U

ultraluminous X-ray source (ULX) An object that gives off more X-rays than most "normal" binary systems in which a companion star is in orbit around a neutron star or black hole.

ultraviolet Region of the electromagnetic spectrum, just outside the visible range, corresponding to wavelengths slightly shorter than blue light. [More Info]

universe The totality of all matter, radiation and space; everything accessible to our observations.

V

Van Allen belts Doughnut-shaped regions of magnetically trapped charged particles high above the Earth's atmosphere. [More Info]

variable star A star whose luminosity changes with time. [More Info]

visible spectrum The small range of the electromagnetic spectrum that human eyes perceive as light. The visible spectrum ranges from about 4000 to 7000 angstroms, corresponding to blue through red light. [More Info]

visible light The small range of the electromagnetic spectrum that human eyes perceive as light. The visible spectrum ranges from about 400 to 700 nm, corresponding to blue through red light. [More Info]

visual binary A binary star system in which both members are resolvable from Earth.

void Large, relatively empty region of the universe around which superclusters of galaxies are organized.