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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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The Columbia Electronic Encyclopedia™ Copyright © 2013, Columbia University Press. Licensed from Columbia University Press. All rights reserved.


(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.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006


[′līt ‚yir]
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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Also, a team of Arizona State University astronomers this month claimed to have found a galaxy from 13 billion light-years away.
Using the shapes and speed of these disturbances to indicate the waves' direction, O'Dell finds that a series of shock waves in the outer part of Orion "trace right back into the middle" of the nebula, about 6 light-years away, he told Science News.
At the Crab's distance of 6,300 light-years, that eyepiece spans about 0.5 x 6,300 / 60 [approximately equal to] 52 light-years.
However, it's often important in modern astrophysics to know the light-crossing time of a given distance--and if the distance is given in light-years, there you are.
Published redshifts (z values) can be converted to light-years with online calculators; note that the resulting distance depends on which cosmological model is adopted.
This ( photo  taken by the Hubble Space Telescope shows the nebula, which is more than 7,000 light-years away and hangs out in the constellation Cassiopeia.
In 2007 a clever statistical technique, based on surface-brightness fluctuations in the cluster's elliptical galaxies, yielded 54 million light-years.
He notes that the separation among the quasars, about 100,000 light-years, is a distance typical of interacting galaxies.
Our Milky Way galaxy may be on a crash course with the Andromeda galaxy, 2.2 million light-years away.
In the first, Istvan Dekany (Millennium Institute of Astrophysics, Chile) and colleagues announced the discovery of two Cepheids 37,000 light-years from Earth and 11,000 light-years from our galaxy's center.
The ring is about 4,900 light-years long and 3,300 light-years wide.
And that's why astronomers measures a star's brightness two ways: by its apparent magnitude (how bright a star looks when seen from Earth) and by its absolute magnitude (how bright a star would look if it were 33 light-years away from Earth).