Parsec

The parsec (short word from English "parallax second", German parallax or parallactic second unit symbol pc) is a unit of length outside the International System of Units.

It is used in astronomy, in addition to the astronomical unit and light year, the distance information of celestial bodies. A parsec is approximately 3.26 light- years or about 30,856,776 million kilometers.

Definition

Parsecs a is the distance from the average distance between the earth and the sun at an angle of one arc-second (1 " = 1 ° / 3600) is displayed.

Considering a star in the course of a year, so this seems to move back and forth because the earth moves around the sun ( see picture). Beginning and end point of this apparent movement span an angle, half of this angle is the angle of parallax. The further a star is removed, the smaller the parallax. Since one can measure the parallax in astronomy, it is useful to moor the distance to distant stars in this angle. However, since the parallax just distant stars (or other objects ) is usually very small, one specifies the Parsec determined as the distance at which the parallax is exactly one arc second.

The distance of a star from Earth in parsecs can thus be calculated from the average distance of the Earth to the Sun and the parallax angle in arc seconds.

Calculation

For the very small angle of an arc second applies with sufficient accuracy:

With this and from the definition arise for conversion to other units of length following values:

In astronomy, the parsec is preferred because it is directly connected to the parallax - one of the basic methods for measuring astronomical distances.

However, the parsec not part of the International System of Units (SI). From the International Astronomical Union ( IAU short ) is defined the parsec in SI units by 30.857 Pm, where

This definition does not correspond exactly to the above geometric interpretation.

In the galactic and extragalactic astronomy, the usual resolutions for units of measurement are used, especially kiloparsecs ( kpc; typical size of galaxies ), megaparsecs ( Mpc; typical size of clusters of galaxies ) and Gigaparsec ( Gpc; diameter of the observable universe ).

Application and measurement

The parallax (above the Earth's orbit ) is the fundamental calibration step to all other astrophysical distance measurements. However, the Earth's atmosphere limits the accuracy of ground-based telescope measurements of the parallax to about 0.01 arc seconds and thus to star at a distance of up to 100 pc. Space telescopes, however, can also measure distances to objects beyond these restrictions. From 1989 to 1993, the Hipparcos satellite of ESA provided parallax measurements for about 100 000 stars with an astrometric accuracy of about one milli- arcsecond ( mas 1 ) and thus distances up to 1000 pc distant stars. In December 2013, the successor Gaia satellite was launched, which in the course of 5 years, a billion stellar distances with a precision of up to 24 micro- arcseconds (24 μas ) is intended to measure. This allows parallax measurements up to about 8000 pc distant galactic center ( in approximately 10% error).

Examples

• Proxima Centauri, the star nearest to our solar system is about 1.3 parsecs (4.2 light years) away.
• The center of our galaxy, the Milky Way is about 8.3 kpc away.
• The nearest radio galaxy (NGC 5128 or Centaurus A) is 3 Mpc away from us.
• The so-called Virgo cluster with about 3000 spiral nebulae is about 22 Mpc away from us, the Comahaufen 120 Mpc. On average, such a bunch of 50 Mpc apart.