[U]
[V]
[W]
[X]
[Y]
[Z]
- Ultraviolet:
- A band of electromagnetic radiation with a higher frequency and shorter wavelength than visible blue light. Most ultraviolet is absorbed by the Earth's atmosphere, so ultraviolet astronomy is normally performed in space.
- Umbra:
- The dark central shadow cast by an illuminated body.
- Uncertainty principle:
- The fact that uncertin values are inevitable at the subatomic level since measuring techniques disrupt the particles being measured. For example, a particle's precise postion and veloci
- Unifield field theory:
- A theory that would explain all the field forces as different manifestations of a single, universal force. Its development is an unralized goal of theoretical physicists.
- Uniform Motion:
- motion at a constant speed in a straight line without rotation. Uniform motion is the only kind of motion addressed by special relativity.
- Universe:
- The totality of all matter and radiation and the space it occupies. The universe is believed to have a finite age of 15 to 20 billion years.
- Vacuum:
- The lowest possible energy state.
- Variable Star:
- A star that changes in luminosity over time. Some variable stars change predictably and repeatedly; others change unpredictably or only once.
- Velocity:
- The speed and direction of motion.
- Violent Relaxation:
- A rapid approach to gravitational equilibrium by multiple bodies, such as the stars in interacting galaxies.
- Violet:
- Radiation appearing at the extreme blue end of the visible spectrum, having a short wavelength and high frequency.
- Virtual particle:
- An extremely short-lived particle created out of nothingness, as permitted by the uncertainty principle. Although it exists too briefly to be directly observed, the effects of its existence may be detected.
- Wave:
- The propagation of a pattern of disturbance.
- Wavelength:
- The distance from crest to crest, or trough to trough, of an electromagnetic or other wave. Wavelengths are related to frequenc: The longer the wavelength, the lower the frequency.
- Weak Force:
- A very short-range force responsible for particle decay.
- White Dwarf:
- An old, extremely dense star about as large as the Earth but with a mass as great as the Sun's; the remains of a star that has completely fused its helium core.
- Wierd terrain:
- The name given to an area on Mercury that is characterized by unusual hill and valley formations, thought to have formed as a result of a single giant impact on the other side of the planet.
- Worldline:
- The path through space and time of a space-time event.
- Wormhole:
- A hypothetical distortion in space-time linking widely separated black holes.
- W particle:
- see Intermediate vector boson.
- X Higgs boson:
- Theoretically, a massive boson capable of releasing the strong force from the previously unified electronuclear force. X Higgs bosons would have existed only during a brief period beginning 10^-35 second after the Big Bang; their decay would have prod
uced an excess of matter over antimatter particles that is still evident.
- X-radiation (x-ray):
- Radiation intermediate in wavelength between ultraviolet and gamma rays.
- X-rays:
- A band of electromagnetic radiation intermediate in wavelength between ultraviolet radiation and gamma rays. Because X-rays are completely absorbed by the atmosphere, x-ray astronomy must be performed in space.
- Yuri Gregarian:
- The first man in space.
- Zeeman effect:
- The splitting of a single spectral line into two or more component lines, caused by the action of a magnetic fields.
- Z particle:
- see Intermediate vector boson.