Glossary -- D

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dark matter

material that does not emit any light (or not detected yet), but has a significant gravitational effect.

declination

(dec): position on the celestial sphere that is the number of degrees an object is north or south of the celestial equator. It is a projection of latitude lines onto the sky. An object's declination is fixed with respect to the stars. Varies from --90° at the SCP to 0° at the celestial equator to +90° at the NCP. Vertical position of an object.

degenerate gas

super-compressed gas that behaves more like a solid and whose properties must be described by quantum mechanics. The pressure of degenerate gas does not depend on the temperature.

density

how much material an object has in the space it occupies: density = mass/volume.

deuterium

an isotope of hydrogen with one proton and one neutron in the nucleus.

differentiated

when the interior of a large object has undergone differentiation so that denser material lies closer to the object's core.

differential rotation

(galaxy): stars closer to a disk galaxy's center complete a greater fraction of their orbit in a given time than stars farther out from the center, so that adjacent parts of the galactic disk do not always stay close to each other.

differentiation

the separation of materials in a liquid medium under the influence of gravity such that the denser material collects at the core while the less dense material rises to the surface.

discrete spectrum

a spectrum that is not continuous---an emission line spectrum or an absorption line spectrum.

dissociation

the splitting apart of a molecule into its constituent atoms, e.g., the splitting of water molecules into hydrogen and oxygen when water vapor is struck by ultraviolet light.

doppler effect

an apparent change in the wavelength of energy produced by an object that is caused by the object's motion towards or away from the observer (along the line of sight). In astronomical spectra, the doppler effect is seen in the shifting of spectral lines.

Drake Equation

an equation that estimates the number of communicating advanced civilizations inhabiting the Galaxy.

dust

one component of the interstellar medium that is made of thin, highly flattened flakes or needles of graphite and silicates coated with water ice and other frozen gases. It is responsible for the reddening and extinction of starlight.

dust tail

(comet): one of the two tails of a comet made of dust grains that curves away from the Sun from the action of the photons in the sunlight pushing the dust grains away from Sun. It has a yellow-white color from reflected sunlight.

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last update: 06 August 1999