Absolute magnitude How bright a star would be in comparison to other stars if all stars were seen at a standard distance of 10 parsecs (about 31 light years) from Earth. Active galaxies Colliding galaxies that produce jet streams of matter and energy. Aerolite Type of meteorite, made up of stony materials such as rocks. Airglow Very dim glow that makes the night sky glow, even when seen from the darkest places on Earth. Caused by weak aurorae. Albedo The amount of sunlight a planet reflects back into space. The moon's albedo is only 7 percent (about the same as blacktop), but Venus's albedo is 80 percent because of its cloud cover. This and its proximity to Earth are the reasons why Venus is so bright. Aldebaran Red giant star in the center of the constellation Taurus, the Bull. A hundred times as luminous as the Sun. Aphelion Point in a planet's orbit at which it is farthest from the Sun. Earth is at aphelion in early July. Andromeda galaxy The nearest spiral galaxy, about 2.2 million light-years away. It contains about 400 billion stars. Visible to the naked eye. Antares Red supergiant star that shines in the constellation of Scorpius. Denotes the heart of the Scorpion. Arcturus Red giant and the brightest star in the constellation of Bootes, the Herdsman. One of the closest stars, at 27 light years. Ariel Moon of Uranus, 720 miles in diameter. Orbits the planet in 2.52 days. Some of the craters on its surface are surrounded by white deposits, like those on our Moon. Aperture The diameter of the lens, or the primary mirror of an optical telescope. Absolute magnitude How bright a star would be in comparison to other stars if all stars Aphrodite terra One of the two "continents" on Venus, named for Aphrodite, the ancient Greek goddess of love.  Apogee The point in a satellite's orbit when it is farthest from the planet it orbits. Apparent magnitude How bright a star appears in comparison to other stars when seen from Earth. Asterisms A grouping of stars. Asteroids Very small planets and even smaller objects made of rock or metal. There are thousands of them in our Solar system, and they mainly orbit the Sun in large numbers between Mars and Jupiter. Some asteroids show up elsewhere in the Solar system as meteoroids and, possibly as captured moons of planets such as Mars. Asteroid celestial Matter that may not necessarily be left over from a planet's formation (as is a planetesimal); asteroids may be parts of planets that were knocked into space by a collision. Asteroid belt A group of asteroids at a certain distance from the Sun, e.g. the belt that orbits the Sun between the orbits of Mars and Jupiter. Astronomical unit CAU A measure used to describe distances within the solar system, equal to the distance from the Earth to the Sun, about 93 million miles. Astronomer A person involved in the scientific study of the Universe and its various bodies. Atmosphere The gases that surround a planet, star, or moon. Aurora Light from the North and South Polar regions caused by the collision of the solar wind with the upper atmosphere. Autumnal Equinox One of two points where the ecliptic crosses the celestial equator. The moment marking the start of autumn. It happens around September 23rd. Equinox means 'equal nights'. See also 'Vernal Equinox' for Spring. Axis The imaginary line through the center of a planet around which the planet rotates. The axis of Mars is tipped so that its seasons change as the planet orbits the Sun. Baryonic matter All known matter in the universe; made from photons, electrons, protons, neutrons, etc. Betelgeuse Red super giant star that shines in Orion. Larger than the orbit of Mars and about 40,000 times as luminous as the Sun. Big bang theory Popular theory of the beginning of the universe that states "all existence began with a vast surge (not really an explosion) of energy and matter from a super dense point, from which all else evolved". Big crunch A nickname for the closed universe model, designating the opposite of the big bang, where the Universe ultimately collapses back to a point. Billion The number represented by 1 followed by nine zeroes: 1,000,000,000. In some places, such as the United Kingdom (Britain), this number is called a "a thousand million. Binary star Two stars in orbit around each other. Black dwarf Dead stars of carbon cores with virtually no remaining helium shell, and no light or heat output. No black dwarf has yet been observed. Black hole A collapsed star of high mass contracted into near zero volume. The gravity near a black hole is so strong that no matter or energy can escape. Brown dwarf An object that is not quite large enough to be star. While it may emit infrared radiation (heat), it does not produce energy by means of nuclear fusion. Callisto Furthest satellite of Jupiter, 3,980 miles (4,800 km) in diameter; orbits the planet in 16.7 days. Canal A waterway made by people to move goods from one place to another. It was once thought that the narrow dark markings on Mars were canals built by Martians to move water from the ice caps to the desert areas. Cannibal galaxies Galaxies that consume smaller galaxies in their proximity. Canopus Giant star that shines in the constellation Carina, the Keel. Much hotter than the Sun and about 400 times as luminous; 74 light-years away.  Capella Yellow giant star that shines in the constellation Auriga, the Charioteer.  Captured rotation Movement of a satellite which keeps one half always facing inward as it orbits the parent planet, such as the Earth's Moon, and many other satellites of the other planets. Carbon dioxide A gas (chemical formula CO2) necessary for plant life. It is a colorless, heavy gas. Carbon dioxide is what gives soda its fizz. And when humans and other animals breathe, they exhale carbon dioxide. Cassegrain A reflecting telescope with a curved, concave secondary mirror that allows for greater flexibility in enlarging and focusing incoming light. Cassini division A gap in Saturn's ring system between the two most prominent rings, rings A and B. A small amateur telescope can usually discern this gap. Celestial bodies Natural objects in space, such as planets, galaxies, comets, and stars; also called heavenly bodies. Celestial globe A spherical map of the night sky as seen from Earth. Celestial horizon The 360 degree encircling line where the sky meets Earth from a stargazer's point of view. Celestial sphere The imaginary dome of the universe surrounding the Earth.  Centaurus A Giant elliptical galaxy, 16 million light-years away. It seems to be erupting.  Cepheids Giant stars, much more luminous than the Sun, which brighten and fade every few days as they swell and shrink. Their extremely regular periods can be used to deduce their distances, even in other galaxies. Ceres The largest asteroid and the first to be discovered. About 600 miles (1,000 km) in diameter. Orbits the Sun every 4.6 Earth years. Charon Pluto's only known satellite, 740 miles (1,200 km) in diameter. Charon is the largest moon in proportion to its parent body. Chiron An object that seems to be intermediate between comets and asteroids. It orbits the Sun between Saturn and Uranus. Chromosphere Lower part of the Sun's atmosphere, about 1,200 miles (2,000 km) thick. This is the part that we see. Circumpolar stars Stars that neither rise nor set, but encircle a celestial pole above a stargazer's celestial horizon. As an example, the Little Dipper is circumpolar for observers in much of the United States. Closed universe model A theory that states that since it has sufficient mass density, the universe will ultimately contract in upon itself through gravitational force, resulting in a so-called 'big crunch'. Colonies Human settlements. Many people have wondered if it might be possible to one day set up colonies on other planets, space stations or spaceships. Coma 1. Hazy head of a comet where gas and solid particles are escaping from the solid nucleus.  2. A distortion that occurs in some low focal-ratio telescopic systems. Comet A frozen mass of primordial gases and rocky debris from space which orbits around the Sun, or sometimes briefly around a planet such as the comet Shoemaker-Levy 9, which orbited Jupiter for a few years before colliding with it in a series of spectacular explosions. Conjunction When two astronomical bodies pass close by each other in the sky. Conjunctions are usually held to occur when 2 bodies have the same celestial longitude (or right ascension), and slightly different celestial latitudes (or declination). An opportunity to observe a major grand conjunction came in the year 2000. This conjunction aligned Mercury to Saturn roughly in a single line. Unfortunately, the Earth was on the other side of Sun. Observers could have seen all the planets in a single location in the sky only during the daytime. Constellations Groups of stars seen from Earth and identified by name, such as Leo, the Big Dipper, and Orion. There are 88 officially recognized constellations, but many people and cultures make up their own. Continents Large land bodies surrounded by water on a planet's surface. Corona Outer atmosphere of the Sun, above the chromosphere; it extends several million miles into space, and can be as hot as 1 Million degrees. The corona is visible from Earth only during a Total Eclipse of the Sun. Cosmology Study of the evolution and structure of the universe. Copernicus, Nicolaus A Polish astronomer who was the first to argue that the Sun, not Earth, was the center of our Solar system and that the planets revolved around the Sun. Core The central part of a celestial body. Earth's core is believed to consist of mainly iron and nickel, while Jupiter's core is believed to be composed of metallic Hydrogen. Crab nebula Expanding cloud of heated gas from an exploding star (supernova). About 3,600 light-years away. The supernova remnant is a pulsar, a star that orbits very rapidly and sends out light beacons into space. This star was so bright when it exploded that it was visible during the day! Crater The depression caused by the impact of a meteorite. Crust The outermost solid layer of the Earth. It includes the surface of Earth. Cygnus A The galaxy that emits the strongest source of radio waves in the entire sky. But, This galaxy is so far away that it appears as a dim speck even with the strongest telescopes. Cygnus X-1 Powerful X-ray source. This radiation is believed to come from gas heated to 18 million degree F (10 million degree C) as it is dragged into a black hole. Dark matter As yet Undetected material that may make up some 90 percent of all matter in the universe. Declination Dec The imaginary parallel horizontal lines on the celestial sphere used along with right ascension to locate the position of celestial objects; corresponds to latitude lines on Earth. Deeps Canyon like valleys on the ocean floor caused by one plate in Earth's crust sliding under another. Deimos Satellite of Mars, measuring about 7 miles by 9 miles (11 km by 15 km) in diameter. Orbits Mars in 1.26 days. Desert Waterless areas on land. Mars is often considered a desert planet. Diameter The distance from a point on the outside of a body, through the center, and back out to a point directly opposite the starting point. Dione Satellite of Saturn with many craters, 700 miles (1,120 km) in diameter, orbiting Saturn every 2.7 days. Double stars Stars that orbit each other. Dwarf star Name given to most ordinary stars. The Sun is a yellow dwarf star. There are other colors of dwarf stars. Earth Third planet from the Sun, at a distance of 92,700,000 miles, measuring 7,926 miles (11,756 km) in diameter. The Earth revolves on its axis once per day, every 24 hours. It takes approximately 365.24 days to orbit the Sun. 75% of the surface is covered with water. Still the only known place in the Universe where life exists. Eclipse To cover or hide. An eclipse occurs when the Moon passes into Earth's shadow (eclipse of the Moon), or when it passes in front of the Sun (eclipse of the Sun). Other bodies in the Solar System, and even binary stars, can eclipse each other. Eclipses may be partial or total. A total solar eclipse occurred on Aug. 11, 1999 in southwestern England, northern France, Hungary, Germany, Romania, and Austria. Ecliptic The path the Sun appears to take around Earth in the sky. In fact, the Earth revolves around the Sun. Einstein, Albert A German-born scientist. His most famous theory is the Theory of Relativity, which among many other things, more accurately predicts the motions of planets than Newtonian mechanics. He is perhaps the best known scientist of the twentieth century. Electromagnetic spectrum The array of energy radiation detected in the universe, separated by frequency and wavelength into (in order of decreasing energy) gamma rays, X rays, ultraviolet rays, light rays, infrared rays, and radio waves. Ellipse The oval shape of the path followed by a planet, satellite, or comet. Ellipses may be almost perfect circles, like the Earth's orbit around the Sun, or very long and narrow, like the orbits of many comets. Elliptical galaxy A galaxy in the shape of an ellipse (an oval anywhere from almost a sphere to almost a flat pancake. Elongation The greatest angular distance a planet appears in the sky east or west of the Sun. Enceladus Satellite of Saturn, about 300 miles (500 km) in diameter; orbits the planet in 1.4 days. Has a smooth, icy surface with few craters. Epoch The specific moment in time depicted on a celestial globe. Most astronomical references now use the epoch of 2000.0. Equator The imaginary line defined by zero latitude. The equator line is tilted from the plane of Earth's orbit because the Earth's axis is tilted from the plane of the solar system by about 23 degrees. Equatorial bulge A spreading out of mass at a celestial body's equator due to the force of the object's rapid rotation. Jupiter shows a slight bulge even in a small telescope. Europa Satellite of Jupiter 1,950 miles (3,138 km) in diameter. It takes 3.6 days to orbit the planet. Europa has a crust of ice and may have a liquid ocean beneath this crust, raising the possibility that life may exist beneath the surface. evaporation The process that turns water into a vapor or gas. Evening Star The name by which Venus has long been known when it appears in the evening sky after sunset. In fact any Planet can appear as the Evening Star. Exosphere The space beyond Earth's atmosphere, i.e. outer space. Extinct No longer living, or no longer active. Both the dinosaurs and inactive volcanoes are said to be extinct.