|
absolute magnitudeThe 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 intrinsic brightness of an object
See also
apparent magnitude
|
absorption line A dark line in a spectrum that represents the absorption of energy at a particular wavelength of light. Each element on the periodic table absorbs energy at unique wavelengths. Astronomers identify which elements are present in an object, and other information about the object, by measuring these absorption lines.
See also
emission line
spectral line
|
accelerationThe rate of change in velocity of an object. |
accretion disk A swirling disk of gas and/or dust orbiting a star or black hole. The material within the disk may generate heat from friction and glow. |
active galaxy A "hyperactive" galaxy that emits more energy than sum of the individual stars within it. These galaxies emit energy at wavelengths across the electromagnetic spectrum, from radio to X-ray, and they have powerful, concentrated, non-thermal energy sources at their cores. Some are also unusually bright at radio, infrared or X-ray wavelengths compared to the Milky Way. Three types of active galaxies are Seyfert Galaxies, quasars, and blazars. Many active galaxies are also radio galaxies. |
alt-az coordintesA system of coordinates that uses altitude and azimuth to locate objects in the night sky. A telescope that uses alt-az coordinates is said to have an alt-az mount. Altitude is an up-down measurement, while azimuth is a left-right measurement.
|
altitudeThe 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
alt-az coordintes
azimuth
|
Andromeda Galaxy The closest spiral galaxy to the Milky Way that is visible in the northern hemisphere, and the most distant object that can be seen with the unaided eye. The Andromeda Galaxy is 2 million light-years away and is located in the constellation Andromeda, the Chained Maiden. |
Angstrom A unit of length used by astronomers and physicists to measure the wavelength of light. One Angstrom equals 0.0000000001 (10^-10) meter, or about the size of an atom. The symbol for Angstrom is Å. |
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
solar eclipse
|
Antarctic CircleOn the Earth, the small circle located at 66.5 degrees south latitude, which is 23.5 degrees latitude north of the Earth's south pole. Below this latitude, the summer Sun never sets, and the winter Sun never rises.
See also
Arctic Circle
|
antimatterA form of matter that is composed of particles that exhibit opposite quantum mechanical 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, the 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
matter
|
aphelion The point of greatest orbital separation between a planet and the Sun. At aphelion, the planet experiences its minimum orbital velocity around the Sun.
See also
orbit
perihelion
|
apogee The point of greatest separation between the Earth and an orbiting satellite, including the Moon. At apogee, the orbiting satellite experiences its minimum orbital velocity around the Earth.
See also
orbit
perigee
|
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
absolute magnitude
|
Arctic Circle On the Earth, the small circle located at 66.5 degrees north latitude, which is 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.
See also
Antarctic Circle
|
asteroidA small, irregularly shaped rock of iron and nickel, many of which are located in the Asteroid Belt. The largest asteroids are Ceres (900-km diameter), Pallas (500-km diameter), and Vesta (500-km diameter). |
Asteroid Belt A ring of asteroids containing perhaps thousands which measure several kilometers in diameter, and millions of which measure several meters in diameter or smaller. The asteroids travel in orbits between 2 and 3.5 A.U. from the Sun, locating them between the orbits of Mars and Jupiter. |
astrologyThe art of studying the relative positions of the Sun, Moon, planets, and constellations in the zodiac in order 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 A unit of length used by astronomers to measure distances in space. One astronomical unit, or A.U., is equal to the average distance between the Earth and Sun, which is 92,955,806 miles or 149,597,870 kilometers. |
astronomyA scientific discipline devoted to the study of the non-terrestrial universe. Not to be confused with astrology. |
atmosphereThe 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 the Earth's magnetic field. These are common near Earth's poles, in both extreme northern latitudes (aurora borealis or Northern Lights) and extreme southern latitudes (aurora australis). |
autumnal equinoxThe equinox that occurs on or near September 21 each year.
See also
equinox
vernal equinox
|
azimuthThe 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
alt-az coordintes
altitude
|
 This program is supported by the National Science Foundation under grant AST 0227870. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.
|
Contact | About | Friends of McDonald | StarDate Online | Sign up for Sky Tips
