ASTR 103 - Astronomy

Glossary - D

dark cloud

Interstellar cloud of gas and dust which is so opaque that stars behind it can not be seen.

dark-line spectrum

see absorption spectrum

dark nebula

Opaque interstellar cloud of gas and dust that dims or obscures the light of stars behind it.

decay series

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deceleration parameter

The rate of which the expansion of the Universe slows with time as a result of self gravitation by the matter of the Universe. It depends on the universal scaling factor and the Hubble constant.

declination

Celestial coordinate that measures the angular distance of a body north or south of the celestial equator along the hour circle passing through the body.

de-excite

The process in which the electron spontaneously gives up energy creating a photon and makes the transition to a lower energy orbit.

deferent

In the Ptolemaic system, the large circle centered on the Earth upon which circumference the center of a smaller circle (epicycle) revolves; the planet moves on the epicycle.

degenerate gas

Very high-density, high-temperature condition of matter (different from the perfect-gas condition) in which the electrons or nucleons are restricted in that the lower energy states are all occupied and the pressure of the degenerate gas does not depend on its temperature.

density

The mass per unit volume of a material body.

density-wave theory

A wave that spirals outward from the center of a spiral galaxy through the interstellar matter of the galactic disk at a constant rotational rate, generated by gravitational variations within the disk.

deuterium

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differential rotation

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differentiation (geologic)

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diffraction

Slight bending of light waves around a sharp edge that is a result of lights wave nature.

diffraction grating

see grating

diffraction pattern

Of a star, bright central disk of a stellar image surrounded by alternating, concentric, bright and dark rings.

diffuse cloud

Interstellar cloud of gas and dust that is not so opaque that stars can not be seen behind it.

diffuse nebula

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dipole

Magnetized object that possesses north and south magnetic poles. A bar magnet and Earth are two examples.

disk population

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dispersion

The separation of composite light into its various wavelengths as, for example, by a prism or grating.

distance modulus

Difference between the apparent magnitude (m) and the absolute magnitude (M) of a celestial body; that is m-M, from which a celestial body's distance may be derived by application of the inverse square law of light.

diurnal motion

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DNA

Deoxyribonucleic acid-the inheritance material consisting of long chains of nucleotides present in the chromosomes of all living matter.

Doppler effect

Apparent shift in wavelength or frequency as a result of relative line-of-sight motion between the observer and the source of radiation.

dust tail

Dusty component of a comet's tail, consisting of dust grains evaporated from the head of the comet.

dwarf star

Star of moderate or lesser mass and luminosity on the main sequence. (The Sun is a yellow dwarf star).

dynamo

The process in which the motion of electrical charges generated a magnetic field.

dyne

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