Glossary -- C

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carbonaceous meteorite

type of stone meteorite containing silicates, carbon compounds (giving them their dark color), around 20% water, and sometimes amino acids (the building blocks of proteins used in biological processes of life).

celestial equator

great circle that is a projection of the Earth's equator onto the sky. Always intercepts horizon at exact East and exact West point. Its meridian altitude = 90\deg -- observer's latitude. We see one-half of its circle at a time (12 hours worth).

celestial sphere

imaginary sphere of extremely large size around the Earth on which the stars appear to be placed.

center of mass

the balance point between two massive objects that is proportionally closer to the more massive object. It is the point where (mass object 1) × (object 1 distance from center of mass) = (mass object 2) × (object 2 distance from center of mass).

centripetal force

a force directed inward.

Cepheid

(variable star): a type of variable star that changes brightness by changing size and temperature with a period that depends on its average luminosity. More luminous Cepheids have longer pulsation periods. Cepheids are particularly valuable for determining distances to the nearby galaxies in which they reside. Distances to Cepheids are derived from measurements of their pulsation periods and apparent brightnesses and application of the inverse square law of light brightness.

chondrule

round glassy structure 0.5 to 5 millimeters in diameter embedded in a primitive stone meteorite. It is a solidified droplet of matter from the early solar nebula and is the very oldest part of the primitive meteorite.

chromatic aberration

a defect in the images from refractor telescopes that is caused by different colors of light focussing to different points behind the glass lens. A rainbow of colors is produced around the image.

chromosphere

the hot, thin layer of the Sun's atmosphere right above its photosphere.

circumpolar

when an object is close enough to either the north celestial pole or south celestial pole (within an angular distance = observer's latitude) such that the object never moves below an observer's horizon or never rises above the horizon as the Earth rotates.

closed universe

a universe with enough matter (gravity) to eventually stop the expansion and recollapse (it has a ``closed future'').

color-magnitude diagram

a plot of the colors (temperatures) and magnitudes of stars. Another name for the Hertzsprung-Russell diagram.

conservation of angular momentum

when an object or system of objects has no net outside forces acting on it, the total amount of its angular momentum does not change.

continuous spectrum

a spectrum that has energy at all wavelengths (a full rainbow). See also thermal spectrum.

coma

(comet): large atmosphere around a comet's nucleus that forms when the nucleus nears the Sun and warms up (usually at around Saturn's or Uranus' distance from the Sun).

convection zone

the region of a star's interior where energy is transported outward using bulk motions of rising hot gas and sinking cool gas. For the Sun, it is the region above the radiative zone.

core

(stellar): the center of a star where the density and temperature are high enough for nuclear fusion to occur.

coriolis effect

the deflection sideways of an object moving across the surface of a rotating body caused by the rotation of the body. The coriolis effect makes storms spiral on the Earth and produces the banded cloud layers on the gas giant planets.

correlation

a mutual relationship between two properties (usually such that an increase in one property is seen when another property increases).

corona

the top layer of the Sun's atmosphere. It is up to a few million degrees in temperature, but has very low density so the amount of heat is small. It is the pearly-white ``crown'' or glow seen around the dark Moon during a total solar eclipse.

cosmic microwave background radiation

radio (microwave) energy that is nearly uniform in all directions and has a nearly perfect thermal spectrum. It is the greatly redshifted remnant of the early hot universe produced several hundred thousand years after the birth of the universe.

cosmological constant

an extra term Albert Einstein put in his equations of General Relativity that would act as a repulsive form of gravity to balance the attractive nature of gravity and keep the universe static.

cosmological principle

an assumption that the universe is everywhere uniform and looks the same in any direction---it is homogeneous and isotropic.

cosmology

the study of the nature and origin of the universe and how it changes over time.

critical density

boundary density between enough mass/volume to eventually stop the expansion of the universe and too little mass/volume to eventually stop the expansion.

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last update: 06 August 1999