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carbonaceous meteorite  - A primitive meteorite made primarily of silicates but often including chemically bound water, free carbon, and complex organic compounds. Also called carbonaceous chondrites. 
carbon-nitrogen-oxygen (CNO) cycle  - A series of nuclear reactions in the interiors of stars involving carbon as a catalyst, by which hydrogen is transformed to helium. 
Cassegrain focus  - An optical arrangement in a reflecting telescope in which light is reflected by a second mirror to a point behind the primary mirror. 
CBR - See cosmic background radiation.
CCD  - See charge-coupled device
cD galaxy  - A supergiant elliptical galaxy frequently found at the center of a cluster of galaxies. 
celestial equator  - A great circle on the celestial sphere 90 from the celestial poles; where the celestial sphere intersects the plane of the Earth's equator. 
celestial meridian  - An imaginary line on the celestial sphere passing through the north and south points on the horizon and through the zenith 
celestial poles  - Points about which the celestial sphere appears to rotate; intersections of the celestial sphere with the Earth's polar axis. 
celestial sphere  - Apparent sphere of the sky, a sphere of large radius centered on the observer.  Directions of objects in the sky can be denoted by their position on the celestial sphere. 
center of gravity  - Center of mass.
center of mass  - The average position of the various mass elements of a body or system, weighted according to their distances from that center of mass; that point in an isolated system that moves with constant velocity, according to Newton's first law of motion.
cepheid variable  - A star that belongs to a class of yellow super-giant pulsating stars. These stars vary periodically in brightness, and the relationship between their periods and luminosities is useful in deriving distances to them. 
Chandrasekhar limit  - The upper limit to the mass of a white dwarf (equals 1.4 times the mass of the sun). 
charge-coupled device (CCD)  - An array of electronic detectors of electromagnetic radiation, used at the focus of a telescope (or camera lens). A CCD acts like a photographic plate of very high sensitivity.
chemical condensation sequence  - The calculated chemical compounds and minerals that would form at different temperatures in a cooling gas of cosmic composition; used to infer the composition of grains that formed in the solar nebula at different distances from the protosun. 
chromosphere  - That part of the solar atmosphere that lies immediately above the photospheric layers. 
circular satellite velocity  - The critical speed that a revolving body must have in order to follow a circular orbit. 
circumpolar zone  - Those portions of the celestial sphere near the celestial poles that are either always above or always below the horizon.
closed universe  - A model of the universe in which the curvature of space is such that straight lines eventually curve back upon themselves; in this model, the universe expands from a big bang, stops, and then contracts to a big crunch. 
cluster of galaxies  - A system of galaxies containing several to thousands of member galaxies. 
color index  - Difference between the magnitudes of a star or other object measured in light of two different spectral regions, for example, blue minus visual (B - V) magnitudes. 
coma (of comet)  - The diffuse gaseous component of the head of a comet; i.e, the cloud of evaporated gas around a comet nucleus. 
comet  - A small body of icy and dusty matter that revolves about the Sun. When a comet comes near the Sun, some of its material vaporizes, forming a large head of tenuous gas, and often a tail.
compound  - A substance composed of two or more chemical elements.
conduction - The transfer of energy by the direct passing of energy or electrons from atom to atom., 
conservation of angular momentum  - The law that the total amount of angular momentum in a system remains the same (in the absence of any force not directed toward or away from the point or axis about which the angular momentum is referred).
constellation - One of 88 sectors into which astronomers divide the celestial sphere; many constellations are named after a prominent group of stars within them that represents a person, animal, or legendary creature from ancient mythology.
continuous spectrum A spectrum of light composed of radiation of a continuous range of wavelengths or colors rather than only certain discrete wavelengths.
convection The transfer of energy by moving currents in a fluid.
core (of a planet) The central part of a planet, consisting of higher density material.
corona (of Galaxy) A region lying above and below the plane of the Galaxy out to much greater distances than the material that gives off electromagnetic radiation.
corona (of Sun) Outer atmosphere of the Sun.
coronal hole A region in the Sun's outer atmosphere where visible coronal radiation is absent.
cosmic background radiation (CBR) The microwave radiation coming from all directions that is believed to be the red-shifted glow of the Big Bang.
cosmic rays Atomic nuclei (mostly protons) the, are observed to strike the Earth's atmosphere with exceedingly high energies.
cosmological constant A term in the equations of general relativity that represents a repulsive force in the universe. The cosmological constant is usually assumed to be zero.
cosmological principle The assumption that, on the large scale, the universe at any given time is the same everywhere-isotropic and homogeneous.
cosmology The study of the organization and evolution of the universe.
critical density In cosmology, the density that provides enough gravity to bring the expansion of the universe just to a stop after infinite time.

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