carbonaceous: Containing a lot of carbon.
carbon cycle: A chain of nuclear reactions, involving carbon as a catalyst at some of its intermediate stages, that transforms four hydrogen atoms into one helium atom with a resulting release in energy. The carbon cycle is important only in stars hotter than the sun.
carbon-nitrogen cycle: The carbon cycle, acknowledging that nitrogen also plays an intermediary role.
carbon-nitrogen-oxygen cycle: A set of variations of the carbon cycle including nitrogen and oxygen isotopes as intermediaries.
Cassegrain: (telescope) A type of reflecting telescope in which the light focused by the primary mirror in intercepted short of its focal point and refocused and reflected by a secondary mirror through a hole in the center of the primary mirror.
Cassini: (a) Jean Dominique Cassini (1625-1712): (b) The NASA mission to orbit Saturn and send a probe into Titan upon its arrival there in 2004.
Cassini’s division: The major division in the rings of Saturn.
catalyst: A substance that participates in a reaction but that is left over in its original form at the end.
CCD: Charge-coupled device, a solid-state imagining device.
celestial equator: The intersection of the celestial sphere with the plane that passes through the Earth’s equator.
celestial poles: The intersection of the celestial sphere with the axis of rotation of the Earth.
celestial sphere: The hypothetical sphere centered at the center of the Earth to which it appears that the stars are affixed.
centaur: A Solar-System object, with 2060 Chiron as the first example, intermediate between comets and icy planets or satellites and orbiting the Sun between the orbits of Jupiter and Neptune.
center of mass: The “average” location of mass; the point in a body or system of bodies at which we may consider all the mass to be located for the purpose of calculating the gravitational effect of that mass or its mean motion when a force is applied.
central star: The hot object at the center of a planetary nebula, which is the remaining core of the original star.
Cepheid variable: A type of supergiant star that oscillates in brightness in a manner similar to the star δ Cephei. The periods of Cepheid variables, which are between 1 and 100 days, are linked to the average luminosity of the stars by known relationships; this allows the distances to Cepheids to be found.
Chandra X-ray Observatory: The major NASA x-ray observatory, the former Advanced X-ray Astrophysics Facility, launched in 1999 to make high-resolution observations in the x0ray part of the spectrum.
Chandrasekhar Limit: The limit in mass, about 1.4 solar masses, above which electron degeneracy cannot support a star, and so the limit above which white dwarfs cannot exist.
chaos: The conditions in which vary slight differences in initial conditions typically lead to very different results. Also, specifically, examples of orbits or rotation directions that change after apparently random intervals of time because of such sensitivity to initial conditions.
charm: An arbitrary name that corresponds to a property that distinguishes certain elementary particles, including types of quarks, from each other.
chromatic aberration: A defect of lens systems in which different colors are focused at different points.
chromosphere: The part of the atmosphere of the Sun (or another star) between the photosphere and the corona. It is probably entirely composed of spicules and probably roughly corresponds to the region in with mechanical energy is deposited.
circle: A conic section formed by cutting a cone perpendicularly to its axis.
circumpolar stars: For a given observing location, stars that are close enough to the celestial pole that they never set.
classical: When discussing atoms, not taking account of quantum mechanical effects.
closed universe: A big-bang universe with positive curvature; it has finite volume and will eventually contract.
cluster: (a) Of stars, a physical grouping of many stars; (b) of galaxies, a physical grouping of at least a few galaxies.
CNO tri-cycle: See carbon-nitrogen-oxygen tri-cycle.
COBE (Cosmic Background Explorer): A spacecraft launched in 1989 to study the cosmic background radiation.
coherent radiation: Radiation in which the phases of waves at different locations in a cross-section of radiation have a definite relation to each other; in noncoherent radiation, the phases are random. Only coherent radiation shows interference.
cold dark matter: Non-luminous matter that moves slowly through the expanding Universe, such as mini black holes and exotic nuclear particles.
color: (a) Of an abject, a visual property that depends on wavelength; (b) an arbitrary name assigned to a property that distinguishes three kinds of quarks.
coma: (a) Of a comet, the region surrounding the head; (b) an arbitrary name assigned to a property that distinguishes three kinds of quarks.
comet: A type of object orbiting the Sun, often in a very elongated orbit, that when relatively near to the Sun shows a coma and may show a tail.
comparative planetology: Studying the properties of Solar-System bodies by comparing them.
comparison spectrum: A spectrum of known elements on Earth photographed in order to provide a known set of wavelengths for zero Doppler shift.
composite spectrum: The spectrum that reveals a star is a binary system, since spectra of more than one object are apparent.
condensation: A region of unusually high mass or brightness.
conic sections: Geometric shapes obtained by slicing a cone.
conjunction: Where two celestial objects reach the same celestial longitude; approximately corresponds to their closest apparent approach in the sky. When only one body is named, it is understood that the second body is the Sun.
conservation law: A statement that the total amount of some property (angular momentum, energy, etc.) of a body or set of bodies does not change.
constellation: One of 88 areas into which the sky has been divided for convenience in referring to the stars or other objects therein.
continental drift: The slow motion of the continents across the Earth’s surface, explained in the theory of plate tectonics as a set of shifting regions called plates.
continuous spectrum: A spectrum with radiation at all wavelengths but with neither absorption nor emission lines.
continuum: (plural;: continua) The continuous spectrum that we would measure from a body if no spectral lines were present.
convection: The method of energy transport in which the raising motion of masses from below carries energy upward in a gravitational field. Boiling is an example.
convection zone: The subsurface zone in certain types of stars in which convection dominates energy transfer.
coordinate systems: Methods of assigning positions with respect to suitable axes.
core: The central region of a star or planet.
corona, solar or stellar: The outmost region of the Sun (or of other stars), characterized by temperatures of millions of kelvins.
coronagraph: A type of telescope with which the corona (and, sometimes, merely the chromosphere) can be seen in visible light at times other than that of a total solar eclipse by occulting (hiding) the bright photosphere. Also, any telescope in which a bright central object is occulted to reveal fainter things.
coronal holes: Relatively dark regions of the corona having low density; they result from open field lines.
correcting plate: A thin lens of complicated shape at the front of a Schmidt camera that compensates for spherical aberration.
cosmic abundances: The overall abundances of elements in the Universe.
cosmic background radiation: The isotopic glow of 3K radiation.
cosmic rays: Nuclear particles or nuclei traveling through space at high velocity.
cosmogony: The study of the origin of the Universe, usually applied in particular to the origin of the Solar System.
cosmological constant: A constant arbitrarily added by Einstein to an equation in his general theory of relatively in order to provide a solution in which the Universe did not expand or contract. It was only subsequently discovered that the Universe does expand after all.
cosmological principle: The principle that on the whole the Universe looks the same in all directions and in all regions.
cosmology: The study of the Universe as a whole.
coudè focus: A focal point of large telescopes in which the light is reflected by a series of mirrors so that it comes to a point at the end of a polar axis. The image does not move even when the telescope moves, which permits the mounting of heavy equipment.
critical density: The density of matter that would allow the Universe to expand forever, but at a rate that would decrease to 0 at infinite time. The density of the Universe with respect to the critical density defines whether the Universe will expand forever or eventually contract.
crust: The outermost solid layer of some objects, including neutron stars and some planets.