valleys: Depressions in the landscapes of solid objects.


Van Allen belts: Regions of high-energy particles trapped by the magnetic field of the Earth.


variable star: A star whose brightness changes over time.


vernal equinox: The equinox crossed by the Sun as it moves to northern declinations.


Very Large Array: The National Radio Astronomy Observatory’s set of radio telescopes in New Mexico, used to gather for interferometry.


Very Large Telescope: The set of instruments, including four 8.2-m reflectors and several smaller telescopes, erected by the European Southern Observatory on a mountaintop of Chile through 2001.


Very-Long-Baseline Array: The National Radio Astronomy Observatory’s set of radio telescopes dedicated to very-long-baseline interferometry and spread over an 8000-km baseline across the United States.


very-long-baseline interferometry: The technique using simultaneous measurements made with radio telescopes at widely separated location to obtain extremely high resolution.


visible light: Light to which the eye is sensitive, 4000-7000 Ĺ.


visual binary: A binary star that can be seen through a telescope to be double.


VLA: See Very Large Array.


VLBA: See Very-Long-Baseline Array.


VLBI: See very-long-baseline interferometry.


VLT: See  Very Large Telescope.


void: A giant region of the Universe in which few galaxies are found.


volatile: Evaporating (changing to a gas) readily.




wave front: A plane in which parallel waves are in step.


wavelength: The distance over which a wave goes through a complete oscillation.


weak nuclear force: One of the four fundamental forces of nature, weaker than the strong force and the electromagnetic force.  It is important only in the decay of certain elementary particles, such as neutrons.


weight : The force of the gravitational pull on a mass.


white dwarf: The final stage of the evolution of a star initially between 0.08 and above 8 solar masses but with a final mass no larger than 1.4 M. It is supported by electron degeneracy white dwarfs are found to the lower left of the main sequence of the temperature-luminosity diagram.


white light: All the light of the visible spectrum together.


Wien’s displacement law: The expression of the inverse relationship of the temperature of a black body and the wavelength of the peak of its emission.


WIMPs: Weakly Interacting Massive Particles, an as yet undiscovered massive particle, interacting with other elementary particles only through the weak nuclear force, that may be a major type of cold dark matter.


winter solstice: For northern-hemisphere observers, the southernmost declination of the Sun, and its date.


Wolf-Rayet Star: A type of O star whose spectrum shows very broad emission lines.


wormhole: The hypothetical connection or bridge between two black holes; they probably do not exist except in idealized black holes.




x-rays: Electromagnetic radiation between 0.1 and 100 Ĺ.



year: The period of revolution of a planet around its central star; more particularly, the Earth’s period of revolution around the Sun.




Zeeman effect: The splitting of certain spectral lines in the presence of a magnetic field.


zenith: The point in the sky directly overhead an observer.


zero-age main sequence: The curve of a temperature-luminosity diagram determined by the locations of stars at the time they begin nuclear fusion.


zodiac: The band of constellations through which the Sun, Moon, and planets move in the course of a year.


zodiacal light: A glow in the nighttime sky near the ecliptic from sunli8ght reflected by interplanetary dust.