The Andromeda Galaxy ( M31) is about 780 kpc (2.5 million light-years) away from the Earth.Ī distance of one million parsecs is commonly denoted by the megaparsec (Mpc).The centre of the Milky Way is more than 8 kiloparsecs ( 26 000 ly) from the Earth, and the Milky Way is roughly 34 kpc ( 110 000 ly) across.The distance to the open cluster Pleiades is 130 ☑0 فخ ( 420 ☓0 ly) from us, per Hipparcos parallax measurement.Proxima Centauri, the nearest known star to earth other than the sun, is about 1.30 parsecs (4.24 light-years) away, by direct parallax measurement.One parsec is approximately 3.26 light-years.Astronomers typically use kiloparsecs to express distances between parts of a galaxy, or within groups of galaxies. A distance of 1000 parsecs (3262 light-years) is commonly denoted by the kiloparsec (kpc). (image from Hubble Space Telescope) Parsecs and kiloparsecsĭistances expressed in parsecs (pc) include distances between nearby stars, such as those in the same spiral arm or globular cluster. The jet erupting from the active galactic nucleus of M87 is thought to be 1.5 kiloparsecs ( 4890 ly) long. To the nearest meter, the small-angle parsec corresponds to 30,856,775,814,913,673 m. As the astronomical unit was defined by the IAU (2012) as an exact SI length in metres, so now the parsec corresponds to an exact SI length in metres. This differs from the classic inverse- tangent definition by about 200 km, i.e. The length of the parsec used in IAU 2015 Resolution B2 (exactly 648 000 / π astronomical units) corresponds exactly to that derived using the small-angle calculation. if Distance earth-sun = 1 AU, unit for Distance star is in astronomical units if Distance earth-sun = 1.5813×10 −5 ly, unit for Distance star is in light years). The calculated stellar distance will be in the same measurement unit as used in Distance earth-sun (e.g.
Where θ is the measured angle in arcseconds, Distance earth-sun is a constant (1 AU or 1.5813×10 −5 ly). Through trigonometry, the distance SD is calculated as follows: The angle SDE is one arcsecond ( 1 / 3600 of a degree) so by definition D is a point in space at a distance of one parsec from the Sun. Thus the distance ES is one astronomical unit (au). In the diagram above (not to scale), S represents the Sun, and E the Earth at one point in its orbit. Stellar parallax motion from annual parallax This corresponds to the small-angle definition of the parsec found in many contemporary astronomical references. In August 2015, the 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 648 000 / π astronomical units, or approximately 3.085 677 581 491 37 ×10 16 metres (based on the IAU 2012 exact SI definition of the astronomical unit). 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. 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. Named as a portmanteau of the parallax of one arc second, it was defined to make calculations of astronomical distances from only their raw observational data quick and easy for astronomers. The parsec unit was probably first suggested in 1913 by the British astronomer Herbert Hall Turner. A parsec is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond (not to scale)
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