The establishment of the relationship between a parameter that is easily determined and a parameter that is more difficult to determine. For example, the periods of Cepheids variables have been calibrated to reveal the absolute magnitudes, which can then be used to find distances. Thus astronomers say Capheids have been calibrated as distance indicators.
Cambrian period
A geological period 0.6-0.5 billion years ago during which life on Earth became diverse and complex.Cambrian rocks caontain the oldest easily identifiable fossiles.
celestial equator
The imaginiary line around the sky directly above the Earth's equator.
celestial pole (north or south)
One of the two points on the celestial sphere directly above Earth's poles.
celestial sphere
An imaginary sphere of very large radius surrounding Earth on which the planets, stars, sun, and moons seem to be moving.
center of mass<
The balance point of a body or system of masses. The point about which a body or system of masses rotates in the absence of external forces.
Cepheid
Variable star with a period of 60 days whose period of variation is related to its luminosity. Collectively, all such stars with similar spectra and period/luminosity realtionships.
charge-coupled device (CCD)
An electronic device consisting of a large array of light-sensitive elements used to record very faint images. Also used as "electronic film" in digital cameras and portable video recorders.
chemical evolution
The chemical process that led to the growth of complex molecules on primitive Earth. This did not involve the reproduction of molecules.
chromatic aberration
A distortion found in refracting telescopes because lenses focus different colors at slightly different distances. Images are consequently surrounded by colored bands.
conservation of energy law
One of the basic laws of stellar structure. The amount of energy flowing out of a given volume must equal the amount coming in plus whatever energy is generated within this volume of space.
constellation
One of the stellar patterns identified by name, usually of mythological gods, people, animals and objects. Also the region of the sky containing that star pattern.
continuous spectrum
A spectrum in which there are no absorption or emission lines.
coronograph
A telescope that blocks light from the disk of a star so that fainter surrounding objects and structures can be observed.
cosmological principle
The assumption that any observer in any galaxy sees the same general features of
the universe.
cosmology
The study of the nature, origin, and evolution of the universe.