| Dark Matter |
A term used to describe matter in the universe that cannot be seen, but can be detected by its gravitational effects on other bodies. |
| Debris Disk |
A ring-shaped circumstellar disk of dust and debris in orbit around a star. Debris disks can be created as the next phase in planetary system development following the protoplanetary disk phase. They can also be formed by collisions between planetesimals. |
| Declination |
The angular distance of an object in the sky from the celestial equator. |
| Degree |
Angular measure, 1/360 of a circle. |
| Density |
The amount of matter contained within a given volume. Density is measured in grams per cubic centimeter (or kilograms per liter). The density of water is 1.0, iron is 7.9, and lead is 11.3. |
| Dioptric |
Refracting telescope |
| Disk |
The surface of the Sun or other celestial body projected against the sky. |
| Dobsonian telescope |
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| Doppler Effect |
The apparent change in wavelength of sound or light emitted by an object in relation to an observer's position. An object approaching the observer will have a shorter wavelength (blue) while an object moving away will have a longer (red) wavelength. The Doppler effect can be used to estimate an object's speed and direction. |
| Double Asteroid |
Two asteroids that revolve around each other and are held together by the gravity between them. Also called a binary asteroid. |
| Double Star |
A grouping of two stars. This grouping can be apparent, where the stars seem close together, or physical, such as a binary system. |
| Dwarf Planet |
A celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite. It has to have sufficient mass to overcome rigid body forces and achieve hydrostatic equilibrium. Pluto is considered to be a dwarf planet. |
