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

ultraviolet telescope A telescope that is designed to collect radiation in the ultraviolet part of the spectrum. The Earth's atmosphere is partially opaque to these wavelengths, so ultraviolet telescopes are put on rockets, balloons, or satellites to get high above most or all of the atmosphere.

umbra Central region of the shadow cast by an eclipsing body. The central region of a sunspot, which is its darkest and coolest part.

unbound An orbit which does not stay in a specific region of space, but where an object escapes the gravitational field of another. Typical unbound orbits are hyperbolic in shape.

universe The totality of all space, time, matter, and energy.