light-year


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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.
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.
More realistically, what would they look like in the company of some stars that you're used to viewing--like the ones in M11, the Wild Duck Cluster in Scutum, which at 6,000 light-years is nearly as far away as the Crab?
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.
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.
html) photo from the Spitzer Space Telescope, to "mountains" and says they are about 50 light-years across.
Ford (Johns Hopkins University) have derived a new distance of 48 [+ or -] 3 million light-years, using planetary nebulae as their distance indicators.
In a study last year, Michael Feast (University of Cape Town, South Africa) and colleagues found five "classical" Cepheids in the outer reaches of the Milky Way's disk, between 42,000 and 72,000 light-years from the galaxy's center.
5 billion light-years from Earth, Djorgovski reported last week at a meeting of the American Astronomical Society in Seattle.
The Milky Way Galaxy is a thin spiral, with 85% of its stars in a disk only 3,000 light-years thick.