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celestial coordinates Pair of quantities-right ascension and declination-similar to longitude and latitude on Earth, used to pinpoint locations of objects on the celestial sphere.

celestial equator The projection of the Earth's equator onto the celestial sphere.

celestial sphere Imaginary sphere surrounding the Earth, to which all objects in the sky were once considered to be attached.

center of mass The "average" position in space of a collection of massive bodies, weighted by their masses. In an isolated system this point moves with constant velocity, according to Newtonian mechanics.

Cepheid variable Star whose luminosity varies in a characteristic way, with a rapid rise in brightness followed by a slower decline. The period of a Cepheid variable star is related to its luminosity, so a determination of this period can be used to obtain an estimate of the star's distance.

Chandrasekhar limit The upper limit to the mass of a white dwarf (equals 1.4 times the mass of the Sun). [More Info: Field Guide]

Chandra X-ray Observatory (CXO) Formerly called AXAF, Chandra was launched July 23, 1999, and is the largest and most sophisticated X-ray observatory to date. [More Info]

charge-coupled device (CCD) An electronic device used to detect photons, composed of many tiny pixels, each of which records a buildup of charge to measure the number of photons striking it. [More Info]

closed universe Geometry that the universe as a whole would have if the density of matter is above the critical value. A closed universe is finite in extent, and has no edge, like the surface of a sphere. It has enough mass to stop the present expansion, and will eventually collapse.

clusters of galaxies Galaxies can swarm together to form groups and clusters of galaxies held together by their mutual galaxy. X-ray observations show that these enormous systems of galaxies are filled with colossal clouds of hot gas. These clouds have temperatures as high as a hundred million degrees and contain as much mass as all the stars in the galaxies in the cluster. [More Info: Field Guide]

cold dark matter Hypothetical class of dark-matter candidates made up of slow-moving subatomic particles, such as supersymmetric relics of a very early stage of the Big Bang.

collecting area The total area of a telescope that is capable of capturing incoming radiation. The larger the telescope, the greater its collecting area, and the fainter the objects it can detect.

Compact Galaxy Group A galaxy group typically contains less than about 50 galaxies bound together by gravity. Although they contain fewer galaxies than the better-known galaxy clusters, they are an important class of objects because most galaxies reside in them, and the typical galaxy group is similar to the Local Group that contains our galaxy.

Compton scattering The scattering, or collision, of a photon with an electron.

conservation of energy A fundamental law of modern physics which states that the sum of the various forms of energy must always remain constant in any physical process.

constellation A grouping of stars in the night sky into a recognizable pattern. Most of the constellations get their name from the Latin translation of one of the ancient Greek star patterns that lies within it. In more recent times, a number of additional groups were introduced by more modern astronomers, and there are now 88 standard configurations recognized. [More Info: Photo Album]

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contact binary A binary star system in which both stars have expanded to fill their Roche lobes and the surfaces of the two stars merge. The binary system now consists of two nuclear burning stellar cores surrounded by a continuous common envelope.

continuous spectrum Spectrum in which the radiation is distributed over all frequencies, not just a few specific frequency ranges. A prime example is the black-body radiation emitted by a hot, dense body.

convection Churning motion resulting from the constant upwelling of warm fluid and the concurrent downward flow of cooler material to take its place.

convection zone Region of a star's interior, lying just below the surface, where the material of the star is in constant convective motion. This region extends into the solar interior to a depth of about 20,000km.

Copernican revolution The realization toward the end of the sixteenth century that Earth is not at the center of the universe.

core The central region of a planet, star, or galaxy.

corona The outermost atmosphere of a star (including the Sun), millions of kilometers in extent, and consisting of highly rarefied gas heated to temperatures of millions of degrees.

cosmic abundances A standard listing of the relative numbers of the various elements, determined by studies of the spectral lines in astronomical objects and averaged for many stars in our cosmic neighborhood.

cosmic microwave background radiation The microwave radiation coming from all directions that is believed to be the redshifted glow of the Big Bang.

cosmic rays Atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere with exceedingly high energies.

cosmic string Cosmic strings are thin strands of ultrahigh density matter that are predicted by some theories to have been left over from an extremely early era of the universe. Cosmic strings would have a width that is far less than an atomic nucleus, and a mass of about 10 million billion tons per centimeter. A kilometer of cosmic string material would weight as much as the Earth! They would make closed loops or stretch across the universe and perhaps have an infinite length. Cosmic strings are not to be confused with superstrings, the tiny subatomic loops of matter that according to superstring theory are the fundamental building blocks of all particles.

cosmological constant A modification of the equations of general relativity that represents a possible repulsive force in the universe. The cosmological constant could be due to the energy density of the vacuum.

cosmology The study of the origin and evolution of the universe as a whole.

critical mass density of universe The cosmic density corresponding to the dividing line between a universe that recollapses and one that expands forever. The universe is infinite in extent, and has zero curvature. The expansion will continue forever, but approach an expansion speed of zero.

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