Parsec
Unit of length used in astronomy / From Wikipedia, the free encyclopedia
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The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 3.26 light-years or 206,265 astronomical units (AU), i.e. 30.9 trillion kilometres (19.2 trillion miles).[lower-alpha 1] The parsec unit is obtained by the use of parallax and trigonometry, and is defined as the distance at which 1 AU subtends an angle of one arcsecond[1] (1/3600 of a degree). The nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun.[2] Most stars visible to the naked eye are within a few hundred parsecs of the Sun, with the most distant at a few thousand.[3]
| Parsec | |
|---|---|
 A parsec is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond (not to scale) | |
| General information | |
| Unit system | astronomical units |
| Unit of | length/distance |
| Symbol | pc |
| Conversions | |
| 1 pc in ... | ... is equal to ... |
| metric (SI) units | 3.0857×1016 m ~31 petametres |
| imperial & US units | 1.9174×1013 mi |
| astronomical units | 2.06265×105 au 3.26156 ly |
The word parsec is a portmanteau of "parallax of one second" and was coined by the British astronomer Herbert Hall Turner in 1913[4] to simplify astronomers' calculations of astronomical distances from only raw observational data. Partly for this reason, it is the unit preferred in astronomy and astrophysics, though the light-year remains prominent in popular science texts and common usage. Although parsecs are used for the shorter distances within the Milky Way, multiples of parsecs are required for the larger scales in the universe, including kiloparsecs (kpc) for the more distant objects within and around the Milky Way, megaparsecs (Mpc) for mid-distance galaxies, and gigaparsecs (Gpc) for many quasars and the most distant galaxies.
In August 2015, the International Astronomical Union (IAU) passed Resolution B2 which, as part of the definition of a standardized absolute and apparent bolometric magnitude scale, mentioned an existing explicit definition of the parsec as exactly 648000/π au, or approximately 30.856775814913673×1015 metres (based on the IAU 2012 definition of the astronomical unit). This corresponds to the small-angle definition of the parsec found in many astronomical references.[5][6]