| Term | Definition |
|---|---|
| Universe | space and all the matter and energy in it |
| Star | a large celestial body that is composed of gas and that emits light |
| Nebula | a large cloud of gas and dust in interstellar space; a region in space where stars are born |
| Galaxy | a collection of stars, dust, and gas bound together by gravity |
| Hertzsprung-Russell diagram | a graph that shows the relationship between a star's surface temperature and absolute magnitude |
| Nuclear reaction | A reaction, as in fission, fusion, or radioactive decay, that alters the energy, composition, or structure of an atomic nucleus. |
| Milky Way Galaxy | the galaxy containing the solar system; consists of millions of stars that can be seen as a diffuse band of light stretching across the night sky |
| Luminosity | the actual brightness of an object such as a star |
| Spectral Class | any of various groups into which stars are classified according to characteristic spectral lines and bands. |
| Kelvin | A unit of absolute temperature equal to 1/273.16 of the absolute temperature of the triple point of water. |
