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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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