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A GLOSSARY of ASTRONOMICAL TERMS

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A GLOSSARY of ASTRONOMICAL TERMS A GLOSSARY of ASTRONOMICAL TERMS Or What every student of astronomy should know. Prepared by Frank Gear. F.R.A.S. February 2004. angular velocity. The speed with which an Astro Glossary: object revolves about its axis. A: annular eclipse. An eclipse of the Sun in A ring. One of the three prominent rings which the Moon is too distant to cover the Sun encircling Saturn. completely, so that a ring of sunlight is seen around the Moon at mid eclipse. absolute magnitude. The apparent magnitude that a star would have if it were at a distance of angular resolution. The angular size of the 10 parsecs. smallest feature that can be distinguished with a telescope absolute zero. The temperature of –273o C (or 0K). Where all molecular motion stops ; the autumnal equinox. The intersection of the lowest possible temperature. ecliptic and the celestial equator where the Sun crosses the equator from north to south. absorption line spectrum. Dark lines superimposed on a continuous spectrum. antielectron. A positron. acceleration. A change in velocity. antimatter. Matter containing antiparticles such as anti protons, anti electrons (positrons) accretion. The gradual accumulation of matter and antineutrons. in one location, typically due to the action of gravity. aperture. The diameter of an opening; the diameter of the primary lens or mirror of a telescope. accretion disc. A disc of gas orbiting a star or black hole. aphelion. The point in its orbit where a planet is farthest from the Sun. active galactic nucleus (AGN). The centre of an active galaxy. apogee.The point in its orbit where a satellite or Moon is farthest from the Earth. Active galaxy. A galaxy that is emitting exceptionally large amounts of energy: a apparent brightness. The flux of a star’s light Seyfert galaxy or a quasar. arriving at the Earth. active Sun. The Sun during times of frequent apparent magnitude. A measure of the solar activity such as Sun spots, flares and brightness of light from a star or other object associated phenomena. as measured from Earth. adaptive optics. A technique of improving a asteroid. One of tens of thousands of small, telescope image by altering the telescope’s rocky planet like objects in orbit around the optics in a way that compensates for the Sun, distortion caused by the Earth’s atmosphere. asteroid belt. A region between the orbits of albedo. The fraction of sunlight that a planet, Mars and Jupiter that encompasses the orbits asteroid or satellite reflects. of many asteroids. angle .The opening between two lines that astronomical Unit (AU).The semimajor axis meet at a point. of the Earth’s orbit; the average distance between the Earth and the Sun. angstrom (A). A unit of length equal to 10 –10 meter. aurora. Light radiated by atoms and ions in the Earth’s upper atmosphere, mostly polar angular diameter. The angle subtended by the regions. diameter of the object. aurora boreealis. Arorae seen from northern angular momentum. A measure of the latitudes momentum associated with rotation. aurora austrailis. Arorae seen from southern brown dwarf . A star like object that is not latitudes. massive enough to ignite hydrogen burning in its core. B: Barnard object. Dark nebulae discovered by butterfly diagram. A plot of Sunspot latitude E.E.Barnard. against time. Barred spiral galaxy. A spiral galaxy in C: which the spiral arms begin from the end of a Caldera. The crater at the summit of a “bar” running through the nucleus rather than volcano. the nucleus itself. carbon burning. The thermonuclear fusion of bazar. A BL Lacertae object. carbon to produce heavier nuclei. Big Bang. An explosion of all space, roughly carbon star. A peculiar red giant star whose 20 billion years ago, from which the universe spectrum shows strong absorption by carbon emerged. and carbon compounds. Big Crunch. The fate of the universe if it is carbonaceous chondrite. A type of meteorite bounded, and ultimately collapses upon itself. that has a height abundance of carbon and volatile compounds. binary star. Two stars revolving about each other. Cassegrain focus. An optical arrangement in a reflecting telescope in which the light rays are bolometric correction. The difference reflected by a secondary mirror to a focus between the visual and bolometric magnitudes behind the primary mirror. of a star. Cassini division. An apparent gap between BL Lacertae object. A type of active galaxy Saturn’s A and B rings. whose nucleus does not exhibit spectral lines. celestial equator. A great circle on the black hole. An object whose gravity is so celestial sphere 90° from the celestial poles. strong that the escape velocity exceeds the speed of light. celestial mechanics. The branch of astronomy dealing with the motions and gravitational black body. A hypothetical perfect radiator interactions of objects in the solar system. that absorbs and re-emits all radiation falling on it. celestial poles. Points about which the celestial sphere appears to rotate. blueshift. A decrease in the wavelength of photons emitted by an approaching source of Celestial sphere. A sphere of very large radius light. centred on the observer; the apparent sphere of the sky. Bode’s Law. A numerical sequence that gives to approximate average distances of the planets centre of mass. That point in an isolated form the sun in astronomical units (AU). system that moves at a constant velocity in accordance with Newton’s first law. Bohr atom. A model of the atom, described by Neils Bohr, in which electrons revolve about central bulge. A spherical distribution of stars the nucleus in certain allowed orbits. around the nucleus of a spiral galaxy. Bock globule. A small, roundish, dark nebular. Cepheid variable. A type of yellow, supergiant, pulsating star. bounded universe. A universe throughout which the average density exceeds the critical Ceres. The largest asteroid and the first to be density. discovered. bolometric magnitude. A measure of the Chandrasekhar limit. The maximum mass of brightness of a star or object as detected by a a white dwarf. device above the Earth’s atmosphere charged-coupled device (CCD). A type of corona. The Sun’s outer atmosphere, which solid-state silicon wafer designed to detect has a high temperature and a low density. photons. chronograph. An instrument for chemical element. A substance that cannot be photographing the solar corona in which a disc decomposed by chemical means into a simpler inside the telescope produces an artificial substances. eclipse. chromatic aberration. An optical defect cosmic microwave background. An isotropic whereby different colours of light passing radiation field with a black body temperature through a lens are focused at different of about 2.7oK that permeates the entire locations. universe. chromosphere. A layer in the solar cosmic particle horizon. An imaginary atmosphere between the photosphere and the sphere, centred on the Earth, whose radius corona. equals the distance light has travelled since the Big Bang. close binary. A double star system in which the stars are separated by a distance roughly cosmic rays. Atomic nuclei (mostly protons) comparable to their diameters. that strike the Earth with extremely high speeds. Cluster of galaxies. A collection of galaxies containing a few to several thousand member cosmological principal. The assumption that galaxies. the universe is homogeneous and isotropic on the largest scale. cold dark matter. Slowly moving, weakly interacting particles presumed to contain the cosmological redshift. A redshift that is bulk of matter in the universe. caused by the expansion of the universe. cosmology. The study of the structure and coma (of a comet). The diffuse gaseous evolution of the universe. component of the head of the comet. code focus. A reflecting telescope in which a coma (optical). The distortion of off-axis series of mirrors direct light to a remote focus images formed by a parabolic mirror. away from the moving parts of the telescope. conjunction. The geometric arrangement of a critical density. The average density planet in the same part of the sky as the Sun, throughout the universe at which space would and the earth. be flat and galaxies would just barley continue receding from each other infinitely far into the conservation of angular momentum. A law future. of physics stating that the total amount of angular momentum in an isolated system D: remains constant. dark matter. Sub luminous matter that seems to be quite abundant in galaxies and conservation of energy. A law of physics throughout the universe. stating that the total energy in an isolated system remains constant. dark-matter problem. The enigma that most of the matter in the universe is severely under conservation of momentum. A law of physics luminous. stating that the total momentum in anisolated system remains constant. dark nebula. A cloud of interstellar gas and dust that obscures the light of more distant constellation. A configuration of stars, often stars. named after an object, person , god or animal. declination. Angular distance of a celestial continuous spectrum. A spectrum of light body north or south of the celestial equator. over a range of wavelengths without any spectral lines. deflagration. A sudden, violent burning. degree. A basic unit of angular measure, dwarf elliptical galaxy. A low-mass galaxy usually designated by the symbol 0 . that only contains a few million stars. density. The ratio of the mass of an object to E: its volume. eclipse. The cutting off of part or all of the light of one celestial object by another. density-wave theory. An explanation of spiral arms in galaxies proposed by C.C.Lin and eclipse path. The track of the Moon’s shadow colleagues. along the Earth’s surface during a total or annular solar eclipse. descending node.
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