Parsec

The parsec (symbol: pc) is a unit of length used to measure very large distances to objects outside our Solar System. One parsec is about equal to 3.26 light-years or 206,265 astronomical units, which is around 30.9 trillion kilometres. This unit helps scientists measure how far away stars and other objects in space are.

The parsec is based on a method called parallax and a math tool called trigonometry. It is defined as the distance at which the space between the Earth and the Sun appears to take up an angle of one arcsecond. The closest star to us, Proxima Centauri, is about 1.3 parsecs from the Sun. Most stars we can see without a telescope are within a few hundred parsecs, while bigger units like kiloparsecs, megaparsecs, and gigaparsecs are used for even farther objects like galaxies and quasars.

The word parsec comes from "a distance corresponding to a parallax of one arcsecond," created by British astronomer Herbert Hall Turner in 1913. It was made to make calculating distances from space observations easier. While the light-year is often used in popular science, astronomers and astrophysicists prefer the parsec because it works better with their calculations. In 2015, the International Astronomical Union gave an exact meaning to the parsec to help make sure everyone uses the same numbers.

History and derivation

See also: Stellar parallax

The parsec is a way to measure very far distances in space. Imagine a triangle where one side is the distance between the Earth and the Sun, called an astronomical unit. If we look at a star from two places — when Earth is on one side of the Sun and then six months later on the other side — the star seems to move a little. This tiny movement helps us measure how far away the star is.

The first person to measure a star’s distance this way was Friedrich Wilhelm Bessel in 1838. He measured the distance to a star called 61 Cygni.

The parsec is based on this method. If a star’s apparent movement, called its parallax, is one arcsecond, then the distance to that star is one parsec. This is a lot — about 3.26 light-years!

The word “parsec” was first used in 1913. An astronomer named Frank Watson Dyson wanted a name for this unit. Another astronomer, Herbert Hall Turner, suggested “parsec,” and that’s the name we use today.

To find the exact length of a parsec, scientists use geometry and careful measurements. They know the exact distance between Earth and the Sun, and they measure angles very precisely. This lets them calculate that one parsec equals about 206,265 astronomical units, or about 30.86 trillion meters.

This way of measuring distances helps astronomers study stars and galaxies that are very, very far away.

Usage and measurement

The parallax method helps us find distances to stars. On Earth, we can measure distances up to about 100 parsecs because of the atmosphere. Space telescopes, like the Hipparcos satellite, can see farther. It measured distances for about 100,000 stars up to 1,000 parsecs away.

The Gaia_ satellite, launched in 2013, aims to measure distances for one billion stars very accurately, even as far as the Galactic Centre in the constellation of Sagittarius.

Distances in parsecs

Distances in space can be measured using a unit called a parsec. One parsec is about three and a half light-years, which is a very big distance! Objects very close to us, like within our solar system, are measured in fractions of a parsec. For example, the distance from the Sun to Earth is only a tiny part of a parsec.

We also use kiloparsecs (kpc) and megaparsecs (Mpc) for even bigger distances. A kiloparsec is one thousand parsecs, and a megaparsec is one million parsecs. These help us talk about distances between stars, galaxies, and groups of galaxies. For example, the nearest star to us, Proxima Centauri, is about 1.3 parsecs away. The center of our galaxy, the Milky Way, is more than 8 kiloparsecs from us. And the Andromeda Galaxy, our nearest big neighbor, is about 780 kiloparsecs away.