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Solar System

Caldera

Crater formed by an explosion or collapse of a volcanic vent.

Calibration
A process for translating the signals produced by a measuring instrument (such as a telescope) into something that is scientifically useful. This procedure removes most of the errors caused by environmental and instrumental instabilities.

Carbon

One of the most common elements in the universe. Produced by stars, carbon is basis of all life.

Carbonaceous Chondrite

A rare type of meteorite containing water, and complex organic compounds. These may be fossils of our early solar system.

Carbonaceous Meteorite
A primitive meteorite made primarily of silicates but often including chemically bound water, free carbon, and complex organic compounds. Also called carbonaceous chondrites.

Carbonate

A compound containing carbon and oxygen.

Carbon-Nitrogen-Oxygen (CNO) Cycle
A series of nuclear reactions in the interiors of stars involving carbon as a catalyst, by which hydrogen is transformed to helium.

Cassegrain Focus
An optical arrangement in a reflecting telescope in which light is reflected by a second mirror to a point behind the primary mirror.

Cassini Division

A 5000-km gap in Saturn's ring system observable through small telescopes and clear skies. G.D. Cassini discovered this dark gap in Saturn's rings in 1675.

Catena

A chain of craters.

Cavus

Hollows, irregular depressions.

cD Galaxy
A very big elliptical galaxy frequently found at the center of a cluster of galaxies.

Celestial Body

A body located in the sky or visible heavens.

Celestial Equator
A great circle on the celestial sphere of 90^o from the celestial poles; where the celestial sphere intersects the plane of the Earth's equator.

Celestial Meridian
An imaginary line on the celestial sphere passing through the north and south points on the horizon and through the zenith.

Celestial Object

Any object seen in the sky, including planets, stars, and galaxies.

Celestial Pole

One of the two points on the celestial sphere around which the diurnal rotation of the stars appears to take place.

Celestial Sphere
Apparent sphere of the sky; a sphere of large radius centered on the observer. Directions of objects in the sky can be denoted by their position on the celestial sphere.

Center of Mass
The average position of the various mass elements of a body or system, weighted according to their distances from that center of mass; that point in an isolated system that moves with constant velocity, according to Newton's first law of motion.

Centrifugal

Moving or directed away from a center or axis.

Centrifugal Force

The apparent force which pushes a body outwards when it is in circular motion. In fact, there is no force acting at all, hence the term 'apparent'. The motion outwards is experienced because of changing frames of reference. For instance, when a car is turning around a corner, the car's frame of reference is changing but the passengers do not want to change their frame of reference and try to continue traveling in a straight line, hence they feel pulled towards to outer side of the car.

Centripetal Force

It can be shown that any mass, m, moving at a velocity v in a circle of radius r must be experiencing a (centripetal) force, mv²/r, towards the center. E.g. a stone tied to the end of a string is constrained to a circular path by a tension in the string. In space a familiar example is when a satellite orbits around a larger body (like the earth, mass M). It is the Newtonian force of gravity acting at a distance, that here provides the radial pull between the center of gravity.

Cepheid Variable Star

Among pulsating variable stars, Cepheids point to W Virgins, RR Lyrae and few other stars. They are all variable stars that show period-luminosity relation needed for the calculation of galaxies. The Cepheid variable star is a pulsating variable star with relatively high absolute magnitude. Scope of variable is 0.2 to 1.5 and its period is 1 day to 50 days. Cepheid’s characteristic is that there is a proportional relation between magnitude and period. Among Cepheids there are stars that belong to young population and aged population. Representative of the former is Cepheid variable stars, and representative of latter is W Virgo variable stars. Incidentally, stars of population 1 are gathered on galactic plain and, stars of population 2 are included in the globular clusters that are distributed around the galaxy.

Ceres

The largest of the asteroids (900-km diameter) and the first to be discovered by Giuseppe Piazzi, Karl Friedrich Gauss, and Baron Franz Xavier von Zach. On December 31, 1801, the efforts of all three confirmed the existence of Ceres. Ceres' orbital period is 4.6 years, at an average distance of 2.7 AU.

Chandrasekhar Limit
The upper limit to the mass of a white dwarf (equals 1.4 times the mass of the Sun).

Chaos

Distinctive area of broken terrain.

Charge-coupled Device (CCD)
An array of electronic detectors of electromagnetic radiation, used at the focus of a telescope (or camera lens). A CCD acts like a photographic plate of very high sensitivity.

Chasma

Canyon.

Chromosphere
The part of the solar atmosphere that lies immediately above the photospheric layers.

Circular Satellite Velocity
The critical speed that a revolving body must have in order to follow a circular orbit.

Circumpolar Star

Any star that does not appear to set from an observer's location on Earth, but instead appears to circle the celestial pole.

Circumpolar Zone
The portions of the celestial sphere near the celestial poles that are either always above or always below the horizon.

Closed Universe
A model of the universe in which the curvature of space is such that straight lines eventually curve back upon themselves; in this model, the universe expands from a big bang, stops, and then contracts to a big crunch.

Cluster of Galaxies
A system of galaxies containing several to thousands of member galaxies.

Collecting Area
The amount of area a telescope has that is capable of collecting electromagnetic radiation. Collecting area is important for a telescope's sensitivity: the more radiation it can collect (that is, the larger its collecting area), the more likely it is to detect dim objects.

Colles

Small hills or knobs.

Color Index
Difference between the magnitudes of a star or other object measured in light of two different spectral regions, for example, blue minus visual (B - V) magnitudes.

Coma

The dust and gas surrounding an active comet's nucleus.

Comet

A small body composed of ice and dust which orbits the sun on an elongated path.

Compound

Composed of or resulting from union of separate elements, ingredients, or parts, Specifically: composed of united similar elements especially of a kind usually independent.

Compton Effect
An effect that demonstrates that photons (the quantum of electromagnetic radiation) have momentum. A photon fired at a stationary particle, such as an electron, will impart momentum to the electron and, since its energy has been decreased, will experience a corresponding decrease in frequency.

Conjunction

An inferior planet is said to be "in inferior conjunction" when it is directly between the Earth and the Sun. It is "in superior conjunction" when it is on the opposite side of the Sun from the Earth. A superior planet is "in conjunction" when it is on the opposite side of the Sun from the Earth. A superior planet obviously cannot have an inferior conjunction. When the Earth is at inferior conjunction with respect to an observer on a superior planet we say that planet is "in opposition" from Earth's perspective.

Conservation of Angular Momentum
The law that the total amount of angular momentum in a system remains the same (in the absence of any force not directed toward or away from the point or axis about which the angular momentum is referred).

Conservation of Energy

The law of science that states that energy (or its equivalent in mass) can neither be created nor destroyed.

Constellation

The night sky has been divided into eighty-eight of these, each a group of bright stars that refers to a historical or mythological figure.

Continuous Spectrum
A spectrum of light composed of radiation of a continuous range of wavelengths or colors rather than only certain discrete wavelengths.

Convection

Fluid circulation driven by large temperature gradients; the transfer of heat by this automatic circulation.

Core (of a planet)
The central part of a planet, consisting of higher density material.

Corona

Plural: Coronae.

1: A usually colored circle often seen around and close to a luminous body (as the sun or moon) caused by diffraction produced by suspended droplets or occasional particles of dust.

2: The tenuous outermost part of the atmosphere of the sun appearing as a gray halo around the moon's black disk during a total eclipse of the sun.

Coronagraph

A special telescope which blocks light from the disk of the Sun in order to study the faint solar atmosphere.

Coronal Hole
A region in the Sun's outer atmosphere where visible coronal radiation is absent.

Cosmic Background Radiation (CBR)
The microwave radiation coming from all directions that is believed to be the red-shifted glow of the Big Bang.

Cosmic Ray

Electromagnetic rays of extremely high frequency and energy; cosmic rays usually interact with the atoms of the atmosphere before reaching the surface of the Earth. Some cosmic rays come from outside the solar system while others are emitted from the Sun and pass through holes in the corona.

Cosmological Constant (Lambda)
A term in the equations of general relativity that represents a repulsive force in the universe. The cosmological constant is usually assumed to be zero. The cosmological constant was found to be unnecessary once observations indicated the Universe was expanding. Had Einstein believed what his equations were telling him, he could have claimed the expansion of the Universe as perhaps the greatest and most convincing prediction of general relativity; he called this the "greatest blunder of my life".

Cosmological Distance
A distance far beyond the boundaries of our Galaxy. When viewing objects at cosmological distances, the curved nature of spacetime could become apparent. Possible cosmological effects include time dilation and redshift.

Cosmological Principle
The assumption that, on the large scale, the universe at any given time is the same everywhere - isotropic and homogeneous.

Cosmological Redshift
An effect where light emitted from a distant source appears redshifted because of the expansion of spacetime itself. Compare: Doppler Effect.

Cosmology
The astrophysical study of the history, organization, structure, dynamics, and evolution of the universe.

Crater

1: A depression formed by the impact of a meteorite.

2: A depression around the orifice of a volcano.

Crescent Moon
One of the phases of the Moon when it appears less than half full.

Critical Density
In cosmology, the density that provides enough gravity to bring the expansion of the universe just to a stop after infinite time.

Dark Matter

Non-luminous (not emitting light or visible) material that cannot be detected by observing the sky, but whose existence is suggested by certain theories.

Dark Nebula
A cloud of interstellar dust that obscures the light of more distant stars and appears as an opaque curtain.

de Broglie Wavelength
The quantum mechanical "wavelength" associated with a particle, named after the scientist who discovered it. In quantum mechanics, all particles also have wave characteristics, where the wavelength of a particle is inversely proportional to its momentum and the constant of proportionality is the Planck constant.

Declination
Angular distance north or south of the celestial equator.

Deconvolution
An image processing technique that removes features in an image that are caused by the telescope itself rather than from actual light coming from the sky. For example, the optical analog would be to remove the spikes and halos which often appear on images of bright stars because of light scatterd by the telescope's internal supports.

Deep-Sky Object

A collective term for nebulas, star clusters, and galaxies.

Degree

1: The basic unit for measuring angles - 1/360 of a full circle.

2: One of the divisions or intervals marked on a scale of a measuring instrument.

Degenerate Gas
A gas in which the allowable states for the electrons have been filled; it behaves according to different laws from those that apply to "perfect" gases and resists further compression.

Density

The quantity per unit volume, unit area, or unit length.

Deuterium

A neutron is added to atomic nucleus of hydrogen. One electron is flying around one proton and one neutron (atomic nucleus). Atomic number (number of protons in atomic nucleus) is 1. Atomic weight (sum of number of protons and neutrons in atomic nucleus, index of weight) is 2. With atoms, placing of electrons hold a great weight in its characteristic , and this matter shows similar characteristic to hydrogen.

Diaplectic Glass

A natural glass formed by shock pressure from any of several minerals without melting; it is found only in association with meteorite impact craters.

Dielectric Constant

The ratio of electric flux density to electric field.

Differential Galactic Rotation
The rotation of the Galaxy, not as a solid wheel, but so that parts adjacent to one another do not always stay close together.

Diffraction

Light interference, due to light interaction with an physical edge. The size of the edge must be the same as the light wavelength in order to produce the interference. Astronomers observe light in specific wavelengths with a diffraction grating, a glass surface with fine grooves cut into it. The groove dimensions correspond to wavelengths of light, around 500 nanometers (green) +/- 150 nanometers (blue to red). Astronomers use the grating to "spread" light into its component colors, to learn about a star's temperature, composition, speed, and distance from Earth.

Dimensionless

Immeasurable in any way.

Direct

Rotation or orbital motion in a counterclockwise direction when viewed looking down from above the north pole of the primary (i.e. in the same sense to most satellites); the opposite of retrograde. The north pole is the one on the same side of the ecliptic as the Earth's north pole. (The word "prograde" is sometimes used to mean "direct" in this sense.)

Disaster

Literally "bad stars"; particularly apt in reference to a major asteroid impact.

Disk (of Galaxy)
The central plane or "wheel" of our Galaxy, where most of the luminous mass is concentrated.

Dispersion
Separation, from white light, of different wavelengths being refracted by different amounts.

Doppler Effect

The apparent change in wavelength of sound or light caused by the motion of the source, observer or both.

Dorsum

Ridge.

Dust