A parsec is the 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, has a parallax of 0.753 second of arc; hence, its distance from the Sun and the Earth is 1.33 parsec. One parsec equals 3.26 light-years, which is equivalent to 3.09x1013 km (1.92x1013 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 3,000 megaparsecs, or 9,000,000,000 to 10,000,000,000 light-years.
Excerpt from the Encyclopedia Britannica without permission.