| watt | A unit of power (energy per unit time). |
| wavelength | The spacing of the crests or troughs in a wave. |
| weak nuclear force or weak interaction | The nuclear force involved in radioactive decay. The weak force is characterized by the slow rate of certain nuclear reactions-such as the decay of the neutron, which occurs with a half-life of 11 min. |
| white dwarf |
A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size; such a star is near its final stage of life. |
| Wien's law | Formula that relates the temperature of a black-body to the wavelength at which it emits the greatest intensity of radiation. |
| winter solstice | Point on the celestial sphere where the Sun reaches its greatest distance south of the celestial equator; the time of the year with the shortest amount of daylight. |
| Wolf-Rayet star | One of a class of very hot stars that eject shells of gas at very high velocity. |