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 Term used to describe the mass in galaxies and clusters whose existence we infer from rotation curves and other techniques, but which has not been confirmed by observations on any electromagnetic wavelength.
declination Celestial coordinate system used to measure latitude above or below the celestial equator on the celestial sphere.
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.
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 An interstellar dust particle, roughly 10-8 m in size, 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).