Astronomy terms starting with D

7 terms

1. Dark Energy An energy causing the acceleration of the expansion of the universe, detectable through its gravitational effects.
2. Dark Matter Matter not visible to us because it emits no radiation that we can observe, but it is detectable gravitationally. A small fraction of it is baryonic, e.g.
3. Deceleration Decreasing the velocity (of expansion).
4. Density Fluctuations in Matter Regions of greater and lesser mass density, which can form into structure such as galaxies, clusters, etc.
5. Deuterium A special form of hydrogen (an isotope called "heavy hydrogen") that has a neutron as well as a proton in its nucleus.
6. Doppler Effect apparent change in frequency of a wave due to the relative motion of the source and observer.
7. Dynamics Changes over a length of time.