An Atlas of the Universe - A Glossary

Abell catalogue (A, ACO): A catalogue of 2712 rich clusters of galaxies produced by George Abell in 1958 from careful examination of the Palomar Sky Survey plates. It was extended in 1989 by George Abell, Harold Corwin and Ron Olowin to include an extra 1364 rich clusters in the far southern hemisphere not covered by the original Palomar Sky Survey. It contains most of the richest clusters of galaxies within 3 billion light years.
Abell cluster: See Rich cluster of galaxies, Abell catalogue.
Absolute magnitude: The apparent magnitude an object would have if placed at a distance of exactly 10 parsecs (=32.6 light years). A supergiant star might have an absolute magnitude of -8 whereas a dim red dwarf might have an absolute magnitude of +16. The Sun has an absolute magnitude of +4.8 - about half way between the two extremes.
Angular Size: The apparent size of an object expressed as an angle. It is measured in degrees, minutes and seconds.
Apparent magnitude: The system used to give the brightness of stars in the sky. Brighter stars have lower numbers and dimmer stars have higher numbers. The dimmest objects visible with giant telescopes have a magnitude of +30. A good portable telescope might see down to magnitude +15. Binoculars can see down to magnitude +9 and the faintest naked eye stars have a magnitude of +6. Very bright objects have a negative magnitude, the brightest star has a magnitude of -1.4, the full Moon has a magnitude of -12.7 and the noon Sun has a magnitude of -26.8.
Arcminute ('): A measure for small angles. 1 arcminute = 1/60 degree.
Arcsecond ("): A measure for very small angles. 1 arcsecond = 1/60 arcminute = 1/3600 degree.
Association: See OB association.
Astronomical Unit (AU): The average distance between the Earth and the Sun. It is equal to 149 597 871 km. It is often used for distances in a solar system or for distances between companion stars.
Barnard catalogue: A catalogue of 349 dark nebulae north of declination -35° produced by E Barnard in 1927.
Barred Galaxy: A galaxy with a bright central bar of stars.
Bayer name: The combination of a Greek letter and the name of a constellation (Alpha Centauri, Epsilon Orionis etc.) used to identify bright stars. The system was first used by Johann Bayer in 1603. Brighter stars in a constellation usually have a letter near the beginning of the alphabet, and dimmer stars usually have a letter nearer the end of the alphabet. A few faint stars were given lower-case Roman letters from a to z or upper-case Roman letters from A to Q (p Eridani, N Velorum etc.) See also Flamsteed number.
Billion: 1 billion = 1 000 000 000.
Black hole: The ultimate cosmic plughole formed when a high mass supergiant star explodes in a supernova explosion at the end of its life creating a super-dense point in space where nothing can escape the gravitational pull. A star probably has to have a mass of more than 40 solar masses to create a black hole which typically have a mass of about 3 solar masses. Black holes can be detected by the disrupting effects they have on neighbouring stars. The centres of most galaxies including our own are believed to have super-massive black holes which have sucked in thousands of stars.
Blue Apparent Magnitude: The apparent magnitude of a star or galaxy when viewed through a blue filter. This magnitude system is commonly used for nearby galaxies because, historically, pictures of galaxies were photographed with photographic plates sensitive to blue light.
Bonner Durchmusterung catalogue (BD, CD, CP): A star catalogue of 325 037 northern stars produced between 1859 and 1862 with a supplement of 134 833 southern stars produced in 1886. Later came two more large southern star catalogues: the Cordoba Durchmusterung (613 959 stars) produced between 1892 and 1932, and the Cape Photographic Durchmusterung (454 877 stars) produced between 1895 and 1900.
Bright Star catalogue (HR, BS, Yale): A star catalogue of nearly all the stars brighter than magnitude +6.5 published by the Yale University Observatory. The original version was published in 1908 as the Harvard Revised Photometry Catalogue and it contains 9096 stars. The current version is the fifth edition.
Brown dwarf star: A failed star that was too small at its birth for nuclear reactions to occur in its core. They may be very common, but because they only glow very dimly they are very hard to detect. Brown dwarfs are not brown, they begin their lives by glowing a dull red and then fade. Brown dwarfs are more massive than planets, and range in mass from 10 to 80 times the mass of Jupiter (or 0.01 to 0.08 times the mass of the Sun). There are two main types of brown dwarfs - the hotter ones (1500K to 2500K) are type L; the cooler ones (below 1500K) are type T. The hottest brown dwarfs are sometimes classified as very cool type M red dwarf stars.
Cluster of galaxies: A concentration of galaxies bound together by gravity. The term 'cluster' usually refers to a large collection of many tens or hundreds of galaxies and the term 'group' is used for a much smaller grouping.
Constellation: Random patterns of stars in the night sky produced by the chance alignment of stars of different luminosities and distances. There are 88 constellations - 48 were listed by the ancient Greeks, and another 40 were added after 1590.
Dark matter: The visible stars and nebulae make up only a small fraction of all the matter in the universe. The rest is in a form that is not easy to detect, but clearly exists because of the effect it has on the motion of stars in galaxies and the motion of galaxies in clusters. Dark matter probably consists of various types of subatomic particles.
Declination (Dec): See Equatorial coordinates.
Dwarf galaxy: A small galaxy usually containing any number of stars between a million and several billion. There is no official size below which a galaxy is designated a dwarf but any galaxy with a diameter below 30 000 light years can be considered a dwarf.
Dwarf star: A normal main sequence star like the Sun that is burning hydrogen in nuclear reactions in its core. The brightest dwarf stars can be much larger than the Sun. See also Giant star, Supergiant star.
Elliptical galaxy: A galaxy with a spherical or oval shape. They are classified as type E and range in shape from type E0 - circular, to type E7 - very elliptical.
Equatorial coordinates (RA, Dec): The most common coordinate system used by astronomers. It is the equivalent of the Earth's latitude and longitude projected onto the sky except that longitude is called Right Ascension and latitude is called Declination. For historical reasons, Right Ascension is not measured in degrees but in 'hours' - 24 hours being equivalent to 360 degrees. Another complication is that this coordinate system is very slowly moving with time - star positions for 1950 are slightly different to those for the year 2000 for example.
ESO catalogue (ESO): The ESO/Uppsala Catalogue of Galaxies. An extension to the UGC catalogue listing 18 422 bright galaxies south of declination -20° published in 1982. Note that the combined UGC and ESO catalogues do not cover the declination zone between -2.5° and -20°.
Flamsteed number: The combination of a number and the name of a constellation (61 Cygni, 36 Ophiuchi etc.) used to identify naked-eye stars. The numbers were applied to John Flamsteed's star catalogue published in 1725. Not all naked-eye stars have a Flamsteed number and most stars in the far southern hemisphere do not have one. See also Bayer name.
Galactic coordinates (l, b): A coordinate system based on the plane of the Galaxy, it is centred on the Sun with the zero point of longitude and latitude pointing directly at the galactic centre. The symbols used for galactic coordinates are l (longitude) and b (latitude). The zero point of Galactic latitude and longitude is at RA=17h45m37s Dec=-28°56'10", and the Galactic north pole is at RA=12h51m26s Dec=+27°07'42", (epoch 2000 coordinates).
Galactocentric coordinates: The same coordinate system as galactic coordinates except that this system is centred on the centre of the Galaxy. The small uncertainty in the distance to the galactic centre prevents this system from being widely used.
Galaxy: A vast concentration of millions or billions of stars. There are four main types of galaxies: Elliptical Galaxies, Lenticular galaxies, Spiral Galaxies and Irregular galaxies. Our galaxy contains 200 billion stars, but the largest galaxies contain many trillions. See also Dwarf Galaxy.
Giant star: A star the size of the Sun will end its life after several billion years by expanding greatly because of changing energy balances at the core of the star. The surface temperature drops and the star becomes redder, this lasts several million years before the star throws off its outer layers and becomes a white dwarf. See also Dwarf star, Supergiant star.
Giclas catalogue (G): The usual name given to the Lowell Proper Motion Survey catalogue produced in the 1970's containing 11 747 stars with a high proper motion.
Gliese catalogue (Gl, Wo, GJ): The usual name given to three catalogues of nearby stars compiled by W Gliese (and later by H Jahreiß) in 1957, 1969 and 1993. The third catalogue which listed 3803 stars within 25 parsecs was only released in a preliminary form.
Globular star cluster: A spherical cluster of many thousands of stars. Globular clusters usually have a size of 50 to 150 light years and are found scattered in a spherical halo surrounding galaxies.
Group of galaxies: A concentration of galaxies bound together by gravity. The term 'group' usually refers to a small collection of a few tens of galaxies and the term 'cluster' is used for a much larger grouping.
Harvard Revised catalogue (HR): See Bright Star catalogue.
Henry-Draper catalogue (HD): A catalogue of the spectral classes of 272 150 stars produced between 1918 and 1924.
Hipparcos catalogue (HIP): A catalogue of 118 218 stars surveyed by the Hipparcos satellite launched in 1989. It collected the parallaxes of these stars providing accurate distances to tens of thousands of stars within 1000 light years.
Hubble constant (H): The value which describes the rate at which the universe is currently expanding. The Hubble constant helps to determine the size and age of the universe, and is also used to convert the redshift of a galaxy into a distance estimate. The Hubble constant is not known precisely but it is somewhere in the range of 60 to 80 km/s/Mpc.
IC catalogue (IC): The Index Catalogue. A supplement to the NGC catalogue listing an extra 5386 star clusters, nebulae and galaxies. It is actually a combination of two catalogues, the first published in 1895 and the second in 1908. See also NGC catalogue.
Irregular galaxy: A galaxy with a very irregular shape and no obvious elliptical or spiral structure. They are classified as type Irr. Irregular galaxies with a crude spiral-like structure are often classified as type Sm (or type SBm if they also have a central bar).
Large Galaxy: See Galaxy.
Lenticular galaxy: A galaxy that is shaped like a lens. They are of an intermediate type between an elliptical galaxy and a spiral galaxy. They are classified as type S0 (or type SB0 if they also have a central bar).
Light year (ly): The distance light travels in a year. It is equal to 0.3066 parsecs. It is 9461 billion km or 63 240 astronomical units.
Luyten, Willem: An astronomer who from the 1930's to the 1980's found thousands of nearby stars in several huge surveys including the Bruce Proper Motion Survey in 1944 and the Luyten-Palomar survey conducted in the 1970's. He produced many proper motion catalogues including the Luyten catalogue (L), the Luyten-Palomar catalogue (LP), the Luyten Two-Tenths Arcsecond catalogue (LTT), the Luyten Four-Tenths Arcsecond catalogue (LFT), and the Luyten Half-Second catalogue (LHS).
Magnitude: See Apparent magnitude, Absolute magnitude.
Main sequence star: A normal star like the Sun that is burning hydrogen in nuclear reactions to produce its energy. Other types of stars include: giant stars, supergiant stars and white dwarfs.
MCG catalogue (MCG): The Morphological Catalogue of Galaxies. A catalogue of 31 000 galaxies north of declination -45° produced between 1962 and 1974.
Minute: See Arcminute.
Nebula: A cloud of interstellar gas and dust. Bright nebulae glow with light emitted by the gas of which they are composed (emission nebulae) or by reflected starlight (reflection nebulae) or both. Dark nebulae consist of clouds of gas and dust that are not illuminated. Planetary nebulae are shells of gas ejected by stars.
Neutron star: The core of a supergiant star which has collapsed during a supernova explosion so much that it consists entirely of neutrons. Most stars between 8 and 60 solar masses end their lives like this usually producing a neutron star with a mass of about 1.4 solar masses. Neutron stars are only 10 kilometres across and have an incredible density - a teaspoon of neutron star material would have a mass of hundreds of millions of tonnes. See also Black hole.
NGC catalogue (NGC): The New General Catalogue. A catalogue of 7840 of the brightest star clusters, nebulae and galaxies published by J Dreyer in 1888. See also IC catalogue.
OB association: A loose group of tens or hundreds of very bright stars scattered over several hundred light years of space. The stars in an OB association were formed in the same star-forming region and are slowly moving apart. They are usually found in the spiral arms of galaxies.
Open star cluster: A cluster of stars usually containing several hundred members packed into a region usually less than 20 light years in size. They are normally found near regions of star formation in the spiral arms of galaxies.
Orange dwarf star: Stars with a luminosity inbetween the Sun-like yellow stars and red dwarfs. They are classified as type K stars.
Parallax: The tiny periodic shift of the apparent positions of nearby stars due to the changing position of the Earth as it orbits the Sun. The nearer the star is, the larger the shift. The distance to stars in parsecs is simply 1/parallax, (or in light years it is 3.2616/parallax) where the parallax is in arcseconds.
Parsec (pc): The distance a star has to be to have a parallax shift of 1 arcsecond. (No star is actually this close). 1 parsec = 3.2616 light years.
PGC catalogue (PGC): The Catalogue of Principal Galaxies. A catalogue of 73 197 of the brightest galaxies published in 1989. Since 1989, galaxies have continued to receive PGC numbers so that there are now over 1 million galaxies with PGC numbers.
Planetary nebula: An expanding envelope of gas surrounding a hot white dwarf. It is formed at the end of a giant star's life when the core contracts ejecting the outer atmosphere of the star creating both the white dwarf and the nebula. The intense radiation from the central white dwarf makes the nebula glow. Planetary nebulae disperse within 50 000 years. They are called planetary nebulae because to early astronomers they looked a bit like planets.
Proper Motion: The slow steady shift of the apparent positions of nearby stars over many years because of their independent motion within the Galaxy. Even the nearest and fastest stars require centuries to move a degree or more.
Quasar: A galaxy with an extremely luminous nucleus outshining the parent galaxy by several hundred times. They lie billions of light years away and were a feature of the early universe. The energy source of a quasar is probably matter falling into a supermassive black hole.
Radial Velocity (RV): The speed of an object in the direction towards or away from the observer. In an expanding universe a galaxy with a larger radial velocity generally lies further from the observer than one with a smaller radial velocity.
Red dwarf star: The smallest and dimmest stars. About 80% of all stars are red dwarfs although none are visible from Earth with the naked eye. Because they shine with less than 1% of the Sun's output they live a long time - the smallest are likely to last trillions of years. They are classified as type M stars.
Recessional Velocity: The radial velocity of a galaxy caused by the expansion of the universe.
Redshift (z): An increase in the wavelength of light caused either by the source of the light moving away from the observer or by the expansion of the universe. In an expanding universe galaxies with large redshifts lie at greater distances than galaxies with small redshifts. Redshifts can also be produce by light climbing out of a strong gravitational field such as a black hole.
Rich cluster of galaxies: A cluster of galaxies containing hundreds of large galaxies. The nearest rich cluster is the Virgo cluster. The richest clusters can contain more than a thousand large galaxies. Most of the richest clusters in the nearby universe are listed in the Abell catalogue.
Right Ascension (RA): See Equatorial coordinates.
Ross, Frank: An astronomer who searched for high proper motion stars between 1925 and 1939, producing a list of 1070 nearby stars.
Second: See Arcsecond.
Solar system: A star together with the planets, moons, asteroids, comets and dust which orbit it.
Spectral Classification: The system used to classify stars. Stars fall into seven main catagories: O, B, A, F, G, K and M ranging from hot blue-white stars to cooler red stars. A number (0 to 9) is usually added to denote a sub-class. A roman numeral is sometimes added to denote the size of the star. Supergiant stars are class I, Giant stars are class III, and ordinary main sequence stars are class V. (Types II and IV are inbetween types). The Sun is type G2V, whereas Arcturus - an orange giant star - is type K2III.
Spiral galaxy: A galaxy with spiral arms. There are two main types, those with central bar - SB, and those without - S (or SA). Spiral galaxies are also subdivided into types a, b, c (and sometimes d), depending on how tightly wound the spiral arms are.
Star classification: See Spectral classification.
Star cluster: See Open star cluster, Globular cluster.
Stellar association: See OB association.
Supercluster: A large concentration of hundreds or thousands of groups of galaxies. Superclusters range in size from 100 million to 500 million light years and are usually embedded in large sheets and walls of galaxies surrounding large voids in which very few galaxies exist. Superclusters formed in the early universe when matter clumped together under the influence of gravity.
Supergalactic coordinates (L, B): A coordinate system based on the approximate plane of the Virgo Supercluster. The supergalactic plane passes through the Sun and the middle of the Virgo cluster. Several nearby superclusters also lie close to this plane. The symbols used for supergalactic coordinates are L (longitude) and B (latitude). The zero point of Supergalactic latitude and longitude is at RA=02h49m14s Dec=+59°31'42", and the Supergalactic north pole is at RA=18h55m01s Dec=+15°42'32", (epoch 2000 coordinates).
Supergiant star: A star bigger than about 10 solar masses will become a supergiant star at the end of its life as hydrogen burning ceases causing the stars outer layers to expand. Supergiant stars are the largest and brightest of all stars and they usually end up exploding in a supernova explosion and creating a neutron star or a black hole. See also Dwarf star, Giant star.
Supernova: A catastrophic stellar explosion that can briefly outshine an entire galaxy of billions of stars. It can occur when a supergiant star exhausts all its nuclear fuel causing the core of the star to collapse releasing a vast amount of energy which blasts away the outer parts of the star and leaves behind a neutron star or in extreme cases a black hole.
Supernova remnant: The remains of a star visible as an expanding nebula of gas that have been ejected at high speed by a supernova explosion.
Temperature: In astronomy, temperature is measured with the Kelvin scale (symbol K) which is equal to °C + 273°. Thus a midday Earth temperature of 20°C is equal to 293K and the Sun's surface temperature of 5500°C is about 5770K.
Trillion: 1 trillion = 1 000 000 000 000.
UGC catalogue (UGC): The Uppsala General Catalogue of Galaxies. A catalogue of 12 939 bright galaxies north of declination -2.5° published in 1973. See also ESO Catalogue.
Variable star: A star that varies in brightness. There are many types, some stars can change in brightness in a matter of minutes whereas others change slowly over many months. The first 334 variable stars discovered in a constellation are given a one or two letter code such as R Scuti or UV Ceti. Other variable stars are designated V335, V336, etc. Proxima Centauri for example is known to variable star astronomers as V645 Centauri.
Visual magnitude: Another name for Apparent magnitude.
White dwarf star: The dying remnant of a giant star that has blown away its outer layers to reveal an intensely hot core. Only the youngest white dwarfs are actually white, over billions of years they slowly cool and change colour to yellow or orange (although the coolest white dwarfs are bluer than expected because the compressed atmosphere of hydrogen filters out the red light). They eventually become a dead black dwarf, although because the universe is less than 15 billion years old none have yet cooled this much. They are classified as type D stars.
Wolf, Max: An astronomer who searched for high proper motion stars between 1919 and 1931, producing a list of 1566 nearby stars.
Yale catalogue: See Bright Star catalogue.
Yellow dwarf star: Any small yellow star like the Sun. They are classified as type G stars.

Thanks to Rob Hribar who created all the links in this page for me.