Descriptionary

big bang the prevailing theoretical model of the cre­ation of the universe, in which it is postulated that an infinitely hot dot of matter smaller than an electron exploded 13.7 billion years ago and rapidly expanded to form all of the gases, matter, stars, and galaxies in existence today.

binary a double star, each revolving around the other.

black hole a star that has exploded and collapsed to infinity, leaving behind a gravity force so powerful that nothing can escape it, including light.

blueshift the decrease in wavelength of light emit­ted by a celestial object that is moving toward an observer.

bolide a bright meteor or fireball.

brane theory a theoretical model in which the uni­verse is postulated to be a three-dimensional mem­brane residing in a four-dimensional space that can­not be perceived and is accessible only to gravity.

brown dwarf a mysterious celestial object com­posed of gas that is smaller than a star but larger than a Jupiter-size planet and that radiates small amounts of energy.

catena a chain of craters.

cephid variable a yellow supergiant star that expands and shrinks every three to 50 days.

closed universe a theoretical model of the universe in the shape of a sphere that closes in on itself.

comet a celestial body composed of ice and rock.

conjunction orbital position of an inferior planet when it is directly between the Earth and the Sun or when it is at the exact opposite side of the Sun from the Earth. Also the orbital position of a superior planet when it is on the opposite side of the Sun from Earth.

conservation of energy and mass law of physics that holds that the amount of energy and mass in the universe remains unchanged.

constellation a group of stars, named for an object, animal, or mythical figure.

cosmic microwave background microwave and other radiation remaining from the big bang; it is present in every part of the sky.

cosmic ray a stream of ionizing radiation from space, largely of protons, alpha particles, and other atomic nuclei.

cosmic year the time it takes the Sun to travel around the center of the galaxy, roughly 225 million years.

cosmogony the study of the creation of the universe.

cosmological principle the principle that states that matter is evenly distributed throughout space.

cosmology the study of the form, content, and evo­lution of the universe.

culmination the point when a celestial object reaches its greatest possible altitude above Earth's horizon.

curvature of space according to Albert Einstein's theory of gravitation, massive objects in space, such as stars, cause space to curve and light to bend.

cyclic model any one of a number of hypothetical models of creation in which the universe expands and then contracts and either collides with an unseen, parallel universe, ultimately forming a new universe, or which re-creates the big bang and expands anew in an endless cycle.

dark matter composing a large portion of the uni­verse, matter that cannot be seen but can be perceived through its gravitational effects.

declination the position of a star as located through the combination of two coordinates, east- west (right ascension) and north-south of the celes­tial equator.

Doppler effect the change in wave frequency, with light or sound, as a source moves toward or away from an observer. An example is the sound of a train whistle as it approaches, passes directly in front of, and recedes from an observer. The pitch sounds as if it is lowered as the train passes and shrinks into the distance. The Doppler effect allows scientists to deter­mine, among other things, if stars are moving away or toward us. It can also be used to determine the veloc­ity of an object detected by radar.

eclipse the obscuring of one celestial body by another, most notably when the Moon passes in front of the Sun.

ephemeris a chart or table providing the future positions of celestial bodies.

event horizon the outer perimeter of a black hole, at which the force of gravity is so powerful that mat­ter would have to exceed the speed of light in order to escape.

evolved star an older star that has converted most or all of its store of hydrogen into helium.

extrasolar planet any planet lying outside of our solar system, of which scores have been discovered.

flare star a star whose brightness can increase by as much as two to 100 times in a matter of minutes, then return to normal.

galaxy a large grouping of stars, sometimes con­sisting of billions of stars. Also known as an island universe.