define light year,astronomical unit and parsec.
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Parsec
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For other uses, see Parsec (disambiguation).
The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System. One parsec is approximately equal to 31 trillion kilometres (19 trillion miles),[a] or 210,000 astronomical units, and equates to about 3.3 light-years. A parsec is obtained by the use of parallax and trigonometry, and is defined as the distance at which one astronomical unit subtends an angle of one arcsecond[1] (1/3600 of a degree). This corresponds to approximately 648000/π astronomical units, i.e. 1 pc = 1 au/tan(1″).[2] The nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun.[3] Most of the stars visible to the unaided eye in the night sky are within 500 parsecs of the Sun.[citation needed]
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 informationUnit systemastronomical unitsUnit oflength/distanceSymbolpc Conversions1 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 parsec unit was probably first suggested by the British astronomer Herbert Hall Turner in 1913.[4] Named as a portmanteau of the parallax of one arcsecond, it was defined to make calculations of astronomical distances from only their raw observational data quick and easy for astronomers. 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 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.[5][6]
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