C ring One of three Saturnian rings visible from Earth. The C ring lies closest to the planet and is relatively thin compared to the A and B rings.

carbon-detonation supernova See type-I supernova.

Cassegrain telescope A type of reflecting telescope in which incoming light hits the primary mirror and is then reflected upward toward the prime focus, where a secondary mirror reflects the light back down through a small hole in the main mirror, into a detector or eyepiece.

Cassini Division A relatively empty gap in Saturn's ring system between the A and B rings, discovered in 1675 by Giovanni Cassini. It is now known to contain a number of thin ringlets.

catastrophic theory A theory that invokes statistically unlikely accidental events to account for observations.

celestial&151;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, taking their masses into account. 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.

chaotic rotation Unpredictable tumbling motion that non-spherical bodies in eccentric orbits, such as Saturn's satellite Hyperion, can exhibit. No amount of observation of an object rotating chaotically will ever show a well-defined period.

charge-coupled device (CCD) Electronic device used for data acquisition, composed of many tiny pixels, each of which records a buildup of charge to measure the amount of light striking it.

chromatic aberration The tendency for a lens to focus red and blue light differently, causing images to become blurred.

chromosphere The Sun's lower atmosphere, lying just above the visible photosphere.

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.

cold dark matter Class of dark-matter candidates made up of very heavy particles, such as supersymmetric relics.

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.

color index A convenient method of quantifying a star's color by comparing its apparent brightness as measured through different filters. If the star's radiation is well described by a black-body spectrum, the ratio of its blue intensity (B) to its visual intensity (V) is a measure of the object's surface temperature.

color-magnitude diagram A way of plotting stellar properties, in which absolute magnitude is plotted against color index.

coma An effect occurring during the formation of an off-axis image in a telescope. Stars whose light enters the telescope at a large angle acquire comet-like tails on their images. The brightest part of a comet, often referred to as the "head."

comet A small body, composed mainly of ice and dust, in an elliptical orbit about the Sun. As it comes close to the Sun, some of its material is vaporized to form a gaseous head and extended tail.

comparative planetology The systematic study of the similarities and differences among the planets, with the goal of obtaining deeper insight into how the solar system formed and has evolved in time.

condensation nuclei Dust grains in the interstellar medium which act as seeds around which other material can coagulate. The presence of dust was very important in causing matter to clump during the formation of the solar system.

condensation theory Currently favored model of solar system formation which combines features of the old nebular theory with new information about interstellar dust grains, which acted as condensation nuclei.

conservation of mass and energy A fundamental law of modern physics which states that the sum of mass and energy must always remain constant in any physical process. In fusion reactions, the lost mass is converted into energy, primarily in the form of electromagnetic radiation.

constellation A human grouping of stars in the night sky into a recognizable pattern.

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 the Sun's interior, lying just below the surface, where the material of the Sun is in constant convective motion. This region extends into the solar interior to a depth of about 200,000 km.

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 Earth, surrounded by the mantle. The central region of the Sun.

core-collapse supernova See type-II supernova.

core hydrogen burning The energy burning stage for main sequence stars, in which the helium is produced by hydrogen fusion in the central region of the star. A typical star spends up to 90% of its lifetime in hydrostatic equilibrium brought about by the balance between gravity and the energy generated by core hydrogen burning.

corona One of numerous large, roughly circular regions on the surface of Venus, thought to have been caused by upwelling mantle material causing the planet's crust to bulge outward.

corona The tenuous outer atmosphere of the Sun, which lies just above the chromosphere, and at great distances turns into the solar wind.

coronal hole Vast regions of the Sun's atmosphere where the density of matter is about 10 times lower than average. The gas there streams freely into space at high speeds, escaping the Sun completely.

cosmic distance scale Collection of indirect distance-measurement techniques that astronomers use to measure the scale of the universe.

cosmic evolution The collection of the seven major phases of the history of the universe, namely galactic, stellar, planetary, chemical, biological, cultural, and future evolution.

cosmic microwave background The almost perfectly isotropic radio signal that is the electro-magnetic remnant of the Big Bang.

cosmological principle Two assumptions which make up the basis of cosmology, namely that the universe is homogeneous and isotropic on sufficiently large scales.

cosmological redshift The component of the redshift of an object which is due only to the Hubble flow of the universe.

cosmology The study of the structure and evolution of the entire universe.

crater Bowl-shaped depression on the surface of a planet or moon, resulting from a collision with interplanetary debris.

critical density The cosmic density corresponding to the dividing line between a universe that recollapses and one that expands forever.

critical universe Geometry that the universe would have if the density of matter is exactly the critical density. The universe is infinite in extent, and has zero curvature. The expansion will continue forever, but approach an expansion speed of zero.

crust Layer of the Earth which contains the solid continents and the seafloor.