| Nuclear Fusion | The process by which two or more small nuclei fuse to make a bigger nucleus. |
| Nuclear Fission | The process by which a large nucleus is split into smaller nuclei. |
| Critical Mass | The amount of isotope necessary to sustain a chain reaction. |
| Absolute Magnitude | The brightness of a star, corrected for distance, on a scale of -8 to +19. The smaller the number, the brighter the star. |
| Apparent Magnitude | The brightness of a star as seen in the night sky. The smaller the number, the brighter the star. |
| Light Year | The distance light could travel along a straight line in one year. |
| Galaxy | A large ensemble of stars, all interacting through the gravitational force and orbiting around a common centre. |
