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:''See 1 E+16 m for comparable lengths, and scientific notation for an explanation of the notation used in this article.''
The 'parsec' (symbol pc) is a unit of length used in astronomy. The length of the parsec is based on the method of trigonometric parallax, one of the oldest methods for measuring the distances to stars.
The name ''parsec'' stands for "'par'allax of one 'sec'ond of arc", and one parsec is defined to be the distance from the Earth to a star that has a parallax of 1 arcsecond. The actual length of a parsec is approximately 3.086×10¹⁶ m, or about 3.262 light-years.

Contents

History

Usage and Measurement

Distances in parsecs

Distances less than a parsec

Parsecs and kiloparsecs

Megaparsecs and gigaparsecs

Calculating the value of a parsec

Parsecs in Pop Culture

References

Notes

See also

History

The first direct measurements of an object at interstellar distances were undertaken by German astronomer Friedrich Wilhelm Bessel in 1838, who used the width of the Earth's orbit as a baseline to calculate the distance of 61 Cygni using parallax and trigonometry.^[1] The parallax of a star is half of the angular distance a star appears to move relative to the celestial sphere as Earth orbits around the Sun; or, equivalently, it is the angle subtended at a star by the semi-major axis of the Earth's orbit.
The use of the parsec as a unit of distance follows naturally from this method, since distance (in parsecs) is simply the reciprocal of the parallax angle (in arcseconds). That is, it is the distance at which the semi-major axis of the Earth's orbit would subtend an angle of one second of arc. (See diagram above.)
Though it had probably been used before, the term parsec was first mentioned in an astronomical publication in 1913, when Astronomer Royal Frank Watson Dyson expressed his concern for the need of a name for that unit of distance: he proposes the name ''astron'', but mentions that Carl Charlier had suggested ''siriometer'', and Herbert Hall Turner had suggested ''parsec''.^[2]

Usage and Measurement

The parallax method is the fundamental calibration step for distance determination in astrophysics, and the natural unit for such measurements, the parsec, has become the most commonly used unit of distance in scholarly astronomical publications. Articles aimed at a wider audience, such as in newspapers and popular science magazines, often use a more intuitive unit, the light-year (ly).
Other than the Sun, which has a parallax of 90 degrees, there is no known star whose parallax is more than 1 arcsecond. The next closest star is Proxima Centauri with a parallax of 0.77233 arcseconds; it is thus 1.295 pc (4.225 LY) away from the Earth.
Refraction caused by the atmosphere, also known as astronomical seeing, limits ground-based telescopes to parallax angle measurement accuracies of less than approximately 0.01 arcsec, so reliable measurements, those with errors of 10% or less, can only be achieved at stellar distances of no more than about 100 pc, or 325 LY. Space-based telescopes are not limited by this effect and can accurately measure distances to objects beyond the limit of ground-based observations.
Between 1989 and 1993, the Hipparcos satellite, launched by the European Space Agency (ESA), measured parallaxes for about 100,000 stars with an astrometric precision of about 0.97 milliarcseconds, and obtained accurate measurements for stellar distances of stars up to 1,000 pc away.^[3] NASA's FAME satellite was due to be launched in 2004, to measure parallaxes for about 40 million stars with sufficient precision to measure stellar distances of up to 2,000 pc. However, the mission's funding was withdrawn by NASA in January 2002.^[4] ESA's GAIA satellite, due to be launched in December 2011, is intended to measure one billion stellar distances to within 20 microarcseconds, producing errors of 10% in measurements as far as the Galactic Center, about 8,000 pc away in the constellation of Sagittarius.^[5]

Distances in parsecs

Distances less than a parsec

Distances measured in fractions of a parsec usually involve objects within a single star system. So, for example:

★ one astronomical unit (AU) - the distance from the Sun to the Earth - is 4.85 pc.

★ the most distant space probe, Voyager 1, was 4.6 pc away from Earth in September 2004. It took Voyager 27 years to cover that distance.

★ the Oort cloud is postulated to be approximately 0.6 pc in diameter.

Parsecs and kiloparsecs

Distances measured in ''parsecs'' include distances between nearby stars, such as those in the same spiral arm or globular cluster. A distance of one thousand parsecs (approximately 3,262 ly) is commonly denoted by the ''kiloparsec'' (kpc). Astronomers typically use kiloparsecs to measure distances between parts of a galaxy, or within groups of galaxies. So, for example:

★ one parsec is approximately 3.262 light years.

★ the nearest known star to the Earth, other than the Sun, is Proxima Centauri, 1.29 parsecs away.

★ the center of the Milky Way is about 8 kpc from the Earth, and the Milky Way is about 30 kpc across.

★ the Andromeda Galaxy (M31), the most distant object visible to the naked eye, is a little under 800 kpc away from the Earth.

Megaparsecs and gigaparsecs

A distance of one million parsecs (approximately 3,262,000 ly or 2×10¹⁹ miles) is commonly denoted by the ''megaparsec'' (Mpc). Astronomers typically measure the distances between neighboring galaxies and galaxy clusters in megaparsecs.
Galactic distances are sometimes given in units of Mpc/h (as in "50/h Mpc"). ''h'' is a parameter in the range [0.5,0.75] reflecting the uncertainty in the value of the Hubble constant for the rate of expansion of the universe (''H'' = 100''h'' km/s/Mpc). The Hubble constant becomes relevant when converting an observed redshift ''z'' into a distance using the formula ''d'' ≈ ''c'' / ''Hz'' (where ''c'' is the velocity of light)^[6].
One ''gigaparsec'' (Gpc) is one billion parsecs — one of the largest distance measures commonly used. One gigaparsec is about 3.262 billion light-years, or roughly one fourteenth of the distance to the horizon of the observable universe (dictated by the cosmic background radiation). Astronomers typically use gigaparsecs to measure large-scale structures such as the size of and distance to the Great Wall, the distances between clusters of galaxies, and the distance to quasars.
For example:

★ the Andromeda Galaxy is 0.77 Mpc away from the Earth.

★ the nearest large galaxy cluster, the Virgo Cluster, is about 18 Mpc away from the Earth.

★ the galaxy RXJ1242-11, observed to have a supermassive black hole core similar to the Milky Way's, is about 200 Mpc away from the Earth.

★ the particle horizon (the observable part of the universe) has a radius of about 14 Gpc. See observable universe.

Calculating the value of a parsec

In the diagram above ''(not to scale)'', 'S' represents the Sun, and 'E' the Earth at one point in its orbit. 'D' is an object at a distance of one parsec from the Sun. By definition, the angle 'D' is one arcsecond, and the distance 'ES' is one astronomical unit (AU). By trigonometry, the distance 'SD' is
:
One AU = 149,597,870.7 km, so 1 parsec = 30,856,775,813,057.3 km ≈ 30.857×10¹⁵ meters ≈ 3.26163626 light-years.

Parsecs in Pop Culture

Parsecs have been mentioned in many science fiction novels (the works of Isaac Asimov for example), television series (''Star Trek'', ''Futurama'', and others), and a number of films.
The most famous use (or misuse) is from ''. In the Mos Eisley Cantina, Han Solo refers to the Millennium Falcon as "the ship that made the Kessel Run in less than 12 parsecs". It is not clear whether the unit is being used to measure time or distance in this context. However, later novels in this fictional universe attributed this claim to the ability of the Millennium Falcon to reach the planet Kessel in the shortest distance despite having to pass near a very dangerous region of space, The Maw, populated by numerous black holes. This shorter course would require them to cut closer to the black holes of The Maw, implying a higher ship speed to avoid the stronger gravitational forces involved in the route.
Stephen Colbert's Tek Jansen character often says he will be back in "three parsecs", incorrectly using the term as a measure of time.
In the novel ''A Wrinkle in Time'' by Madeleine L'Engle, Dr. Alexander Murry's nickname for his daughter Meg (Margaret) Murry is "Megaparsec".

References

★ Astronomical Distance Scales Dr. Michael Guidry

Notes

1. Bessel, FW, "Bestimmung der Entfernung des 61sten Sterns des Schwans" (1838) ''Astronomische Nachrichten'', Vol.16, p.65-96.
2. Dyson, F. W., "The distribution in space of the stars in Carrington's Circumpolar Catalogue" (1913) ''Monthly Notices of the Royal Astronomical Society'', Vol. 73, p.334-342; see footnote on p.342.
3. The Hipparcos Space Astrometry Mission
4. FAME news, 25 January 2002.
5. GAIA from ESA.
6. Galaxy structures: the large scale structure of the nearby universe

See also

★ Conversion of units

★ Small-angle approximation

★ Attoparsec: a unit humorously used by some hackers. It is approximately 3.1 centimetres — just a little over an inch.

★ Light-year

★ Cosmic Distance Ladder