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dark dust cloud (dark nebula) A region of interstellar space containing a rich concentration of gas and dust in an irregular but well-defined cloud that obscures the light from stars beyond it.

dark energy A mysterious source of energy introduced to explain the acceleration of the expansion of the Universe. One possible explanation is that it is the energy present in empty space - "vacuum energy" - which has the properties of a repulsive force. [More Info: Field Guide]

dark halo A large envelope of dark matter around a galaxy that is postulated to explain the rapid rotation of galaxies and other observations.

dark matter A term used to describe the mass in galaxies and clusters of galaxies that can be inferred to exist from its gravitational effects, but has not been directly detected by electromagnetic radiation. [More Info: Field Guide]

declination Celestial coordinate system used to measure latitude above or below the celestial equator on the celestial sphere. [More Info: Field Guide]

decoupling Event in the early universe when atoms first formed, and after which photons could propagate freely through space.

Deep Space Network NASA's three ground stations (in Spain, Australia, and California) that are used to relay commands to an orbiting spacecraft, such as the Chandra X-ray Observatory. [More Info]

degree A unit of angular measure. There are 360 degrees in a complete circle.

density A measure of the compactness of the matter within an object, computed by dividing the mass by the volume of the object.

deuterium An isotope of hydrogen in which there is a neutron bound to the proton in the nucleus. Often called "heavy hydrogen" because of the extra mass of the neutron.

differential rotation The tendency for a gaseous sphere, such as a jovian planet or the Sun, to rotate at a different rate at the equator than at the poles. For a galaxy or other object, a condition where the angular speed varies with location within the object.

diffraction The ability that waves have to bend around corners. The diffraction of light establishes its nature as a wave.

Doppler Effect Apparent change in wavelength of the radiation from a source due to its relative motion away from or towards the observer.

dust grain A term for dust particles in the space between the stars. Sizes vary over a wide range but are typically about one micrometer, comparable to the wavelength of visible light.

dust lane A lane of dark, obscuring interstellar dust in an emission nebula or galaxy.

dwarf Any star with a radius comparable to, or smaller than that of the Sun (including the sun itself).

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