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light-year,

in astronomy, unit of length equal to the distance lightlight,
visible electromagnetic radiation. Of the entire electromagnetic spectrum, the human eye is sensitive to only a tiny part, the part that is called light. The wavelengths of visible light range from about 350 or 400 nm to about 750 or 800 nm.
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 travels in one sidereal yearsidereal year,
time required for the earth to complete an orbit of the sun relative to the stars. The sidereal year is 365 days, 6 hr, 9 min, 9.5 sec of mean solar time (see solar time).
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. It is 9.461 × 1012 km (about 6 million million mi). Alpha CentauriAlpha Centauri
, brightest star in the constellation Centaurus and 3d-brightest star in the sky; also known as Rigil Kent or Rigil Kentaurus; 1992 position R.A. 14h39.1m, Dec. −60°49'. Its apparent magnitude is −0.26.
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 and Proxima Centauri, the stars nearest our solar system, are about 4.3 light-years distant. See also parsecparsec
[parallax + second], in astronomy, basic unit of length for measuring interstellar and intergalactic distances, equal to 206,265 times the distance from the earth to the sun, 3.26 light-years, or 3.08 × 1013 km (about 19 million million mi).
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.

light-year

(l.y.) A unit of distance equal to the distance traveled through space in one year by light, radio waves, or any other form of electromagnetic radiation. Since all electromagnetic radiation travels in a vacuum at the speed of light, one light-year equals about 9.4605 × 1012 km. Distances expressed in light-years give the time that radiation would take to cross that distance. One light-year equals 0.3066 parsecs, 63 240 astronomical units, or a parallax of 3.259 arc seconds.

Analogous but smaller units of distance, such as the light-month, light-week, light-day, and light-second are also used, often to indicate the size of an object (such as the core of an active galaxy) whose output is varying: the timescale of the variations imposes an upper limit on the size, the conditions being unlikely to change more quickly than the time it takes for light to travel across the region.

light-year

[′līt ‚yir]
(astrophysics)
A unit of measurement of astronomical distance; it is the distance light travels in one sidereal year and is equivalent to 9.461 × 1012 kilometers or 5.879 × 1012 miles.
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