parsec

/pahr"sek'/, n. Astron.

a unit of distance equal to that required to cause a heliocentric parallax of one second of an arc, equivalent to 206,265 times the distance from the earth to the sun, or 3.26 light-years.

[1910-15; PAR(ALLAX) + SEC(OND)²]

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Unit of measure used by astronomers to express distances to stars and galaxies.

It is the distance at which the radius of Earth's orbit would subtend an angle of one second of arc, so an object one parsec away would have a parallax of one second. An object's distance in parsecs is the reciprocal of its parallax in seconds of arc. For example, Alpha Centauri, with a parallax of 0.76 second, is 1.33 parsecs from the Sun and Earth. One parsec equals 3.26 light-years, or 19.2 trillion mi (30.9 trillion km).

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▪ unit of measurement

      unit for expressing distances to stars and galaxies, used by professional astronomers. It represents the distance at which the radius of the Earth's orbit subtends an angle of one second of arc; thus a star at a distance of one parsec would have a parallax of one second, and the distance of an object in parsecs is the reciprocal of its parallax in seconds of arc. For example, the nearest triple-star system, Alpha Centauri A and B and Proxima Centauri, has a parallax of 0.747 second of arc; hence, its distance from the Sun and the Earth is 1.34 parsec. One parsec equals 3.26 light-years (light-year), which is equivalent to 3.09 × 10¹³ km (1.92 × 10¹³ miles). In the Milky Way Galaxy, wherein the Earth is located, distances to remote stars are measured in terms of kiloparsecs (1 kiloparsec = 1,000 parsecs). The Sun is at a distance of 8.5 kiloparsecs from the centre of the Milky Way system. When dealing with other galaxies or clusters of galaxies, the convenient unit is the megaparsec (1 megaparsec = 1,000,000 parsecs). The distance to the Andromeda Galaxy (Messier 31) is about 0.7 megaparsec. Some galaxies and quasars have likely distances on the order of about 4,000 megaparsecs, or 13,000,000,000 light-years.

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