water hole The radio interval between 18 cm and 21 cm, the wavelengths at which hydroxyl (OH) and hydrogen (H) radiate, respectively, in which intelligent civilizations might conceivably send their communication signals.

wave A pattern that repeats itself cyclically in both time and space. Waves are characterized by the velocity with which they move, their frequency, and their wavelength.

wave period The amount of time required for a wave to repeat itself at a specific point in space.

wavelength The length from one point on a wave to the point where it is repeated exactly in space, at a given time.

weak nuclear force Short-range force, weaker than both electromagnetism and the strong force, but much stronger than gravity, responsible for certain nuclear reactions and radioactive decays.

weird terrain A region on the surface of Mercury of oddly rippled features. This feature is thought to be the result of a strong impact which occurred on the other side of the planet, and sent seismic waves traveling around the planet, converging in the weird region.

white dwarf A dwarf star with a surface temperature that is hot, so that the object glows white.

white dwarf region The bottom left-hand corner of the Hertzsprung–Russell diagram, where white dwarf stars are found.

white light Visible light that contains approximately equal proportions of all colors.

white oval Light-colored region near the Great Red Spot in Jupiter's atmosphere. Like the red spot, such regions are apparently rotating storm systems.

Wien's law Relation which gives the connection between the wavelength at which a black-body curve peaks and the temperature of the emitter. The temperature is inversely proportional to the peak wavelength, so the hotter the object, the bluer its radiation.

winter solstice Point on the ecliptic where the Sun is at its southernmost point below the celestial equator, occurring on or near December 21.