ASTR 103 - Astronomy

Glossary - C

Cambrian

Paleontological/geological 100 million-year period that began about 600 million years ago. It was characterized by an explosive growth of marine life, principally the invertebrates.

canals

(of Mars)

the straight-line markings on Mars thought to be actual water canals, but found not to exist after spacecraft visits to Mars.

capture theory

the formation of a planet-satellite or binary system in which one body captures the other by gravity.

carbonaceous

refers to carbon-bearing material.

carbon burning

The thermonuclear process in which two carbon nuclei (¹²C) fuse to produce primarily neon (²⁰Ne), sodium (²³Na), or magnesium (²⁴Mg) at temperatures above 600 million degrees K and densities between 10⁵ and 10⁸ g/cm³; occurs after hydrogen and helium burning in later stages of evolution for high-mass stars.

carbon cycle

see CNO cycle

Cassegrain focus

optical arrangement that permits light from the primary mirror of a reflecting telescope to be reflected back through the central hole of the mirror to a focus behind the primary mirror.

Cassini division

5000-kilometer gap between the outer and middle rings of Saturn.

celestial equator

great circle that represents the projection of Earth's equator onto the sky (celestial sphere).

celestial mechanics

field of astronomy that deals with the gravitational motions of celestial bodies, primarily in the Solar System.

celestial meridian

the great circle on the celestial sphere containing the north and south points of the horizon and the north and south celestial poles and the observers zenith and nadir.

celestial pole

extension of Earth's axis of rotation on the sky (celestial sphere).

celestial sphere

the imaginary sphere centered on the Earth to which the stars are affixed.

cell

the smallest unit of biological organisms that are capable of independent functions or possess the characteristics of life.

center of mass

the location in a body at which all its mass can be concentrated without affecting its response to gravity.

centimeter (cm)

central force

the name for a force located at the center of a system to which it gives definition. Example:the gravitational attraction of the Sun is a central force for the orbital motion of the planets.

Cepheid variable

pulsating variable star of the giant or supergiant luminosity classes with a period of pulsation between a fraction of a day and 50 days; its spectral class is between F and early K.

Chandrasekhar limit

theoretical limit below which a low-mass star can evolve into a white dwarf at the end of its life. Its value is 1.4 solar masses.

channel

riverbedlike valleys on Mars, which are possibly sites of ancient Martian rivers.

chemical differentiation

the process in the formation of a planet in which, if the planet is molten, the heavy elements sink to the center and the light elements rise to the surface.

chondrite

stony type of meteorite which contain large amounts of carbon, water and other volatile compounds and believed to be the most primitive condensed matter in the solar nebula.

chondrule

small, glasslike, round body found in stony meteorites. It is believed to have crystallized from molten droplets present during the initial stages of condensation of the solar nebula.

chromatic aberration

defect in an optical system which is the failure to bring different wavelengths to a common focus.

chromosome

threadlike string of genes containing DNA, which is part of every nucleus in biological cells.

chromosphere

narrow pinkish portion of the Sun's atmosphere lying immediately above the photosphere and below the corona.

circle

one member of a family of plane curves called conic sections for which every point on th curve is equidistant from its center.

circumpolar stars

for a given latitude, those stars close enough to the observable celestial pole that never set, or those stars close enough to the unobservable celestial pole that never rise.

circumstellar shell

a shell of gas and dust surrounding a star.

clock paradox

also known as the twin paradox, in which one of the twins stay home while the other makes a high-speed, roundtrip journey to a distant star and returns younger in age than the twin who stayed on Earth; see time dilation.

closed universe

a cosmological model of the Universe with a finite volume in which space is positively curved and for which sufficient matter exists to stop the expansion of the Universe by mutual gravitational attraction.

cluster

a physical grouping of stars or galaxies bound either temporarily or permanently by gravity.

cluster of galaxies

CNO cycle

carbon-nitrogen-oxygen thermonuclear fusion process, which results in hydrogen nuclei being converted into helium nuclei plus energy with the carbon isotope 12C6 as a catalyst.

color

see color index

color excess

difference between the observed color index of a star, whose light has been reddened in passing through interstellar matter, and the normal unreddened color index.

color index

difference in magnitude between two spectral regions of a celestial object (most often between the blue and yellow wavelength regions.

color-magnitude diagram

Hertzsprung-Russell diagram for stars in which the horizontal axis is the color index and vertical axis is the apparent (of a stellar cluster) or absolute magnitude.

coma

(of a comet)

gaseous envelope immediately surrounding the nucleus of a comet.

comet

small interplanetary body consisting of ices and dust that is in orbit around the Sun. As it nears the Sun, its surface material vaporizes to form a large head, or coma, at whose center lies a bright nucleus where its mass is concentrated.

comet tail

diffuse streams of gas and dust liberated from a comet and driven away from the direction of the Sun by the solar wind and solar radiation.

comparison spectrum

the emission spectrum of a known element placed above and below a stellar spectrum as a reference for wavelength determination.

condensation theory

the formation of Solar System bodies by the accretion of planetesimals, particularly the Earth and Moon.

conduction

movement of thermal energy from one point to another by collisions of more energetic particles with adjacent, less energetic particles.

configuration

(of the planets)

the positions of the planets relative to the Earth-Sun line (see also elongation).

conic section

plane curve formed by the intersection of a plane and a circular cone. The angle at which the plane cuts the cone determines whether the curve is a circle, ellipse, parabola, or hyperbola.

conjunction

the planetary configuration in which the planet's angular distance (elongation) from the Sun is either zero or a minimum.

conservation of angular momentum

conservation of energy

principle stating that energy can be converted from one form into another but it cannot be destroyed or created; the total amount of energy remains constant.

conservation of momentum

constellation

arbitrary grouping of stars within a bounded area of the sky named after mythological heroes, animals, or other objects.

continental drift

gradual separation of the continents produced by convective motions of lithospheric plates at a rate of a few centimeters per year.

continuous spectrum

uninterrupted band of emission produced by a body radiating energy over a continuous range of wavelengths; it contains no absorption or emission lines.

continuum

(1) atomic: the continuous spectral region toward the violet, adjacent to the head of a series limit of an atom's spectral lines. (2) Space: the space-time environment in four dimensional space.

convection

process in which energy is transferred from one point to another point by currents in a gas or liquid moving in a closed loop.

core

the central part of a planet, star or other astronomical body which is surrounded by some outer layers.

Coriolis effect

corona

outermost portion of the Sun's atmosphere best observed during a total eclipse of the Sun.

coronagraph

special telescope carefully designed to photograph the chromosphere and inner corona of the Sun without the intervention of an eclipse of the Sun.

coronal hole

regions in the solar corona of exceptionally low density and temperature and weak magnetic fields.

cosmic background radiation

An isotropic microwave radiation thought to be the remnant of the big bang; the radiation has a blackbody spectrum at a temperature of 3^oK.

cosmic rays

highly energetic particles incident on the Earth's atmosphere. Originating in outer space, they consist mostly of protons with a sprinkling of heavier atomic nuclei.

cosmic void

large regions about the size of superclusters of galaxies lying between the superclusters that appear to be devoid of any significant number of galaxies.

cosmogony

The study of the origin of individual celestial bodies, such as the Solar System.

cosmological constant

Mathematical term involving a repulsive force, which Einstein introduced into his field equations in the general theory of relativity to counteract the self gravitation of the Universe.

cosmological principle

Assumption that the Universe-at-large looks the same to all observers.

cosmological model

 

cosmological redshift

Galactic redshifts that indicate the expansion of space (i.e., the expansion of the Universe).

cosmology

Branch of astronomy concerned with the origin, evolution, and structure of the Universe.

coude focus

Optical system that permits the beam of light from the primary mirror of the reflecting telescope to be directed down the hollow polar axis of the instrument to a remote focal position that remains fixed regardless of the position of the telescope.

crater

see impact cratering

crust

Outermost layer of a terrestrial planet.