asterism — The most prominent pattern of stars Glossary within a constellation; e.g., the Big Dipper is the asterism for the constellation Ursa Major. asteroid belt — The region between the planets AAVSO — The American Association of Mars and Jupiter where the majority of asteroids Variable Star Observers, located in Cambridge, are located. Massachusetts, is an astronomical organization asteroids — Chunks of rocky debris (sometimes specializing in variable stars. It maintains the called “minor planets”) which are mostly largest variable star database in the world. contained within the asteroid belt. absolute error —See percentage error. astrometry — The branch of astronomy which absolute magnitude — The apparent brightness measures the positions of celestial bodies in right of a star computed as if placed at a standard ascension and declination coordinates. distance of 10 parsecs from the Earth. See also astronomical unit — The value of one luminosity. astronomical unit (AU) is the distance between absorption lines — Dark lines which appear in the Earth and the Sun. a continuous spectrum where light with specific average deviation — A mathematical method of wavelengths has been removed, or absorbed. assessing the spread, or range, of a set of values accretion disk — Material from the atmosphere or of a data set. of a star which spirals around the surface of a azimuth — A coordinate system to measure more dense companion in a binary star system, angular distances along the horizon, with North forming a disk. as the zero point. accuracy — How closely a measurement agrees Balmer series — Hydrogen emission and with the true or accepted value of the quantity absorption lines that fall in the visible part of the being measured. spectrum. actual brightness —The absolute magnitude of barycenter — The common center of mass a star (see above). around which two gravitationally-bound objects alias, aliases — A false period which seems to orbit. be significant in a period search. bell curve — See normal curve. amplitude — The difference between the bin, bin value — A subset of values, useful in maximum and minimum brightness in a light determining the frequency with which a particular curve. value appears within a set of values. antisolar point — A straight line from the Sun, binary system — A system of two or more through an observer’s eyes, to the center point, gravitationally-associated stars in orbit around or radius, of the arc of a rainbow. their barycenter. apogee — The point in the Moon’s orbit at black body — A hypothetical ideal radiator which it is at its greatest distance from Earth. which absorbs all radiation without reflecting or apparent brightness — The brightness that a transmitting any of it, and then radiates it all star appears to have for an observer on Earth; away. same as apparent magnitude. black body radiation — The continuous apparent magnitude —The brightness that a distribution of wavelengths of thermal radiation star appears to have for an observer on Earth. emitted from a black body at a particular temperature. apsidal motion — The rotation of the major axis of the elliptical orbit of stars in a binary star black dwarf — The cold carbon core left after a system, such that there is a gradual shift of the white dwarf radiates all of its energy into space. point where the two stars reach their closest approach to each other. black hole — The end product of the death of constellation — Organized patterns of stars in the most massive stars, leaving a region of space the night sky. so gravitationally compact that light cannot continuous spectrum — A spectrum in which escape. the electromagnetic radiation is distributed over bright line spectrum — See emission all frequencies. spectrum. convective zone — The zone beneath the brown dwarf — A dim object about 80 times photosphere of the Sun where convection more massive than Jupiter, but not massive currents transport the energy produced by nuclear enough for continuous fusion to occur within its fusion towards the surface. core. cosmology — The study of the origin, evolution, cataclysmic variable — A binary system large-scale structure, and possible futures of the consisting of a Sun-like or larger star and a white universe. dwarf. Matter from the larger star accretes onto a cycle — The time interval required for a disk surrounding the gravitationally-stronger particular behavior to be completed once. white dwarf. Instabilities in the accretion disk cause eruptions, which appear as visible declination — The extension of the coordinate brightenings to observers on Earth. See also of latitude on Earth to the celestial sphere. It is accretion disk, and eruptive variable. measured in degrees, the same as latitude on Earth. celestial sphere — An imaginary transparent hollow sphere centered on the Earth with Earth’s dispersion — The separation of a beam of light coordinate system of longitude and latitude into its component colors, i.e., component extended outward and superimposed onto the wavelengths, so that a spectrum is formed. sphere (right ascension and declination, distance modulus — A mathematical respectively). relationship among the absolute magnitude, Cepheid variable — A pulsating variable star apparent magnitude, and distance of a star. with a period from 1 to 70 days, and an double blind — An experiment in which amplitude of light variation from 0.1 to 2.0 neither the test subject nor the scientist recording magnitudes. Cepheids have high luminosity, and data knows which experiment is being performed are of F spectral class at maximum and G to K at on that test subject. minimum. Cepheids obey the period-luminosity relationship dwarf nova — A cataclysmic variable that has eruptions at intervals of 10 to several hundreds of clusters — Groups of galaxies which are days, resulting in light increases of 2 to 6 gravitationally associated. The Milky Way magnitudes. Galaxy is one of ~24 galaxies which belong to the Local Group cluster. eclipsing binary — Binary system of stars with an orbital plane lying near the line of sight of an comets — Small bodies of ice and dust travelling observer on Earth. The components periodically in an elliptical orbit about the Sun. As they near eclipse each other, causing a decrease in the the Sun, they begin to vaporize, thus forming apparent brightness of the system as seen by the extended tails of ions and dust. observer. The period of the eclipse, which comparison stars — Stars of known magnitude coincides with the orbital period of the system, which are used to estimate the varying brightness can range from minutes to years. of a nearby variable star. ecliptic — The apparent path of the Sun. It is composite-spectrum binary — A system of two represented on the celestial sphere as a dotted line o stars so close together that their individual which extends 23½ into the northern hemisphere o features can be revealed only by spectroscopic and 23½ into the southern hemisphere. It is a o analysis. Also called spectroscopic binary. reflection of the 23½ tilt of the Earth’s axis. conjunction — The alignment of two or more celestial bodies in the Solar System so that they have the same longitude as seen from the Earth. effective temperature — The surface temperature finder charts — Star maps used to locate fields of a star, expressed as the temperature of a black of variable stars. body having the same radius as the star and folded light curve — A plot of magnitude as a radiating the same total amount of energy per function of phase, rather than as a function of unit area per second. time. For a periodic light curve, this allows electromagnetic radiation (electromagnetic successive cycles to be “folded” on top of each spectrum) — The entire spectrum of radiation other. See also phase diagram. from radio waves to gamma rays, which consists galaxy — A gravitationally-bound conglomerate of alternating electric and magnetic energy fields of stars, dust, and gas. that transfer energy and information without a medium. General Catalogue of Variable Stars (GCVS) — A catalogue which lists the relevant electron degeneracy pressure — The repulsive information pertaining to all known variable force between electrons which keeps white dwarfs stars. in equilibrium and prevents further gravitational general relativity — A theory of gravitation collapse. which concludes that gravitational fields change emission lines — Bright lines in specific the geometry of spacetime, causing it to become locations of the spectra of radiating materials, curved. The curvature of spacetime controls the corresponding to the emission of light at specific natural motions of bodies. Matter affects how wavelengths and frequencies. spacetime curves, and spacetime affects how matter moves. emission spectrum — The pattern of spectral lines produced by an element. globular cluster — Tightly-bound spherical groups of hundreds of thousands of stars which envelope — A band on a graph which encloses reside in galactic halos. the error bars and represents 68% of the graphed data. gnomon [“ˈnō mon”] — A vertical shaft whose ephemeris (plural: ephemerides) — A list of shadow is used to measure the altitude of the Sun predicted positions of the Sun, Moon, and to determine the time of day, and the day of the planets, as well as information relating to times year. of maxima or minima of variable stars. Greenwich Mean Astronomical Time (GMAT) epoch — A precise instant that can be used as a A time-keeping system used by astronomers, in fixed reference point of time, such as a time of which each day begins at 12 noon in Greenwich maximum magnitude for a variable star. [“ˈgren ich”], England (at 0° longitude). error bar — A line drawn on a graph to ground state — The first orbital level, or lowest represent the range of error for a data point. energy state, of an electron in orbit around an atomic nucleus. eruptive variable — A star whose variability is caused by eruptions in the star or stellar system, H-R diagram (Hertzsprung-Russell diagram) i.e., supernovae, novae, recurrent novae, dwarf — A stellar plot of luminosity or absolute novae, and symbiotic stars.