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D

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.

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.

Disk
The surface of the Sun or other celestial body projected against the sky.

Double Asteroid
Two asteroids that revolve around each other and are held together by the gravity between them. Also called a binary asteroid.

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 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 neighbouring 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.

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