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.
galaxy cluster a grouping of galaxies held in close association by the strength of their collective gravity. A super cluster may hold as many as tens of thousands of galaxies across 100 million light-years of space.
geocentric relating to the Earth as a center; relative to the Earth.
geosynchronous orbit an orbit in which a satellite or other object keeps pace with the Earth's rotation. Also known as a geostationary orbit.
globular cluster a spherelike cluster of old stars, sometimes numbering in the hundreds of thousands.
gravitational clustering the natural tendency for a large mass, such as a galaxy, to attract other masses, including stars and galaxies, and to grow ever larger over time.
gravitational lens a massive object in space, such as a galaxy, that distorts, bends, or magnifies the light from objects behind it.
gravitational wave as predicted by Albert Einstein's general theory of relativity, a hypothetical wave, oscillation, or disturbance originating in a black hole or other source and thought to travel unimpeded across space.
Great Red Spot a massive perpetual storm on the surface of Jupiter.
Hubble constant the ratio of a galaxy's velocity in traveling away from the Earth divided by its distance from the Earth.
Hubble's law a law that states that due to the expanding universe, the velocity of a galaxy moving away from Earth is directly proportional to its distance from Earth.
hydrogen the Sun's primary gas and the most common element throughout space.
inferior planets the planets Venus and Mercury, whose orbits are closer to the Sun than Earth's.
interferometer the combination of two or more optical telescopes to produce sharper focus. Also, two or more radio telescopes combined to magnify radio signals.
interstellar dust dust particles between the stars.
interstellar space the vast regions of empty space between the stars.
inverse square law a law of physics that holds that gravity decreases with the square of the distance between two masses. Doubling the distance between two masses, for example, would reduce gravity by three-quarters. The same law applies to the magnitude or brightness of stars. Doubling a star's distance reduces brightness by three-quarters.
irregular galaxy a galaxy without an organized form, such as a spiral or globe.
island universe see galaxy.
Kuiper belt a disk-shaped swarm of 200 million comets and comet fragments located from just beyond the orbit of Neptune and extending past Pluto.
light-year an astronomical unit of measurement, specifically the distance light travels in a year, approximately 5,880,000,000,000 miles.
Local Group, the a cluster of more than 30 galaxies, including Andromeda, the Milky Way, and the Magellanic Clouds.
Magellanic Clouds two irregular galaxies that can be seen with the naked eye in the southern sky.
magnetosphere the magnetic field that surrounds a planet.
magnetotail the outer portion of a planet's magnetosphere, which is pushed away from the Sun by the solar wind. It is so named because it forms a cometlike tail that extends away from a planet's night side.
magnitude a scale for measuring the apparent brightness of celestial bodies, the brightest being negative, zero, or first magnitude, the dimmest visible to the naked eye being sixth magnitude.
meridian the great circle passing through the sky's zenith and touching the north and south horizons.
meteor a rock or metal fragment entering Earth's atmosphere and burning up. Popularly known as a shooting star.
meteorite a meteor that is not completely burned away by the atmosphere and strikes the Earth.