- U -
ultraviolet radiation  Electromagnetic radiation of wavelengths shorter than the shortest visible wavelengths; radiation of wavelengths in the approximate range 10 to 400 nm.
umbra The central, completely dark part of a shadow.
uncertainty principle Heisenberg uncertainty principle. It is fundamentally impossible to make simultaneous measurements of a particle's position and velocity with infinite accuracy.
universe The totality of all matter, radiation and space; everything accessible to our observations.
variable star A star that varies in luminosity.
vernal equinox The point on the celestial sphere where the Sun crosses the celestial equator passing from south to north; a time in the course of the year when the day and night are roughly equal.
very-long-baseline interferometry (VLBI) A technique of radio astronomy whereby signals from telescopes thousands of kilometers apart combined to obtain very high resolution by letting waves from different sites interfere with each other.
visual binary star A binary star in which the two components are telescopically resolved.
void A region between clusters and superclusters of galaxies that appears relatively empty of galaxies.
- V -

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