Parsecs
| Parsec | |
|---|---|
A parsec is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond (the diagram is not to scale).
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| Unit system | astronomical units |
| Unit of | length |
| Symbol | pc |
| 1 pc in ... | ... is equal to ... |
| metric (SI) units |
3.0857×1016 m ~31 Petametres |
| imperial & US units | 1.9174×1013 mi |
| astronomical units |
2.0626×105 au 3.26156 ly |
A parsec (symbol: pc) is a unit of length used to measure large distances to objects outside the Solar System. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. A parsec is equal to about 3.26 light-years (31 trillion kilometres or 19 trillion miles) in length. The nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun. Most of the stars visible to the unaided eye in the nighttime sky are within 500 parsecs of the Sun.
The parsec unit was likely first suggested in 1913 by the British astronomer Herbert Hall Turner. Named from an abbreviation of the parallax of one arcsecond, it was defined so as to make calculations of astronomical distances quick and easy for astronomers from only their raw observational data. Partly for this reason, it is still the unit preferred in astronomy and astrophysics, though the light-year remains prominent in popular science texts and everyday 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 all but the closest galaxies, and gigaparsecs (Gpc) for many quasars and the most distant galaxies.
In August 2015, the IAU passed Resolution B2, which as part of the definition of a standardized absolute and apparent bolometric magnitude scale, included an explicit definition of the parsec as exactly 648000/π astronomical units, or approximately 3.08567758149137×1016 metres (based on the IAU 2012 exact SI definition of the astronomical unit). This corresponds to the small-angle definition of the parsec found in many contemporary astronomical references.
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