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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.
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