AstroGlossary - A
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
absolute magnitude
The apparent magnitude that a given star would have if that star were placed 10 parsecs, or 32.6 light years, away from an observer. Absolute magnitude is a measure of the true brightness of an object. See also: parsec, starabsorption line
Dark narrow lines that represent absorption of energy at a particular wavelength of light. Each element on the periodic chart absorbs energy at specific wavelengths of light; this unique pattern looks like a bar code. See also: wavelengthacceleration
The rate of change in velocity of an object. accretion disk
A swirling disk of gas and dust orbiting a star or black hole. The material within the disk may generate heat from friction; the hottest accretion disks produce enormous amounts of X-rays. See also: staractive galaxy
A galaxy that emits more energy than sum of all its stars. These galaxies emit energy at wavelengths across the electromagnetic spectrum, from radio to X rays, and they have powerful, concentrated energy sources at their cores. Three types of active galaxies are Seyfert Galaxies, quasars, and blazars. Their energy comes disks of superhot gas that encircle supermassive black holes in the galaxies' cores. Magnetic fields shoot some of the material back into space as high-speed "jets," which produce radio waves as they ram into the gas around the black holes. altitude
The measurement, usually in degrees, of an object's apparent height above the horizon. An object on the horizon has an altitude of 0 degrees, and an object at zenith has an altitude of 90 degrees. Altitude and azimuth together are one of several types of coordinate systems that astronomers use to locate astronomical objects in the sky. See also: Andromeda Galaxy
The closest large galaxy to the Milky Way, and the most distant object that is readily visible to the unaided eye. The Andromeda Galaxy is about 2.5 million light-years away, in the constellation Andromeda, the princess or chained maiden. Also known as M31, the galaxy is a grand spiral, like the Milky Way. It is about 125,000 light-years in diameter, which is larger than the Milky Way, but it is less massive. angstrom
A unit of length used by astronomers and physicists to measure the wavelength of light. One angstrom equals 0.0000000001 meter, about the size of an atom. annular eclipse
An eclipse of the Sun in which the Moon is slightly farther away from the Earth than in a total solar eclipse, so it appears too small to completely cover the Sun's disk. The result is a ring of light (an annulus) around the Moon at the peak of the eclipse. See also: antimatter
A form of matter that is composed of particles that exhibit opposite properties from particles of normal matter. As an example, positrons are the antimatter particles that correspond to electrons, which are made of regular matter. A positron is simply an electron with a positive, rather than negative, charge. When matter and antimatter particles encounter each other, they annihilate to become energy according to Einstein's famous equation E=mc^2. Pairs of matter and antimatter particles can also be created from energy. Most of the matter in the universe is normal matter; there has been very little antimatter present in the universe since the Big Bang. See also: aphelion
The point of greatest separation between a planet or other body and the Sun. See also: apogee
The point of greatest separation between Earth and the Moon or an orbiting satellite. See also: apparent magnitude
The magnitude of a star or other celestial body as measured from Earth. Apparent magnitude depends upon the instrinsic brightness of the object and on its distance; that is, near-by objects appear brighter than more distant objects of the same intrinsic brightness. See also: Arctic Circle
66.5 degrees north latitude, 23.5 degrees latitude south of the Earth's north pole. Above this latitude, the summer sun never sets, and the winter sun never rises. asteroid
A small, irregularly shaped object made of metal (iron and nickel), rocky materials, or a combination of the two that orbits the Sun or another planet. In our solar system, most of the asteroids orbit in a broad band between the orbits of Mars and Jupiter. The largest is Ceres, which is about 600 miles (900 km) in diameter. asteroid belt
A ring of asteroids containing perhaps thousands of big rocks several miles in diameter, and millions of smaller chunks. The asteroid belt is between 2 and 3.5 AU from the Sun, between the orbits of Mars and Jupiter. astrology
The art of studying the relative positions of the Sun, Moon, planets, and constellations in the zodiac to explain and predict events on Earth. Not to be confused with astronomy. Astronomical Almanac
A book of astronomical facts and tables published annually by the United States Naval Observatory. Astronomers use the information, for example, to locate stars, planets, and asteroids, and to forecast eclipses and lunar phases. astronomical unit (AU)
A unit of length used by astronomers to measure distances in space. One astronomical unit, or AU, is equal to the average distance between the Earth and Sun, which is 92,955,806 miles or 149,597,870 kilometers. astronomy
A scientific discipline devoted to the study of the non-terrestrial universe. Not to be confused with astrology. atmosphere
The layers of gas that are gravitationally bound above the surface of a planet, moon, or outer layers of a star. atom
The smallest part of an element of matter that retains the basic characteristics of the element. An atom consists of a tightly packed nucleus composed of protons and neutrons surrounded by a "cloud" of electrons that arrange themselves in an orderly pattern according to the rules of quantum mechanics. Atoms with equal numbers of protons and electrons are electrically neutral; ions are simply atoms that contain more or fewer electrons than protons. Atoms are very small -- much smaller than the wavelength of visible light. The simplest and most abundant atom in the universe is hydrogen, which contains one proton and one electron. aurora
Beautiful ribbons of light caused by the interaction of high-energy particles in the solar wind and Earth's magnetic field. These are common in both extreme northern (aurora borealis or northern lights) and southern latitudes (aurora australis/southern lights), near Earth's magnetic poles. azimuth
The measurement, usually in degrees, of an object's apparent angular separation from north. An object that lies due north has an azimuth of 0 degrees, an object that lies due east has an azimuth of 90 degrees, one that lies due south has an azimuth of 180 degrees, and one that lies due west has an azimuth of 270 degrees. Altitude and azimuth together are one of several types of coordinate systems that astronomers use to locate astronomical objects in the sky. See also:
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