O2 can be A u+ , A u or c u− (figure 3) The main precursor of O(1 S) could be O2 (c u− ), but it is still not certain what the nature of the precursor is, and that is still an active field of investigation today In Earth’s atmosphere, the Herzberg I system is the major cause of the blue nightglow continuum, with an average emission of 400 R From spaceborne observations of the Venus airglow by the Soviet Venera probes, similar oxygen airglows were found, although in quite different amounts The Venus molecular oxygen nightglow shows a strong pre-eminence of the Herzberg II system (2700 R) Such large differences are due to the different chemical compositions of the atmospheric environment for Earth and Venus (see VENUS: ATMOSPHERE) In the luminous layers, the ambient air (N2 +O2 for Earth, CO2 for Venus) density is nearly 100 times larger on Venus than on Earth, the reverse situation occurring for O (three times less on Venus than on Earth) Because the collisional quenching of O2 (A u+ ) is mainly realized by air molecules, whereas O2 (c u− ) is preferentially quenched by collisions with O, Herzberg I bands are weak on Venus and Herzberg II bands are strongly enhanced with respect to the case of Earth (2700 R on Venus versus 100 R on Earth) Another interesting feature is the extreme weakness of the oxygen green line in Venus nightglow (5 R versus 120 R on Earth), which was an important element in favoring the Barth mechanism over to the Chapman process As seen in figure 3, a glow of the infrared atmospheric system of a g → X g− is expected in the infrared, at 1.27 µm This glow, first detected on Venus by the French spectroscopist Pierre Connes and his collaborators at the end of the 1970s is quite intense (>1 MR, that is 106 R) Such a strong emission is also measured on Earth Due to the long radiative lifetime of O2 (a g ) (1 h), the analysis is complicated by transport across the terminator Photolysis of O3 under sunlit conditions according to the following reaction O3 + hν → O2 (a g ) + O(1 D) constitutes part of the source in the dayglows of Earth and Mars, but it is negligible on Venus The major source in the nightglows of all the terrestrial planets, the dayglow of Venus and a part of the dayglows of Earth and Mars, is the recombination of O atoms produced in the photolysis of O2 on Earth and CO2 on Venus and Mars (see MARS: ATMOSPHERE) Figure The six lowest levels of molecular oxygen and the corresponding band systems (from Krasnopolsky 1983) Connes measured individual rotational lines of this band in the Martian spectrum with the m telescope on Mount Palomar in 1973 and 1975 Rotational temperatures and airglow intensities were derived from the spectra In each spectrum, equivalent widths of about 30 lines were measured to be typically 0.005–0.01 nm By ratioing the equivalent width to the line strength, and by examining the dependence of this ratio on the energy of the upper rotational state of the transition, a rotational temperature of 200 K was deduced, which corresponds to the real temperature averaged over the airglow altitude profile Typical airglow intensities of to 30 MR, depending on latitude, were found Assuming the quenching rate coefficient by air molecules (CO2 on Mars) is known, it is possible to infer the ozone amount from 1.27 µm airglow measurements, although in a rather inaccurate way Bibliography Barth C A, Stewart A I F, Bougher S W, Hunten D M, Bauer S J and Nagy A F 1992 Aeronomy of the current Martian atmosphere Mars ed H H Kieffer, B M Jakosky, C W Snyder and M S Matthews (Tucson, AZ: The University of Arizona Press) pp 1054–89 Chamberlain J W 1995 Physics or the Aurora and Airglow (Classics in Geophysics volume 1) (Washington, DC: American Geophysical Union) Fox J L 1997 Airglow Encyclopaedia of Planetary Sciences ed J H Shirley and R W Fairbridge (London: Chapman and Hall) Krasnopolsky V A 1983 Venus spectroscopy in the 3000– 8000 Å region by Veneras and 10 Venus ed D M Hunten, L Colin, T M Donahue and V I Moroz (Tucson, AZ: The University of Arizona Press) pp 459–83 Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Airglow E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Rees M H 1989 Physics and Chemistry of the Upper Atmosphere (Cambridge: Cambridge University Press) Solomon S C 1991 Optical aeronomy Rev Geophys Suppl 1089–109 Eric Chassefi´ere Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Airy Disk E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Airy Disk The bright spot at the center of the diffraction pattern that is formed when a point source, such as a star, is imaged at the focus of a telescope Light waves from a distant star arrive at the Earth as a series of parallel wavefronts (rather like waves rolling onto a beach) When these wavefronts are interrupted at the edge of a telescope’s aperture (the rim of the objective or primary mirror), and are brought to a focus, interference effects between different parts of each wavefront result in the formation of a diffraction pattern For a point source, the resulting image (assuming a perfect optical system) consists of a central spot of light surrounded by a series of light and dark fringes, or rings According to theory, 84% of the light energy is concentrated into the central spot (the Airy disk), the diameter of which depends on the aperture of the telescope and the wavelength of the light The radius of the first dark minimum in light intensity, that lies immediately outside the Airy disk, is given, in radians, by 1.22λ/D, where λ denotes wavelength and D the aperture of the telescope The central spot is known as the Airy disk because this type of diffraction pattern was first investigated by Sir George Biddell Airy (1801–92), the seventh Astronomer Royal See also: aperture, Rayleigh limit, resolving power Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Airy, Sir George Biddell (1801–92) E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Airy, Sir George Biddell (1801–92) A brilliant Cambridge mathematician (Senior Wrangler 1823, i.e leader of the graduating mathematics class), Airy became the seventh Astronomer Royal in 1835 after a brief period as Lucasian Professor at Cambridge His output was prodigious, and he published nearly 400 scientific papers and 150 reports on various scientific issues, such as the gauge of railways, spectacles to correct astigmatism, and methods to correct for compass readings in ships made of iron His work on optics is recognized by the use of the term Airy disc for the resolution element of a telescope due to diffraction at its aperture, which he studied As Astronomer Royal, he saw that the Royal Observatory at Greenwich was re-equipped with modern instruments and that its work was carried out punctiliously by its many human ‘calculators’ and observers, insisting that everything was carried out in the best way, namely his way This regime was effective in raising standards He himself determined the density of the Earth, the mass of the planet Jupiter and its rotation period He calculated the orbits of comets and cataloged stars He made numerous contributions to the prediction of the motion of the Moon, and analysed transits of the planet Venus and eclipses, including the eclipse predicted by THALES As a result of the accuracy of the observations made under Airy at Greenwich, and his practical exploitation of the railway telegraph to distribute it, Greenwich Mean Time was established in 1880 as the official time service throughout Britain, and afterwards became the basis for the timekeeping of the world Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Aitken, Robert Grant (1864–1951) E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Aitken, Robert Grant (1864–1951) Astronomer, born in California, Aitken became director of the Lick Observatory He made systematic visual surveys of binary stars, measuring their positions visually His massive New General Catalogue of Double Stars (1932) contained data on 17 000 stars that allowed orbit determinations of many, from which it was possible to calculate data on the masses of the stars’ components He also measured positions of comets and planetary satellites and computed orbits Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Akebono (EXOS-D) E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Akebono (EXOS-D) Japanese satellite launched in 1989 to study Earth’s auroras Named from the Japanese for the ‘rising Sun’ Auroral image capability was lost in early 1995, but other instruments continue to operate Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK al-Battani, Abu Abdullah [known as Albategnius] (c 868–c 929) E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S al-Battani, Abu Abdullah [known as Albategnius] (c 868–c 929) Born in Harran, in present-day Syria, al-Battani spent most of his life in Raqqa, situated on the bank of the Euphrates, where he was educated and flourished as a scholar and a Sabean which means ‘worshipper of the stars’ His best-known achievement was the determination of the solar year as being 365 days, hours, 46 minutes and 24 seconds, an extremely accurate value that was used in the Gregorian Calendar reform of the Julian Calendar Using trigonometrical rather than geometrical methods, he also determined the true and mean orbit of the Sun, proving the variation of the apparent angular diameter of the Sun (an indication of the variable distance between Sun and Earth) and the possibility of annular eclipses He wrote a number of books on astronomy and trigonometry, his most famous book being his astronomical treatise with tables (a zij), which was translated into Latin in the twelfth century as De Scienta Stellarum—De Numeris Stellarum et Motibus His treatise on astronomy was extremely influential in Europe till the Renaissance, with translations available for centuries in several languages Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Albedo E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Albedo A measure of the reflectivity of a material or object For bodies in the solar system, albedo is the proportion of sunlight falling on them that is reflected away It is measured on a scale from (a perfectly absorbing black surface) to (a perfectly reflecting white surface) Albedo may be defined in various ways The most common is geometrical albedo, sometimes called physical albedo This is the ratio of the amount of light reflected from an object, as viewed from the direction of the Sun, to the amount that would be reflected by a perfectly diffusely reflecting sphere (which by definition has an albedo of 1) of the same size and at the same distance as the object As well as applying to visible light, geometrical albedo may be defined for radiation in any specified wavelength range; bolometric geometrical albedo applies to all wavelengths Bond albedo, also known as spherical albedo, is defined similarly but for radiation of all wavelengths reflected in all directions; it is named after George Phillips Bond Another measure is hemispherical albedo: the ratio of scattered to incident light as a function of the angle of incidence, and again the body is assumed to be a perfectly diffusely reflecting sphere Of the major planets, Mercury has the lowest geometrical albedo, at 0.11 (comparable to the Moon’s 0.12), while Venus, by virtue of its blanket of highly reflective clouds, has the highest, 0.65 The lowest albedo in the solar system seems to be possessed by the particles that make up the rings of Neptune, which probably have a value close to the theoretical lower limit of zero Asteroids which originally formed between about 2.5 and 3.5 AU from the Sun, where it was cool enough for dark carbonaceous compounds to condense from the solar nebula, but not water ice, have albedo values as low as 0.02 The highest measured albedo is that of Saturn’s satellite Enceladus, to which some sources assign a value of 1.0 Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Albireo E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Albireo The star β Cygni, said to represent the eye of the swan in the ancient constellation figure It is seen as a single star of apparent magnitude about 2.9 with the unaided eye, but with good binoculars or a small telescope it appears as a beautiful double, comprising an orange star (β Cyg) of apparent magnitude 3.05, spectral type K3II, with a bluishwhite companion (β Cyg) of apparent magnitude 5.17 and spectral type B8V Their separation is 34.4 at position angle 54◦ The system is 380 light-years distant, having a parallax of 0.008 The absolute magnitudes of the two stars are β Cyg −2.3, β Cyg −0.2 With more powerful instruments β Cyg can be further resolved: it is a binary system with components of apparent magnitude 3.4 and 5.5, separated by 0.39 at position angle 152◦ Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK al-Biruni, Abu Raihan (973–1048) E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S al-Biruni, Abu Raihan (973–1048) Born in Kheva near Ural, present-day Uzbekhistan, alBiruni was a polymath and traveler (to India), making contributions in mathematics, geography and geology, natural history, calendars and astronomy His book Qanun-i Masoodi, which he dedicated to his patron Sultan Masood, discusses astronomy, trigonometry, solar, lunar and planetary motions, including the question whether the Earth rotates or not He undertook experiments (observations) related to astronomical phenomena, for example eclipses and the dates of the equinoxes, and determined that, compared with the speed of sound, the speed of light is immense He recognized the Milky Way as ‘a collection of countless fragments of the nature of nebulous stars’ Al-Biruni’s quest for scientific knowledge is epitomized by his statement that the phrase Allah is omniscient does not justify ignorance Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Albumazar [Abu Mashar al-Balkhi; Abu-Mashar; Abu Maschar] (787–885) E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Albumazar [Abu Mashar al-Balkhi; Abu-Mashar; Abu Maschar] (787–885) Astronomer and astrologer, born in Balkh, Afghanistan, flourished in Baghdad An eleventh century Latin translation of his Introduction to Astrology introduced the West to Aristotelian physics Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK Aldebaran E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S Aldebaran The star α Tauri, a red giant of spectral type K5III, apparent magnitude 0.87 Its distance is 65 light-years, its parallax 0.050 The absolute magnitude is −0.6 Aldebaran appears at the eastward corner of the striking triangular open cluster the Hyades, although it is not actually a member of the cluster which is much more distant (about 100 light-years) The name derives from the Arabic Al Dabaran, ‘the Follower’—presumably of the Pleiades Aldebaran was one of the ‘Royal Stars’ or ‘Guardians of the Sky’ of the Persian astronomer/astrologers c 3000 BC Although not the brightest stars these four were carefully chosen, apparently to mark the seasons, as they are approximately h apart in right ascension Aldebaran (ancient Persian name Tascheter) was prominent in the evening sky in March and was associated with the vernal equinox; the others were Regulus (summer solstice), Antares (autumnal equinox) and Fomalhaut (winter solstice) Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK ... clusters of galaxies Astrophys J Suppl 211 –88 Abell G O, Corwin H G and Olowin R P 19 89 A catalog of rich clusters of galaxies Astrophys J Suppl 70 1 13 8 Reviews: Bahcall N A 19 77 Clusters of galaxies... Hampshire, RG 21 6XS, UK Registered No 785998 and Institute of Physics Publishing 20 01 Dirac House, Temple Back, Bristol, BS1 6BE, UK Abetti, Antonio (18 46 19 28) and Abetti, Giorgio (18 82– 19 82) E N... (18 46 19 28) and Abetti, Giorgio (18 82 19 82) Antonio was born in San Pietro di Gorizia, Italy A civil engineer, he turned to astronomy and became director of the observatory in Arcetri and professor