Parsec
The parsec (abbreviated pc) is a unit of length used in astronomy. It stands for "parallax of one arc second". One parsec is the distance at which the Earth's orbit around the Sun appears to have an angular radius of one arcsecond. It is thus 1296000/2π = 206265 AU.
One parsec is equal to 3.085 677 580 666 31 × 1016 m, or 3.26 light years. For historical reasons, astronomers usually express distances to astronomical objects in units of parsecs, instead of light years; the first direct measurements of an object at interstellar distances (of the star 61 Cygni, by Friedrich Wilhelm Bessel in 1838) were done via trigonometry using the width of the Earth's orbit as a baseline. The parsec follows naturally from this method.
One kiloparsec, denoted kpc, is one thousand parsecs. One megaparsec, denoted Mpc, is one million parsecs. The distance from Earth to the center of the Milky Way is about 8kpc and the distance from Earth to M100 is about 17Mpc.
A parsec is the fundamental unit of distance in astronomy. It is based on the method of trigonometric parallax, the most ancient and standard method of determining stellar distances.
The angle subtended at a star by the mean radius of the Earth's orbit around the Sun is called the parallax. Given two points on opposite ends of the orbit, the parallax is half the maximum parallactic shift evident from the star viewed from the two points. The parsec is simply the reciprocal of a parallax of 1 arcsec.
There is no star whose parallax is 1 arcsec. The greater the parallax of the star the closer it is to the Earth, and the smaller its distance in parsecs. Therefore the closest star to the Earth will have the largest measured parallax. This belongs to the star Proxima Centauri, with a parallax of 0.762 arcsecs, and lying approximately 4.28 light years, or 1.3 parsecs, away from us.
The measurement of distances of celestial bodies from the Earth in parsecs is a key aspect of Astrometry, the science of making positional measurements of celestial bodies.
Because of the extremely small scale of parallactic shifts, ground-based parallax methods provide reliable measurements of stellar distances of no more than 325 light years, or about 100 parsecs, corresponding to parallaxes of no less than 1/100 of 1 arcsec, or 10 mas (1 mas or milli arc sec = 1/1000 arc sec).
Between 1989 and 1993 the Hipparcos satellite, launched by the European Space Agency (ESA) in 1989, measured parallaxes for about 100,000 stars, with a precision of about 0.97 mas (almost 1 mas), and obtained accurate measurements for stellar distances of around 1000 pc. NASA's FAME satellite was due to be launched in 2004, to measure parallaxes for about 40 million stars as small as 50 micro arcsecs, corresponding to stellar distances of up to 2000 pcs. However, the mission's funding was withdrawn by NASA in January 2002.
The ESA's GAIA satellite, due to be launched in mid-2012, will be of sufficiently high astrometric precision to measure stellar distances to within 10% accuracy as far as the galactic centre over 10 kpc away.
How to calculate the value of a parsec:
One astronomical unit is equal to approximately 1.49568 × 10 8 km
Using a triangle (with angles A, B, C and sides (oppose to respective angle) a, b, c): angle B = 1/3600 degrees (one second, is 1/60 of a minute which is 1/60 of a degree) angles A = C = 89 7199/7200 (all angles of a triangle add upto 180 degrees)
side B = 1 AU or 1.49568 × 10 8 km
Using the sine law:
a/b = (sin A)/(sin B)
You can then get the value of 30850614540795.9km
Calculate value of a parsec:
Form a right-angled triangle with base being the mean radius of the Earth's orbit around the Sun (this being the astronomical unit (AU) of value approximately 149,568,000 Km), and height being the distance from the Sun to a hypothetical star which is such that the angle, say A, subtended at the star (parallax) by the base (1 AU) is equal to 1 arcsec (1/3600 of 1 degree). The parsec is of course the unknown hypotenuse of the triangle, x Km, representing the distance from the star to the Earth. We have sin A = 1 AU/x Km. or x Km = 1 AU/sin (1/3600)degrees, which is approximately 3.085 x 10^13 Km, or 30.85 x 10^12 Km. This is the approximate distance from the Earth to a star whose parallax is 1 arc second. 1 ly is approximately 9.461 x 10^12 Km, therefore this distance is equal to about 3.26 light years. This unit is the parsec.
See also: conversion of units.
External links
In computer science, a parsec is an XML syntax analizer, like Lark