Glossary
Astronomy glossary
514 definitions · page 9 of 9.
- tropical year
- the time it takes Earth to revolve around the sun with respect to the vernal equinox
- true field of view
- the angle of sky seen through an eyepiece when it is attached to a telescope; the true field equals the apparent field divided by the magnification
- type Ia supernova
- the explosion of a white dwarf that occurs when it accretes enough mass from a companion star to go above the Chandrasekhar limit
- type II quasars
- a quasar enshrouded in gas and dust that emits very little visibile light, however, is easily seen in the infrared and x-ray region of the electromagnetic spectrum
- type II supernova
- the explosion of a massive star that occurs when its core runs out of nuclear fuel; these explosions leave behind a neutron star or a black hole
- ultraviolet light
- radiation with higher energy than visible light, but without as much energy as x rays
- ultraviolet radiation
- radiation with higher energy than visible light, but without as much energy as x rays
- umbra
- 1. the dark, central region of a shadow from which none of the light source can be seen2. the dark center of a sunspot
- unidentified infrared bands (UIBs)
- mysterious objects in space that give off as yet unidentified infrared emission patterns
- Universal Time
- the local time of day on a line of longitude centered on Greenwich, England; also known as Greenwich Mean Time, it forms the basis for all civil timekeeping
- Universe
- everything that exists
- UT
- abbreviation for Universal Time, the local time of day on a line of longitude centered on Greenwich, England; also known as Greenwich Mean Time, it forms the basis for all civil timekeeping
- UV
- short for ultraviolet; UV radiation has more energy than visible light but less energy than x rays
- Van Allen belts
- two belts of charged particles from the solar wind that have been trapped by Earth’s magnetic field above Earth’s atmosphere
- variable star
- a star that varies in luminosity
- vernal equinox
- the time of year around March 21 when the sun crosses the celestial equator heading north
- vignetting
- uneven or reduced illumination over the image plane in a telescope or camera, causing distortion such as dimming near the edge of an image
- Virgo Cluster
- a group of about 2,500 known galaxies lying near the north galactic pole in the constellation Virgo
- visible light
- the portion of the electromagnetic spectrum visible to the human eye
- voids
- enormous regions of relatively empty space between galaxy superclusters
- volatiles
- chemical compounds that are gaseous at low temperatures.
- waning
- the period between full moon and new moon
- wavelength
- the distance between two successive wave crests or troughs
- waxing
- the period between new moon and full moon
- weight
- the force exerted on an object due to gravity
- white dwarf
- the dense, collapsed, Earth-sized remnant of an intermediate-mass star like the sun
- winter
- a season that begins around December 21 in the Northern Hemisphere
- Wolf-Rayet star
- a very luminous and very hot star with temperatures reaching as high as 90,000 kelvins
- X rays
- electromagnetic radiation more energetic than ultraviolet light but less energetic than gamma rays
- X-class flares
- the brightest and most energetic type of solar flares
- zenith
- the point on the celestial sphere directly over the head of an observer
- zenithal hourly rate
- the number of meteorites expected to be seen per hour when a meteor shower's radiant is at an observer's zenith; abbreviated ZHR
- zodiac
- a band around the celestial sphere 18° in width and centered on the ecliptic
- zodiacal light
- a faint, cone-shaped glow of light seen in the west after nightfall or in the east before dawn, caused by sunlight reflecting and scattering off interplanetary dust particles lying along the ecliptic plane