# What is a parsec and how is it measured?

Astronomical units of measurement are mostly pretty straight-forward:

• Astronomical Units refer to the mean Earth-Sun distance (~150 million km or 93 million miles)

• Light years are the distance light travels in a year (~9.46 × 10^12 km)

Another astronomical unit of measurement is the parsec. What is a parsec and how is it measured?

A parsec (abbreviated pc) is a unit of distance used by astronomers, cosmologists, and astrophysicists.

1 parsec is equal to $3.08567758 \times10^{16}$ meters, or $3.26163344$ light years (ly).

A few typical scales to keep in mind:

1) The disks of galaxies like the Milky Way are a few 10's of kpc (that's pronounced kiloparsecs, which are 1000's of parsecs) in size. The dark matter halos surrounding them extend out to almost an order of magnitude further. 2) Galaxy clusters come in at around 1 Mpc (that's pronounced megaparsecs, or millions of parsecs), and are the largest bound objects in the universe. Pictured below is the galaxy cluster Abell 2218. 3) The closest star to the planet Earth (not counting the sun, of course) is the star system Alpha Centauri, at a distance of just about 1.3 parsecs. While you may think that this is incredibly close (and it is by cosmological standards), it would nonetheless still take us 4.24 years to travel to it if we traveled at the speed of light. A parsec is approximately 3.26 times a light year or 206,265 Astronomical Units (AU).

According to "Cosmic Reference Guide" (Caltech), the word 'parsec' stands for "parallax of one arc second", and is (according to the website):

the length of the long leg of a right triangle, whose short leg is one astronomical unit when the angle between the Sun and the Earth, as seen from an object in space (a star for example), is one arcsecond 