D ring Collection of very faint, thin rings, extending from the inner edge of the C ring down nearly to the cloud tops of Saturn. This region contains so few particles that it is completely invisible from Earth.

dark dust cloud A large cloud, often many parsecs across, which contains gas and dust in a ratio of about 1012 gas atoms for every dust particle. Typical densities are a few tens or hundreds of millions of particles per cubic meter.

dark energy

dark halo Region of a galaxy beyond the visible halo where dark matter is believed to reside.

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 at any electromagnetic wavelength.

declination Celestial coordinate 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.

deferent A construct of the geocentric model of the solar system which was needed to explain observed planetary motions. A deferent is a large circle encircling the Earth, on which an epicycle moves.

degree See arc degree.

density A measure of the compactness of the matter within an object, computed by dividing the mass by the volume of the object. Units are kilograms per cubic meter (kg/m3), or grams per cubic centimeter (g/cm3).

deuteron 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 or for the rotation rate to vary with depth. For a galaxy or other object, a condition where the angular speed varies with location within the object.

differentiation Variation with depth in the density and composition of a body, such as Earth, with low-density material on the surface and higher density material in the core.

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

Doppler effect Any motion-induced change in the observed wavelength (or frequency) of a wave.

Drake Equation Expression that gives an estimate of the probability that intelligence exists elsewhere in the galaxy, based on a number of supposedly necessary conditions for intelligent life to develop.

dust grain An interstellar dust particle, roughly 10&150;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.

dust tail The component of a comet's tail that is composed of dust particles.

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

dynamo theory Theory that explains planetary and stellar magnetic fields in terms of rotating, conducting material flowing in an object's interior.