Astronomy Encyclopedia

Astronomy Encyclopedia

If you would like to prepare for school subjects or simply increase your general knowledge, then enjoy our astronomy encyclopedia. We tried to focus only on very important terms and definitions. We also kept our terminology very brief so that you absorb the concept more quickly and easily.

Astronomy Glossary (Page 1)


21 CENTIMETER EMISSION: A radio wavelength emission that originates with a neutral hydrogen atom, i.e., a single proton, or hydrogen nucleus, with its accompanying electron. The proton and the electron each have a quantum "spin", which points either "up" or "down." They can both be "up" or "down" (Parallel spins), or one can be "up" and the other "down" (antiparallel spins). The antiparallel state has slightly less energy than the parallel state, so if an atom in the parallel state changes to antiparallel, a 21 cm radio photon is emitted. Thus, cold neutral hydrogen in space emits this radiation which can be detected using a radio telescope.
ABLATION: A process by where the atmosphere melts away and removes the surface material of an incoming meteorite.
ABSOLUTE MAGNITUDE: The brightness a star would be as seen from a distance of 10 parsecs.
ABSOLUTE ZERO: The lowest possible temperature, at which substances contain no heat energy, and atomic movement has stopped.
ABSORPTION LINES: Dark lines superimposed over a bright continuous spectrum background, created when a cooler gas absorbs photons from a hotter source.
ACCRETION: Accumulation of dust and gas into larger bodies such as stars, planets, and moons.
ACCRETION DISK: A disk of hot, glowing matter spiraling into a black hole.
ACHONDRITE: A stone meteorite that contains no chondrules.
ACTIVE GALACTIC NUCLEUS (AGN): The central region of an active galaxy where the energtic activity is concentrated. The active galactic nuclei are believed to contain supermassive black holes that power the nonstellar phenomena associated with active galaxies.
ACTIVE GALAXY: A galaxy under-going a violent outburst in its central regions.
ADAPTIVE OPTICS: The technology that allows, based on a laser beam aimed through the atmosphere, a computer to make very slight modifications to a telescope's mirror, which will correct for atmospheric distortions.
ALBEDO: The reflecting power of a non-luminous body; a perfect reflector would have an albedo of 100 per cent.
ALBEDO FEATURE: A dark or light marking on the surface of an object that may not be a geological or topographical feature.
ALTAZIMUTH MOUNTING: A telescope mounting that swings from side to side parallel to the horizon, and up and down.
ALTITUDE: The angular distance from the observer's horizon, usually taken to be that horizon that is unobstructed by natural or artificial features (such as mountains or buildings), measured directly up from the horizon toward the zenith; positive numbers indicate values of altitude above the horizon, and negative numbers indicate below the horizon - with negative numbers usually being used in terms of how far below the horizon the sun is situated at a given time [for example, the boundary between civil twilight and nautical twilight is when the sun is at altitude -6 degrees].
ANGSTROM UNIT: The hundred-millionth part of a centimeter (10-10 m).
ANGULAR MOMENTUM: Angular momentum is a measure of the rotational property of motion. It is defined in terms of the motion of a body with respect to some point in space and the angle between the direction of the motion and the direction toward that defining point. An important principle of physics is the "conservation of angular momentum" which means that the angular momentum of a system (the momentum of rotation about a point) remains the same as long as no external torque acts.
ANNIHILATION: Total destruction of matter in a burst of energy.
ANTHROPIC PRINCIPLE: We see the universe the way it is because if it were different we would not be here to observe it.
ANTIGRAVITY: A gravitational field that repels, rather than attracts, matter and light rays.
ANTIMATTER: The exact opposite of matter; when matter meets the tiny amount of antimatter in the universe, the two annihilate each other (see antiparticle).
ANTIPARTICLE: An atomic particle that has exactly the opposite properties of its counter-part (e.g. A positron and an electron).
ANTIPODAL POINT: The point that is directly on the opposite side of the planet.
ANTONIADI SCALE: A roman numeral indicates the quality of seeing according to the following scale:.
APASTRON: The point of greatest separation of two stars, such as in a binary star system.
APERTURE: The diameter of an opening through which light passes in an optical instrument.
APHELION: The position of a planet (or other body) when it is at its furthest from the sun.
APOGEE: The point in the orbit of the Moon or other satellite where it is farthest from the Earth.
APPARENT MAGNITUDE: The brightness of an object as seen from Earth.
ARC MINUTES: There are 60 minutes (denoted as 60') of arc in 1 degree. In the sky, with an unobstructed horizon (as on the ocean), one can see about 180 degrees of sky at once, and there are 90 degrees from the true horizon to the zenith. The full moon is about 30' (30 arc minutes) across, or half a degree. There are 60 seconds (denoted 60") of arc in one minute of arc.
ARCUATE: Having the form of a bow; curved; arc-shaped.
ASTEROID: A small rocky object orbiting the sun, less than 1,000 kilometers in diameter.
ASTROCHEMISTRY: The branch of science that explores the chemical interactions between dust and gas interspersed between the stars.
ASTROMETRY: The careful, precise measurement of astronomical objects, usually made with respect to standard catalogues of star positions. For comet orbit computations, astrometry good to 1" or 2" (1 or 2 arc seconds), or better, is the standard nowadays.
ASTRONOMICAL UNIT: A unit of distance equal to the average spacing between the Earth and the Sun. Usually abbreviated A.U., it is equal to about 150 million kilometers (93 million miles), and is a distance that light takes about 8 minutes to cover. It is a handy size for use for expressing distances in the solar system. For example, the diameter of the orbit of the most distant planet, Pluto, is about 80 A.U.
ASYMPTOTIC GIANT BRANCH: The part of the HR diagram (in the upper right hand corner) where stars move to after Helium burning ceases in their cores. The carbon core of the star shrinks, the outer layers expand, and the star becomes a large red giant.
ATMOSPHERE: The layer of gases enveloping a celestial object.
ATOM: The smallest part of an element that can take part in a chemical reaction; most of the mass of the an atom is concentrated in its nucleus, which is about .000000000001 meters (.01 angstroms) across.
AU: See Astronomical Unit.
AURORA: Curtains and arks of light in the sky visible over middle and high latitudes; they are caused by particles from the sun hitting the Earth's atmosphere and causing some of its gases to glow.
AURORA AUSTRALIS: Also known as the southern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the southern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Borealis in the northern hemisphere.
AURORA BOREALIS: Also known as the northern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the northern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Australis in the southern hemisphere.
AXIS: The imaginary line through the center of a planet, star, or galaxy around which it rotates; also, a similar line through a telescope mounting.
AZIMUTH: Angular distance measured clockwise around the observer's horizon in units of degrees; astronomers usually take north to be 0 degrees, east to be 90 degrees, south to be 180 degrees, and west to be 270 degrees.
BAR: Equals 0.987 atmosphere = 1.02 kg/cm2= 100 kilopascal = 14.5 lbs/inch2.
BARYCENTER: The center of mass of a system of bodies, such as the solar system. When a comet, for example, is well outside the orbit of Neptune (the farthest major planet), it sees the sun and major planets essentially as a single object of summed mass, and the center of this mass (called the barycenter of the solar system) is offset somewhat from the sun; "original" and "future" orbits of long-period comets are computed for this barycenter, while perturbed, osculating orbits of currently-observed objects in the inner solar system are computed for heliocentric orbits.
BARYCENTRIC DYNAMICAL TIME (TDB): Differing from TDT only via periodic variations, TDB is used in ephemerides and equations of motion that refer to the barycenter of the solar system.
BARYON: Baryons are composed of three quarks; they include protons and neutrons.
BESSELIAN YEAR: A quantity introduced by F. W. Bessel in the nineteenth century that has been used into the twentieth century. Bessel introduced a system whereby it would be convenient to identify any instant of time by giving the year and the decimal fraction of the year to a few places, but the starting time of the year was not convenient for dynamical studies that utilize Julian dates (see definition for Julian date), differing by 0.5 day, and the Besselian year varies slowly. The recent change to Julian year usage in dynamical astronomy (and the J2000.0 equinox) took effect in solar-system ephemerides of the Minor Planet Center and Central Bureau for Astronomical Telegrams on Jan. 1, 1992. (See Julian year.).
BIG BANG: The violent event that gave birth to our universe.
BIG CRUNCH: The ultimate collapse of the universe that may take place in the future if the universe starts to contract.
BINARY: A system of two stars that revolve around a common center of gravity.
BINARY STAR: A system of two stars, orbiting around one another. Binary (and triple and even higher multiples) stars are very common; astronomers estimate that about half of all stars are members of multiple-star systems. The nearest star to our solar system, Alpha Centauri, is actually our nearest example of a multiple star system - it consists of three stars - two very similar to our Sun and one dim, small, red star - orbiting around one another.
BINARY SYSTEM: System of two stars that orbit around each other.
BL LAC OBJECT (ALSO BLAZAR): A type of active galaxy characterized by very rapid (day to day) variability by large percentages in total luminosity, no emission lines, strong nonthermal radiation, and starlike appearance. A BL Lac object is a radio galaxy aligned so that we are looking down the jet into the very heart of the system, right into the nucleus. Since we are looking right along the jet we see very rapid, highly luminous radiation.
BLACK DWARF: A dead star with a maximum possible mass of 1.4 solar masses that has cooled to a point where it no longer glows with residual heat.
BLACK HOLE: A collapsed object with such strong gravity that nothing can escape it; as a result, the object is black, and it is a hole because nothing can escape from it.
BLACKBODY: An ideal object that is a perfect absorber of light (hence the name since it would appear completely black if it were cold), and also a perfect emitter of light. Light is emitted by solid objects because those objects are composed of atoms and molecules which can emit and absorb light. They emit light because they are wiggling around due to their heat content (thermal energy). So a blackbody emits a certain spectrum of light that depends only on its temperature. The higher the temperature, the more light energy is emitted and the higher the frequency (shorter the wavelength) of the peak of the spectrum.
BLAZAR: A type of active galaxy that is angled in such a way to us that we look almost directly at its accretion disk and jet.
BLUE SHIFT: A shift in the wave-length of radiation emitted by an object when it is approaching us; the Doppler shift makes the wave fronts bunch closer together, causing the light to appear of a shorter wavelength, and hence bluer.
BLUESHIFT: A shift in the lines of an object's spectrum toward the blue end. Blueshift indicates that an object is moving toward the observer. The larger the blueshift, the faster the object is moving.
BOLIDE: A fireball that produces a sonic boom.
BOLOMETRIC MAGNITUDE: The magnitude that a star would have if all of its energetic emissions were included in the measurement. For example when you look at a star and observe its brightness with your eye, your eye is only detecting the brightness from the visible portion of the spectrum. If your eye could see into the ultraviolet the star might appear much brighter. The bolometric magnitude is the estimate of how bright the star would be if all of its light, from the entire spectrum, was observed.
BRIGHTNESS: Like flux brightness is energy per unit time per unit area (e.g. Ergs per second per square centimeter). How bright something appears to you depends on how much energy (light) it is giving off per second, and how spread out it is over your viewing area. A certain amount of light energy will appear much brighter if concentrated into a small region of emission than when spread out over a large emission region. A tiny lightbulb can seem very bright, even when its total light is small. The apparent brightness of a star is called the apparent magnitude and that is what is measured by a telescope:, how much energy does the star put into the telescope's collecting area per second.
BROWN DWARF: A "failed star" in the sense that when it was finished forming, it did not have enough mass to begin fusion; it does not shine as a star does, but can generate heat through very slow gravitational contraction - this works because when a gas is compressed, it gains temperature.
BULGE, GALACTIC: A thick region around the center of the Galaxy that spheroidal in shape, containing warm gas and metal-rich older stars.
CALDERA: A volcanic crater.
CARBONATE: A compound containing carbon and oxygen (e.g. Calcium carbonate AKA limestone).
CASIMIR EFFECT: The attractive pressure between two flat, parallel metal plates placed very near to each other in a vacuum; the pressure is due to a reduction in the usual number of virtual particles in the space between the plates.
CATADIOPTRIC TELESCOPE: A telescope that uses both mirrors and lenses to form and image.
CATENA: A chain of craters.
CAVUS: A hollow, irregular depression.
CCD: Charge-coupled device, a very sensitive electronic device that is revolutionizing astronomy in the 1990s. CCD cameras are composed of silicon chips that are sensitive to light, changing detected photons of light into electronic signals that can then be used to make images of astronomical objects or to analyze how much light is being received from such objects. Ccds require computers for reduction of data, so the expense can be much greater than for, say, photography - but ccds can detect much fainter objects than can photographs. Unfiltered ccds tend to be more red-sensitive than the human eye.
CELESTIAL EQUATOR: The imaginary line encircling the sky midway between the two celestial poles.
CELESTIAL POLES: The imaginary points on the sky where Earth's rotation axis, extended infinitely, would touch the imaginary celestial sphere.
CELESTIAL SPHERE: The imaginary sphere enveloping the Earth upon which the stars, galaxies, and other celestial objects all appear to lie.
CEPHEID VARIABLE: A variable star of short period; the fluctuations are regular and are linked with its real luminosity; the longer the period, the more luminous the star.
CEPHEID VARIABLE STARS: A type of luminous giant star whose luminosity varies in a periodic fashion. Cepheids are characterized by a rapid rise in luminosity followed by a slow decline. The period of the cycle is related to the luminosity of the Cepheid by the Period-Luminosity relationship. The more luminous the Cepheid, the longer the period. This property makes Cepheids useful for obtaining distances. Cepheids come in two types, Type I which are metal rich and Type II which are metal poor. Type I Cepheids are more luminous than Type II.
CHANDRASEKHAR LIMIT: The maximum possible mass of a stable cold star, above which it must collapse into a black hole.
CHANDRASEKHAR MASS: The maximum mass, approximately 1.4 solar masses above which an object has too much mass to support itself against collapse by electron degeneracy pressure. Hence, the maximum mass of a white dwarf.
CHAOS: A distinctive area of broken terrain.
CHARGE-COUPLED DEVICE (CCD): A computer-controlled electronic detector that can record an image.
CHASMA: A canyon.
CHONDRITE: A meteorite that contains chondrules.
CHONDRULE: Small, glassy spheres commonly found in meteorites.
CHROMOSPHERE: That part of the sun's atmosphere that lies just above its visible surface, or photosphere.
CIRCUMPOLAR STAR: A star that never sets but always stays above the horizon. This depends on the location of the observer. The further South you go the fewer stars will be circumpolar. Polaris, the North Star, is circumpolar in most of the northern hemisphere.
CIRCUMPOLAR STARS: Stars that never set when seen from a given location.
CIRCUMSTELLAR DISK: A torus or ring-shaped accumulation of gas, dust, or other debris in orbit around a star in different phases of its life cycle.
CLOSED UNIVERSE: A model of the universe that has spherical geometry (hence is finite in space) and which will eventually stop expanding and recollapse (and hence is finite in time as well).
CNO CYCLE: A series of nuclear reactions that convert 4 hydrogen into 1 helium nucleus. The process starts with the capture of a proton (hydrogen nucleus) onto Carbon turning it into Nitrogen. Eventually further reactions turn the Nitrogen into an Oxygen, and the process is completed by the ejection of an alpha particle (helium nucleus) and the return to Carbon. Thus, carbon acts as a catalyst - it is neither destroyed nor created but simply facilitates the H -> He process. The CNO cycle is important only in stars more massive than the Sun.
COLLES: Small hills or knobs.
COLLIMATION: The procedure of aligning a telescope's optics.
COLOR INDEX: The difference in a star's brightness (magnitude) as measured in two different wavelength bands. For example, suppose one of the two bands were centered on red and the other on blue. Suppose blue was larger, taking the difference of blue minus red brightness would give you a number that quickly shows that the star is blue. Since color indices are measured in units of magnitudes their use can be somewhat confusing since the larger the magnitude the fainter the brightness.
COMA: The dust and gas surrounding an active comet's nucleus.
COMET: A small body composed of ices and dust which orbits the sun on an elongated path.
COMPACT RADIO SOURCE: An object emitting radio wavelength emission from a small unresolved region. An example would be the very core of a radio galaxy.
CONDUCTION: Conduction is a process of heat transport through the physical collisions of the particles making up a substance. Similar to electrical conductivity, substances have heat conductivity. Substances with large heat conductivity can transfer heat rapidly (e.g. A hot metal plate). Some substances have low conductivity; they are insulators (e.g., an insulating blanket of foam). Conductivity is an important heat transfer mechanism within white dwarf stars, but not in stars such as the Sun.
CONJUNCTION: The moment when two celestial objects lie closest together in the sky.
CONSERVATION OF ANGULAR MOMENTUM: The law of science that states that momentum must be conserved within a system.

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