Parsecs

A parsec is a unit of length used in astronomy to express distances to stars and other objects outside the Solar System. It is defined as the distance at which one astronomical unit (the average Earth-Sun distance) subtends an angle of one arcsecond. In other words, a star at a distance of 1 parsec would have a parallax of 1 arcsecond as observed from Earth. The term parsec is short for parallax arcsecond and was introduced by astronomer Herbert Hall Turner in 1913.

The distance d to a star measured by parallax p (in arcseconds) is d = 1/p parsecs. This relation follows from simple trigonometry using the 1 AU baseline corresponding to Earth's orbit. One parsec equals approximately 206,265 AU, which converts to about 3.086e16 meters or about 3.26156 light-years (roughly 3.26 ly).

Parsecs are commonly used for stellar distances within the Milky Way. Distances to other galaxies or large-scale structures are usually given in kiloparsecs (kpc) or megaparsecs (Mpc). For example, Proxima Centauri is about 1.3 parsecs away. The accuracy of parallax measurements improved with space-based astrometry from missions such as Hipparcos and Gaia, enabling reliable distances to far more stars than was possible from the ground.