A GLOSSARY of ASTRONOMICAL TERMS
What every student of astronomy should know.
Prepared by Frank Gear. F.R.A.S.
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
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. A point along an orbit where an object crosses a reference plane eclipse year. The interval between successive (usually the ecliptic or celestial equator) from passages of the Sun through the same node of north to south. the Moon’s orbit. deuterium. An isotope of hydrogen whose eclipsing binary. A binary system in which, as nucleus contains one proton and one neutron, seen from Earth the stars pass in front of each heavy hydrogen. other. diffraction. The spreading out of light passing ecliptic. The apparent annual path of the Sun the edge of an opaque object. on the celestial sphere. diffraction grating. A piece of glass or metal, Einstein ring. The circular image of a remote containing thousands of closely spaced lines light source produced by gravitational lens in that is used to disperse light into a spectrum. which the source, the observer, and the deflecting mass are nearly perfectly aligned. diffuse nebula. A reflection or emission nebula consisting of interstellar gas and dust. electromagnetic radiation. Radiation consisting of oscillating electric and magnetic dilation of time. The slowing of time due to fields including gamma rays, X rays, visible relativistic motion. light, ultraviolet and infrared radiation, radio waves, and microwaves. direct motion. The apparent eastward movement of a planet seen against the electromagnetic spectrum. The entire array background of stars. or family of electromagnetic radiation. disc (of a galaxy). The disc-shaped element. A chemical that cannot be broken distribution of population 1 stars that down into more basic chemicals. dominates the appearance of a spiral galaxy. elliptical galaxy. A galaxy with an elliptical distance modulus. The difference between shape and no conspicuous interstellar material. the apparent and absolute magnitudes of an object. elongation. The angular distance between a planet and the Sun as viewed from Earth. diurnal. Daily. emission line. A bright spectral line. diurnal motion. Motion in one day. emission line spectrum. A spectrum that Doppler effect. The apparent change in contains emission lines. wavelength of a radiation due to relative motion between the source and the observer emission nebula. A glowing gaseous nebula along the line of sight. whose light comes from a fluorescence caused by a nearby star. double star. A pair of stars in orbit about each other and held together by their mutual Encke division. A narrow gap in Saturn’s A gravitational attraction; a binary star. ring. dust tail. The tail of a comet that is composed epoch. A date and time selected as a fixed primarily of dust particles. reference. equation of time. The difference between full moon. A phase of the Moon during which apparent and mean solar time. its full daylight hemisphere can be seen from Earth. equinox. One of the intersections of the ecliptic and the celestial equator. G: galactic cannibalism. A collision between two ergosphere. The region of space immediately galaxies of unequal mass and size in which the outside the event horizon of a rotating black smaller galaxy seems to be absorbed by the hole where it is impossible to remain at rest. larger galaxy. escape speed. The speed needed by one galactic cluster. A loose association of young object to achieve a parabolic orbit away from a stars in the disc of our galaxy. second object and thereby permanently move away from the second object. galactic equator. The intersection of the principal plane of the Milky Way with the event horizon. The location around a black celestial plane. hole where the escape velocity equals the speed of light; The surface of a black hole. galactic nucleus. The centre of a galaxy. extragalactic. Beyond our galaxy. galaxy. A large assemblage of stars, nebulae, and interstellar gas and dust. eyepiece. A magnifying lens used to view the image produced at the focus of a telescope. galaxy merger. A collision between two galaxies that cause them to coalesce. F: favourable opposition. A Martian opposition Galilean satellite. Any one of the four large that affords good Earth-based views of the moons of Jupiter. planet. gamma rays. The most energetic form of first quarter moon. The phase of the Moon electromagnetic radiation. that occurs when the moon is 90o east of the Sun. general theory of relativity. A description of gravity formulated by Albert Einstein, which flare. A sudden, temporary outburst of light explains that gravity affects the geometry of from an extended region of the solar surface. space and the flow of time. flat space. Space that is not curved; space with geocentric cosmology. An Earth centred zero curvature. theory of the universe. flocculant spiral galaxy. A spiral galaxy with giant star. A star whose diameter is typically fuzzy, poorly defined spiral arms. 10 to 100 times that of the sun and whose luminosity is roughly that of 100 Suns. flux. The number of particles or the amount of energy flowing across a given area per unit of giant elliptical galaxy. A large, massive time. elliptical galaxy containing many billions of stars. focal length. The distance from a lens or mirror to the point where converging light rays giant molecular cloud. A large cloud of meet. interstellar gas and dust. focus (optical). The point where light rays gibbous moon. A phase of the Moon in which converged by a lens or mirror meet. more than one half , but not all, of the Moon’s force. That which can change the momentum daylight hemisphere is visible from Earth. of an object. glitch. A sudden speedup in the period of a frequency. The number of wave crests or pulsar. troughs that cross a given point per unit of time ; the number of vibrations per unit of globular cluster. A large spherical cluster of time. stars, typically found in the outlying regions of a galaxy. globule. A small, dense, dark nebula. helio-. A prefix referring to the Sun. gluon. A particle that is exchanged between heliocentric cosmology. A Sun centred theory quarks. of the universe. granulation. The rice-grain-like structure of helium burning. The thermonuclear fusion of the solar photosphere. helium to form carbon and oxygen. grating. An optical device, consisting of Hertzsprung-Russell (H-R) diagram. A plot thousands of closely spaced lines etched on of the absolute magnitude (or luminosity) of glass or metal, that disperses light into a stars against their spectral type (or surface spectrum. temperature). gravitation. The tendency of matter to attract heterogeneous accretion theory. A theory of matter. planetary formation which argues that the composition of planetesimals changed as the gravitational lens. The deflection of light planets formed. from a remote source caused by the presence of an intervening mass. homogeneous accretion theory. A theory of planetary formation which argues that the gravitational radiation/waves. Oscillations of planets formed from planetesimals of generally space produced by changes in the distribution the same composition. of matter. horizontal branch. A group of stars on the graviton. The particle that is responsible for Hertzsprung-Russell diagram of a typical the gravitational force. globular cluster near the main sequence and having roughly constant absolute magnitude. gravity. The force with which matter attracts matter. hot dark matter. Dark matter consisting of particles moving at high speeds. Great Dark Spot. A prominent high-pressure system in Neptune’s southern hemisphere. Hubble classification scheme. A method of classifying galaxies as spirals, barred spirals, Great Red Spot. A prominent high-pressure ellipticals, or irregulars according to their system in Jupiter’s southern hemisphere. appearance. greatest elongation. The largest possible Hubble constant (HO) The constant of angle between the Sun and an inferior planet. proportionality in the relation between the recessional velocities of remote galaxies and Greenwich meridian. The meridian of their distances. longitude that passes through the Old Greenwich Observatory near London; the Hubble flow. The recessional motions of longitude of 0°. remote galaxies caused by the expansion of the universe. group (of galaxies). A poor cluster of galaxies. Hubble law. The empirical relationship stating that the redshifts of remote galaxies are H: directly proportional to their distances from H l region. A region of neutral hydrogen in Earth. interstellar space. hyperbolic space. Space with a negative H ll region. A region of lionized hydrogen in curvature. interstellar space. hypothesis. An idea or collection of ideas that hadron. A particle composed of quarks. seem to explain a specific pheromone; a conjecture. halo (of a galaxy). A spherical distribution of globular clusters of population ll stars that surround a spiral galaxy.
I: inverse-square law. The statement that the impact crater. A crater formed by the impact apparent brightness of a light source varies of a meteoroid. inversely with the square of the distance from that source. inertia. The property of matter that requires a force to act on it to change its state of motion. ion. An atom that has become electrically charged due to the addition or loss of one or inferior conjunction. The configuration when more electrons. an inferior planet is between the Sun and Earth. ion tail (of a comet). The relatively straight tail of a comet produced by the solar wind inferior planet. A planet that is closer to the acting on ions. Sun than the Earth is. ionisation. The process by which an atom inflation. A sudden expansion of space. loses electrons. inflationary epoch. A brief period shortly ionosphere. A layer in the Earth’s upper after the Big Bang during which the scale of atmosphere in which many of the atoms are the universe increased very rapidly. lionized. infrared radiation. Electromagnetic radiation iron meteorite. A meteorite composed of wavelength longer than visible light, yet primarily of iron. shorter than radio waves. irregular cluster (of galaxies). A sprawling inner core. The solid portion of the Earth’s collection of galaxies whose overall iron core. distribution in space does not exhibit any noticeable spherical symmetry. inner Lagrangian point. The point between two stars comprising a binary star where their irregular galaxy. An asymmetrical galaxy Roche lobes touch; the point across which having neither spiral arms nor elliptical shape. mass transfer can occur. isotope. Any of several forms of the same interferometry. A technique of combining the chemical element whose nuclei all have the observations of two or more telescopes to same number of protons but different numbers produce images better than one telescope alone of neutrons. could make. isotropic. Having the same property in all interplanetary medium. The sparse directions. distribution of gas and dust particles in interplanetary space. J: juole (J). A unit of energy. interstellar dust. Microscopic solid grains of various compounds in interstellar space. Jovian planet. Any of the four largest planets; Jupiter, Saturn, Uranus, or Neptune. interstellar extinction. The dimming of starlight passing through the interstellar K: medium. kelvin (K). A unit of temperature on the Kelvin temperature scale. interstellar gas. Sparse gas in interstellar space. Kepler’s laws. Three statements, formulated by Johannes Kepler, that describes the motion interstellar medium. Interstellar gas and dust. of the planets. interstellar reddening. The reddening of kilo parsec (kpc). One thousand parsecs; starlight passing through interstellar medium, about 3,260 light-years. caused by the fact that blue light is scattered more than red. kinetic energy. The energy possessed by an object because of its motion.
Kirchhoff’s laws. Three statements about liquid metallic hydrogen. Hydrogen circumstances that produce absorption lines, compressed to such a density that it behaves emission lines, and continuous spectra. like a liquid metal.
L: LMC. The Large Magellanic Cloud. Lagrangian points. Five points in the orbital plane of two bodies revolving about each other Local Bubble. A large cavity in the interstellar in circular orbits where a third object of medium in which the Sun and nearby Stars are negligible mass can remain in equilibrium. located, last quarter moon. The phase of the Local Group. The cluster of galaxies of which moon that occurs when the Moon is our Galaxy is a member. 90o west of the Sun. long-period comet. A comet that takes law of equal areas. Kepler’s second law. hundreds of thousands of years to complete one orbit of the sun. law of inertia. Newton’s first law. long-period variable. A variable star with a law of physics. A set of physical principals period longer than about 100 days. with which we can understand natural phenomena and the nature of the universe. luminosity. The rate at which electromagnetic radiation is emitted from a star or other object. leap year. A calendar year with 366days. luminosity class. A classification of a star of a lenticular galaxy. A galaxy with a central given spectral type according to its luminosity. bulge and a disc but no spiral arms. luminosity function. The numbers of stars lepton. Any member of a class of particles that of differing brightness per cubic parsec. includes the electron and neutrino. lunar. Referring to the Moon. libration. A slight rocking of the Moon in its orbit whereby an Earth-based observer can, lunar eclipse. An eclipse of the Moon over time, see slightly more than one half of by the Earth; a passage of the Moon the Moon’s surface. through the Earth’s shadow. light. Electromagnetic radiation. lunar month. The time the Moon takes to complete one cycle of its light curve. A graph that displays variations in phases; the synodic month. the brightness of a star or other astronomical object. lunar phase. The appearance of the illuminated area of the Moon as seen light-gathering power. A measure of the from Earth . amount of radiation brought to the focus by a telescope. M: Magellanic clouds. Two nearby galaxies light year (ly). The distance light travels in a visible to the naked eye from southern vacuum in one year. latitudes. limb (of Sun or Moon). The apparent edge of magnetosphere. The region around a planet the Sun or Moon as seen in the sky. occupied by its magnetic field. limb darkening. The phenomenon whereby magnification. The factor by which the the Sun is darker near its limb than near the angular size of an object is apparently centre of its disc. increased when viewed through a telescope. limiting magnitude. The faintest magnitude magnifying power. The number of times that can be observed with a certain telescope larger in angular diameter an object appears under certain conditions. through a telescope than when viewed with the naked eye. magnitude. A measure of the amount of light Maunder minimum. An interval of about 70 received from a star or other luminous object. years, starting around 1645, during which very few sun spots were seen. magnitude scale. A system for denoting the brightness of astronomical objects. maximum eastern elongation. The configuration of an inferior planet at its main sequence. A grouping of stars on the greatest angular distance east of the Sun. Hertzspruung-Russell diagram extending diagonally across the graph from the hottest, maximum western elongation. The brightest stars to the dimmest, coolest stars. configuration of an inferior planet at its greatest angular distance west of the Sun. main sequence star. A star whose luminosity and surface temperature place it on the main mean solar day. The interval between sequence on a H-R diagram; a star that derives successive meridian passages of the mean Sun; its energy from core hydrogen burning. the average length of a solar day. major axis (of an ellipse). The longest mechanics. The branch of physics dealing diameter of an ellipse. with the behaviour and motions of objects acted upon by forces. Mantle (of a planet) .That portion of a terrestrial planet located between the crust and mega parsec (Mpc). One million parsecs. the core. meridian (local). The great circle on the mare. Latin for “sea”; a large, relatively celestial sphere the passes through an crater-free plain on the Moon. plural maria. observer’s zenith and the North and south celestial poles. marginally bounded universe. A universe throughout which the average density equals metal poor star. A star which, compared with the critical density. the Sun, is under abundant in elements heavier than helium. mascon. A localised concentration of dense material beneath the lunar surface. metal rich star. A star whose abundance of heavy elements is roughly comparable to that mass. A measure of the total amount of of the Sun. material in an object. meteor. The luminous phenomenon seen when mass density of radiation. The energy a meteoroid enters the Earth’s atmosphere; a possessed by a radiation field per unit volume “shooting star”. divided by the square of the speed of light. meteor shower. Many meteors that seem to mass function. A numerical relationship radiate from a common point in the sky. involving the masses of the stars in a binary system and the angle of inclination of their meteorite.. A fragment of a meteoroid that has orbits in the sky. survived passage through the Earth’s atmosphere. mass loss. A process by which a star gently loses matter. meteoroid. A small rock in interplanetary space. mass-luminosity relation. A relationship between the masses and luminosity of main micrometeorite. A very small meteoroid; a sequence stars. grain of interplanetary dust. mass transfer. The flow of gases from one microwaves. Short wavelength radio waves. star in a binary to another. Milky Way. Our Galaxy; the band of feint Maunder butterfly diagram. A plot of stars seen from the Earth in the plane of our sunspot latitude versus time . Galaxy’s disc.
minor planet. An asteroid. minute of arc. One-sixtieth of a degree, node. The intersection of an orbit with a designated by the symbol ‘. reference plane such as the plane of the celestial equator or the ecliptic. molecule. A combination of two or more atoms. north celestial pole. The point directly above the Earth’s north pole where the Earth’s axis of moving cluster method. A technique for rotation, if extended, would intersect the determining the distance of a cluster of stars celestial sphere. from the motions of the cluster’s members northern lights . Aurorae; aurorae borealis. muon. A subatomic particle that behaves like a heavy electron. nova. A star that experiences a sudden outburst of radiant energy, temporarily N. increasing its luminosity roughly a thousand nadir. The point on the local meridian 180° fold. from the zenith. nuclear bulge. The central region of our nanosecond. One-billionth (10-9 ) second. Galaxy; the central bulge. nebula. A cloud of interstellar gas and dust. nuclear fusion. The combining of lighter nuclei to make heavier ones. neutrino, A subatomic particle with no electrical charge and little or no mass, yet one nucleus (of an atom). The massive part of an that is important in many nuclear reactions. atom, composed of protons and neutrons, about which electrons revolve. neutron. A subatomic particle with no electrical charge and with a mass nearly equal nucleus (of a comet). A collection of ices and to that of the proton. dust that constitutes the solid part of a comet. neutron star. A very compact, dense star nucleus (of a galaxy). The concentration of composed almost entirely of neutrons. stars and dust at the centre of a galaxy.
New General Catalogue (NGC). A famous mutation. A small periodic wobbling of the nineteenth-century catalogue of nebulae, Earth’s axis superimposed on precession. galaxies, and star clusters. O. new moon. The phase of the Moon when the OB association. A grouping of hot, young, dark hemisphere faces the Earth. massive stars, predominantly of spectral type O and B. new technology telescope. A telescope whose design incorporates innovative features, such OBAFGKM. The temperature sequence of as adaptive optics. spectral types .
Newtonian focus. An optical arrangement in a objective lens. The principal lens of a reflecting telescope in which a small mirror refracting telescope. reflects converging light rays to a focus to one side of the telescope tube. oblate. Flattened at the poles.
Newtonian mechanics. The branch of physics oblatness. A measure of how much a flattened based on Newton’s law that deals with sphere (or spheroid) differs from a perfect gravitation. sphere.
Newtonian reflector. A reflecting telescope Obliquity (of the ecliptic). The angle between that uses a small mirror to deflect the image to the planes of the celestial equator and the one side of the telescope tube. ecliptic (about 23 1/2 °).
Newton’s laws of motion. Three statements obscuration (interstellar). The absorption of about the nature of physical reality on which starlight by interstellar dust. Newtonian mechanics is based. observable universe. That portion of the positron. An electron with a positive rather universe inside the cosmic horizon. than a negative charge; an antielectron.
Occam’s razor. The notion that a potential energy. The energy possessed by an straightforward explanation of a phenomenon object because of its elevated position in a is more likely to be correct than a convoluted gravitational field. one . precession (of the Earth). A slow, conical occultation. The eclipsing of an astronomical motion of the Earth’s axis of rotation caused object by the Moon or planet. by the gravitational pull of the Moon and Sun on the Earth’s equatorial bulge. Olber’s paradox. The dilemma associated with the fact that the night sky is dark. precession (of the equinoxes). The slow westward motion of the equinoxes along the Oort cloud. A presumed accumulation of ecliptic due to the precession of the Earth. comets and cometary material surrounding the Sun at a distance of roughly 50,000 to 100,000 prime focus. The point in a telescope where AU. the objective focuses light. opacity. The ability of a material to impede primordial black hole. A hypothetical black the passage of light. hole that may have been created immediately after the Big Bang. open cluster. A loose association of young stars in the disc of our Galaxy; a galactic prism. A wedge shaped piece of glass that is cluster. used to disperse white light into a spectrum. opposition . The configuration of a planet prominence. Flame like protrusions seen near when it is at an elongation of 180O and thus the limb of the Sun and extending into the appears opposite the Sun in the sky. solar corona. optical window. The range of visible proper mass. The mass of an object measured wavelengths to which the Earth’s atmosphere at rest. is transparent. proper motion. The angular rate of change in optics. The branch of physics dealing with the the location of a star on the celestial sphere, behaviour and properties of light. usually expressed in seconds of arc a year. orbit. The path of an object that is moving proto-. A prefix referring to the embryonic about a second object or point. stage of a young astronomical object (planet, star etc), that is still in the process of outer core. The molten portion of the Earth’s formation. core. proton. A heavy, positively charged plasma. Hot ionised gas. subatomic particle that is one of two principal constituents of the atomic nuclei. poor cluster (of Galaxies). A cluster of galaxies with a very few members; a group of protoplanetary disc. A disc of material galaxies. encircling a proto star or a newborn star. population l star. A star whose spectrum pulsar. A pulsating radio source believed to be exhibits spectral lines of many elements associated with a rapidly rotating neutron star. heavier than Helium; a metal rich star. pulsating variable. A star that pulsates in size population ll star. A star whose spectrum and luminosity. exhibits comparatively few spectral lines of Q. many elements heavier than Helium; a metal quantum mechanics. The branch of physics poor star. dealing with the structure and behaviour of atoms and their interaction with light population lll star. A star virtually devoid of elements heavier than Helium. quark. One of several hypothetical particles reddening (interstellar) . The reddening of presumed to be the internal constituents of starlight as it passes through the interstellar certain heavy subatomic particles such as medium. protons and neutrons. redshift. The shifting to longer wavelengths of quarter moon. A phase of the Moon when it the light from remote galaxies and quasars; is located 90O from the Sun in the sky, so that The doppler shift of light from a receding one half of its day lit hemisphere is visible source. from earth. reflecting telescope. A telescope in which the quasar. A star like object with a very large principal optical component is a concave redshift; a quasi-stellar object or quasi-stellar mirror. radio source. reflection grating. A diffraction grating that quasi-stellar object. A starlike object with a produces a spectrum when a light is reflected very large redshift; a quasar. off it. quasi-stellar radio source. A quasar that reflection nebula. A comparatively dense emits detectable radio radiation. cloud of dust in interstellar space that is illuminated by a star. R. radar. A technique of reflecting radio waves refracting telescope. A telescope in which the from a distant object. principal optical element is a lens. radial velocity. The portion of an object’s refraction. The bending of light rays passing velocity parallel to the line of sight. from one transparent medium to another. radiant (of a meteor shower). The point in refractor. A refracting telescope . the sky from which meteors of a particular shower seem to originate. regolith. The layer of rock fragments covering the lunar surface. radiation. Electromagnetic energy; photons. regular cluster (of Galaxies ). A spherical radiation dominated universe. A universe in cluster of galaxies. which the mass density of radiation exceeds the average density of matter. regular orbit. An orbit in the plane of a planet’s equator along which a satellite travels radio astronomy. That branch of astronomy in the same direction that the planet rotates. dealing with observations of radio wavelength. residual polar cap. Ice-covered polar regions radio galaxy. A galaxy that emits an unusually on Mars that do not completely evaporate large amount of radio waves. during the Martian summer. radio telescope. A telescope designed to resolution. The degree to which fine details in detect radio waves. an optical image can be seen. radio waves. The longest wavelength elerto resolving power. A measure of the ability of magnetic radiation. an optical system to distinguish, or resolve, fine details in the image it produces. radioactivity. The process whereby certain atomic nuclei naturally decompose by retrograde motion. The apparent westward spontaneously emitting particles . motion of a planet with respect to background stars. ray (lunar). Any one of a system of bright, elongated streaks on the lunar surface. retrograde rotation. The rotation of an object in the direction opposite to which it is red giant. A large, cool star of high luminosity revolving about another object. red supergiant. An extremely large, cool star revolution. The motion of one body about of luminosity class 1. another. rich cluster (of galaxies) . A cluster of sidereal year. The orbital period of the earth galaxies containing many members. about the Sun with respect to the Sun right ascension. A coordinate for measuring singularity. A place of infinite space-time the east-west positions of objects on the curvature; the centre of a black hole . celestial sphere. SMC. The Small Magellanic Cloud. rille. A trench like depression on the lunar surface solar activity. Phenomena that occur in the solar atmosphere such as sunspots, plagues, rotation. The turning of a body about an axis flares, and so forth. passing through the body. Solar atmosphere. The outer layers of the RR lyrae variable . A class of pulsating stars Sun, consisting of the photosphere, with periods less than one day. chromosphere, and corona.
S. solar constant. The average amount of energy satellite. A body that revolves about a larger received from the sun per square metre per one . second, measured just above the earth’s atmosphere. Schmidt telescope. A reflecting telescope invented by Bernard Schmidt that is used to solar corona. Hot faintly glowing gasses seen photograph large areas of the sky. around the Sun during a total solar eclipse; the uppermost regions of the solar atmosphere second of arc. One-sixtieth of an arc minute, designated by the symbol “. solar cycle. The semiregular 22-year interval between successive appearances of sunspots at self-propagating star formation. The the same latitude and with the same magnetic process by which the formation of stars in one polarity. location in a galaxy stimulates the formation of stars in a neighbouring location. solar solar eclipse. An eclipse of the sun by the Moon ; a passage of the Earth through the Seyfert galaxy. A spiral galaxy with a bright Moon’s shadow. nucleus whose spectrum exhibits emission lines. solar flare. A violent outburst on the Sun’s surface. shell star. A star, usually of spectral type A to F, that is surrounded by a shell of gas. solar interior. Everything below the solar atmosphere; the inside of the Sun. Short-period comet. A comet that orbits the Sun with a period of less than about 200 years. solar nebula. The cloud of gas and dust from SI. The international System of units based on which the Sun and solar system formed. the meter (m), the second (s), and the kilogram (kg). solar system . The Sun, planets, and their satellites, asteroids, comets and related objects sidereal clock. A clock that measurers sidereal that orbit the Sun. time. solar transient. A short lived eruption that sidereal day. The interval between successive moves rapidly outwards through the solar meridian passages of the vernal equinox. corona. sidereal month. The period of the Moon’s solar transit. The passage of an object in front revolution about the Earth with respect to the of the Sun. stars. solar wind. A radial flow of particles (mostly sidereal period. The orbital period of one electrons and protons) from the Sun. object about another with respect to the stars. solstice. Either of two points along the ecliptic sidereal time . Time reckoned by the location at which the sun reaches its maximum distance of the vernal equinox. north or south of the celestial equator. south celestial pole. The point directly above star burst galaxy . A galaxy that is the Earth’s south pole where the Earth’s axis experiencing an exceptionally high rate of star of rotation, if extended, would intersect the formation. celestial sphere. stellar association. A loose grouping of southern lights. Aurorae; aurorae australis. young stars. space-time. A four dimensional combination of stellar evolution . The changes in size, the three dimensions of space and time luminosity, temperature, and so forth that . occur as a star ages . special theory of relativity. A description of the mechanics and electromagnetic theory stony iron meteorite. A meteorite composed formulated by Albert Einstein, which explains of both stone and iron. that measurements of distance, time, and mass are affected by the observer’s motion. stony meteorite. A meteorite composed of stone . spectral analysis. The identification of chemical substances from the patterns of lines summer solstice. The point on the ecliptic in their spectra. where the Sun is farthest north of the celestial equator. spectral class/type. A classification of stars according to the appearance of their spectra. Sun. A star about which the Earth and other planets revolve . spectral line. In a spectrum, an absorption or emission feature that is at a particular sun-grazing comet. A comet that passes quite wavelength. near to the Sun. spectrogram. The photograph of a spectrum. sun spot. A temporary cool region in the solar spectrograph. An instrument for photosphere. photographing a spectrum. sunspot cycle. The semiregular 11 year period spectroheliograph. An instrument for with which the number of sunspots fluctuates . photographing the Sun in the monochromatic light of one spectral line . sunspot maximum/minimum. The time during the sun spot cycle when the number of spectroscope . An instrument for directly sunspots is highest/lowest. viewing a spectrum. super cluster. A collection of clusters of spectroscopic binary. A binary star whose galaxies. binary nature is deduced from the periodic Doppler shifting of lines in its spectrum. super giant. A very large, extremely luminous star; stars of luminosity class 1. spectrum. The result of dispersing a beam of electromagnetic radiation so that components superior conjunction. The configuration of a with different wavelengths are separated in planet being behind the Sun as viewed from space. Earth. spiral arms. Lanes of interstellar gas, and superior planet. A planet that is more distant dust, and young stars that wind outwards in a from the sun than the Earth is . plane from the central regions of a galaxy. supermassive black hole. A black hole with a spiral galaxy. A flattened, rotating galaxy mass in the range of a million to a billion solar with pinwheel like spiral arms winding masses. outwards from the galaxy’s nucleus. supernova. A stellar outburst during which a standard candle. An astronomical object of star suddenly increases its brightness roughly a known intrinsic brightness that can be used to million fold. determine extragalactic distances. Supernova explosion. The detonation of a Star. A self-luminous sphere of gas. supernova. supernova remnant. The gasses ejected by a thermonuclear reaction. A reaction resulting super nova. from the high speed collision of nuclear particles that are moving rapidly because they synchronous rotation The rotation of a body are at a high temperature. with a period equal to its orbital period . third quarter moon. The phase of the moon synodic month. The period of revolution of that occurs when the Moon is 90° west of the the Moon with respect to the Sun.; the length Sun; last quarter Moon. of one cycle of lunar phases . time zone. A region on the Earth where, by synodic period. The period between agreement, all clocks have the same time. successive occurrences of the same configuration of a planet. total eclipse. A solar eclipse during which the Sun is completely hidden by the Moon, or a T. lunar eclipse during which the Moon is tail (of a comet). Gas and dust particles from a completely immersed in the Earth’s umbra. comet’s nucleus that have been swept away from the comet’s head by radiation pressure of total lunar eclipse. A lunar eclipse during sunlight and the solar wind. which the Moon is completely immersed in the Earth’s umbra. tangential velocity. That portion of an object’s velocity perpendicular to the line of total solar eclipse. A solar eclipse during sight. which the Sun is completely hidden by the Moon. tektites. Rounded glassy objects believed to have a meteoritic origin . transit. The passage of a celestial body across the meridian; the passage of a small object in telescope. An instrument for viewing remote front of a larger object. objects. transmission grating . A diffraction grating temperature Kelvin. Absolute temperature that produces a spectrum when light is shone measured in units (kelvin abbreviated “K”) through it . equivalent to the degree Celsius. Trojan asteroid. One of several asteroids that terminator. The line dividing day and night share Jupiter’s orbit about the sun. on the surface of the Moon or a planet; the line of sunset or sunrise . Type l Cepheid. A metal rich Cepheid variable . terra. Cratered lunar highlands. Type l Seyfert galaxy. A Seyfert galaxy terrestrial planet. Mercury, Venus, Earth, and whose Balmer lines are significantly broader Mars; the classification sometimes also than any other of its other emission lines. includes the Galilian satellites and Pluto. Type ll Cepheid. A metal poor Cepheid theory of everything. (TOE). A supergrand variable . unified theory that completely describes all particles and forces as well as the structure of Type ll Seyfert galaxy. A Setfert galaxy space and time. whose Balmer lines have about the same width as its other emission lines. thermal energy. The energy associated with heat stemming from the motions of atoms or U. molecules in a substance. UBV filters. Three coloured filters that are transparent to ultra violet (U), blue (B), and thermodynamics. The branch of physics visible (V) light. dealing with heat and the transfer of heat between bodies. UBV system. A system of stellar magnitude involving measurements of starlight intensity thermonuclear fusion. The combining of in the ultra violet, blue, and visible spectral nuclei under conditions of high temperature in regions. a process that release substantial energy. Ultra violet radiation. Electromagnetic radiation of wavelengths shorter than those of wavelength. The distance between two visible light but longer than those of X rays. successive wave crests. umbra. The central, completely dark portion waxing crescent moon. The phase of the of a shadow. Moon that occurs between new moon and the first quarter. umbra (of a sunspot). The dark, central region of a sunspot. waxing gibbous moon. The phase of the moon that occurs between first quarter and full unbounded universe. A universe throughout moon. which the average density is less than the critical density. white dwarf. A low mass star that has exhausted all its thermonuclear fuel and universal law of gravitation. A formula contracted to a size roughly equal to the size of derived by Isaac Newton. That expresses the the Earth. strength of the force of gravity that two masses exert on each other. white hole. A black hole from which matter and radiation emerge. universal time (UT). Local mean time at the prime meridian. white oval . A round whitish feature usually seen in Jupiter’s southern hemisphere. universe. All space, along with all the matter and radiation in space. WIMP. A hypothetical massive particle proposed to explain the low neutrino flux from V. the sun Van Allen belts. Two doughnut-shaped regions around the Earth where many charged Wolf-Rayet star. A class of very hot stars that particles (protons and electrons) are trapped by eject shells of gas at high velocity . the Earth’s magnetic field. wormhole. A speculative, topological feature variable star. A star whose luminosity varies. of a black hole that connects our universe with another universe. velocity. The speed and direction with which an object moves. X. X rays. Electromagnetic radiation whose vernal equinox. The point on the ecliptic wavelength is between that of ultraviolet and where the sun crosses the celestial equator gamma rays. from south to north. Y. very-long-baseline interferometry (VLBI). Year (yr). The period of revolution of the A method of connecting widely separated Earth about the sun. radio telescopes to make observations of very high resolution. Z. ZAMS. Zero-age main sequence. visible light. Photons detectable by the human eye. zenith The point on the celestial sphere opposite to the direction of gravity. void. A large volume of space, typically 100 to 400 million light years in diameter, that Zero-age main sequence. The main sequence contains few galaxies. of young stars that have just begun to burn hydrogen at their cores. W. waning crescent moon. The phase of the zodiac. A band of constellations around the moon that occurs between third quarter and sky centred on the ecliptic. new Moon. zone. A light coloured band in Jupiter’s waning gibbous moon. The phase of the atmosphere. moon that occurs between full moon and third quarter. Prepared by Frank Gear F.R.A.S.