| A | B |
|---|
| corona | top layer of the Sun's atmosphere |
| variable star | stars in the giant branch of the Hertsprung-Russell diagram |
| RR Lyrae variable | star with a period of pulsation between 1.5 hours and 1 day |
| Cepheid variable | star with a period of pulsation between 1 and 100 days |
| Population I stars | stars in the disk; have small amount of heavy elements |
| Population II stars | stars in the halo and bulge; contain only traces of heavy elements |
| halo | spherical region where globular clusters are located |
| spiral density wave | spiral arm that alternates between dense and less dense regions |
| interpret | to explain or tell the meaning of |
| mass | the amount of matter in an object |
| dark matter | invisible material believed to be made up of a form of subatomic particle that interacts only weakly with other matter |
| supercluster | clusters of galaxies |
| Hubble constant | formulaic method of representing the expansion of the universe |
| radio galaxy | giant galaxy that emits more radio waves than waves of visible light |
| active galactic nucleus | core of a galaxy where a highly energized object is located |
| quasar | star-like objects with emission lines in their spectra |
| positive | characterized by or displaying affirmation or acceptance or certainty |
| radiation | energy transfer through space by visible light, ultraviolet radiation, and other forms of electromagnetic waves |
| cosmology | study of the universe, its current nature, and its origin and evolution |
| Big Bang theory | theory that the universe began as a single point and is continually expanding |
| cosmic background radiation | weak radiation that is left over from the early, hot stages of the Big Bang expansion of the universe |
| inflationary universe | theory that the universe began as a fluctuation in a vacuum and expanded very rapidly for a fraction of a second |
| correspond | to compare closely |
