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Dictionary of Astronomy Terms


Accretion Disks - A disk of interstellar material surrounding a celestial object with an intense gravitational field, such as a black hole.

Asteroids - Any of numerous small celestial bodies that revolve around the sun, with orbits lying chiefly between Mars and Jupiter and characteristic diameters between a few and several hundred kilometers.

Astrology - The study of the positions and aspects of celestial bodies in the belief that they have an influence on the course of natural earthly occurrences and human affairs.

Astronomy - The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena.

Astronomer - One who specializes in astronomy.

Atmosphere - The gaseous mass or envelope surrounding a celestial body, especially the one surrounding the earth, and retained by the celestial body's gravitational field.

Black Hole - An area of space-time with a gravitational field so intense that its escape velocity is equal to or exceeds the speed of light.

Brown Dwarf - A celestial body that resembles a star but does not emit light because it is too small to ignite internal nuclear fusion. The planet Jupiter is a small brown dwarf.

Celestial Equator - A great circle on the celestial sphere in the same plane as the earth's equator.

Celestial Sphere - An imaginary sphere of infinite extent with the earth at its center on which the stars, planets, and other heavenly bodies appear to be located.

Comets - A celestial body, observed only in that part of its orbit that is relatively close to the sun, having a head consisting of a solid nucleus surrounded by a nebulous coma up to 2.4 million kilometers (1.5 million miles) in diameter and an elongated curved vapor tail arising from the coma when sufficiently close to the sun. Comets are thought to consist chiefly of ammonia, methane, carbon dioxide, and water.

Contraction - To be reducde in size by drawing together; to shrink.

Convection - Heat transfer in a gas or liquid by the circulation of currents from one region to another.

Core - The central or innermost part; for example, of planets or stars.

Corona - faintly colored luminous ring appearing to surround a celestial body visible through a haze or thin cloud, especially such a ring around the moon or sun, caused by diffraction of light from suspended matter in the intervening medium.

Ecliptic - The intersection plane of the earth's orbit with the celestial sphere, along which the sun appears to move as viewed from the earth.

Ellipse - a plane curve, The locus of points for which the sum of the distances from each point to two fixed points is equal.

Event Horizon - The region, usually described as spherical, marking the outer boundary of a black hole, inside which the gravitational force is strong enough to prevent matter or radiation from escaping.

Extra Terrestrial - Originating, located, or occurring outside Earth or its atmosphere.

Galactic Center - The center of a galaxy; in the Milky Way Galaxy, the galactic center is thought to be a black hole.

Galaxy - Any of numerous large-scale aggregates of stars, gas, and dust that constitute the universe, containing an average of 100 billion (1011) solar masses and ranging in diameter from 1,500 to 300,000 light-years.

Gravity - The natural force of attraction exerted by a celestial body, such as Earth, upon objects at or near its surface, tending to draw them toward the center of the body.


Horizon - The apparent intersection of the earth and sky as seen by an observer.

Magnetic Field - A condition found in the region around a magnet or an electric current, characterized by the existence of a detectable magnetic force at every point in the region and by the existence of magnetic poles.

Mass - A property of matter equal to the measure of an object's resistance to changes in either the speed or direction of its motion. The mass of an object is not dependent on gravity and therefore is different from but proportional to its weight.

Meteor - A bright trail or streak that appears in the sky when a meteoroid is heated to incandescence by friction with the earth's atmosphere.

Meteoroid - A solid body, moving in space, that is smaller than an asteroid and at least as large as a speck of dust.

Molecular Clouds - a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2).

Moons - A natural satellite revolving around a planet.

Planets - A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

Orbit - The path of a celestial body or an artificial satellite as it revolves around another body.

Planetary Nebula - A nebula, such as the Ring Nebula, consisting of a hot, blue-white, central star surrounded by an envelope of expanding gas.

Protoplanet - the first stage in planetary development.

Protostar - any early stage in the formation of a star when an interstellar cloud of gas and dust starts to collapse but before nuclear synthesis has begun at its core.

Red Giant - A star of great size and brightness that has a relatively low surface temperature.

Satellite - A celestial body that orbits a planet; a moon.

Singularity - A point in space-time at which gravitational forces cause matter to have infinite density and infinitesimal volume, and space and time to become infinitely distorted.

Solar Wind - A stream of high-speed, ionized particles ejected primarily from a star's corona.

Star - A self-luminous celestial body consisting of a mass of gas held together by its own gravity in which the energy generated by nuclear reactions in the interior is balanced by the outflow of energy to the surface, and the inward-directed gravitational forces are balanced by the outward-directed gas and radiation pressures.

Supernova - A rare celestial phenomenon involving the explosion of most of the material in a star, resulting in an extremely bright, short-lived object that emits vast amounts of energy.

Terrestrial - Of or relating to the earth or its inhabitants.

Universe - All matter and energy, including the earth, the galaxies, and the contents of intergalactic space, regarded as a whole.

Wavelength - The distance between one peak or crest of a wave of light, heat, or other energy and the next corresponding peak or crest.

White Dwarf - The remnant of a star that has collapsed, having an extremely dense state with no empty space between its atoms, but not reaching the extremely dense state of a neutron star or black hole.

X-ray Radiation a stream of relatively high-energy photons having wavelengths in the approximate range from 0.01 to 10 nanometers.

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