Measure of angle. A right angle equals 90 degree. The apparent diameter of the full moon is about half a degree.
density wave theory
Theory proposed to account for spiral arms as compressions of the interstellar medium in the disk of galaxies.
differential rotation
The rotation of a body in which different parts of the body have different periods of rotation. This is true of the Sun, the Jovian planets, and the disk of the galaxy.
differentiation
The separation of planetary material according to density.
diffraction fringe
Blurred fringe surrounding any image, caused by the wave properties of light. Because of this, image detail smaller than the fringe can be seen.
distance indicator
Objects whose luminosity of diameter is known. Used to find the distance to a star cluster or galaxy.
distance modulus
The different between the apparent and absolute magnitude of a star. A measure of how far away the star is.
DNA (deoxyribonucleic acid)
The long carbon-chain molecule that records information to govern the biological activity of the organism. DNA carries the genetic data passed to offspring.
Doppler effect
The change in wavelength of radiation due to relative radial motion of source and observer.