The Parsec

We have seen how we can use parallax to find the distance to nearby stars. We do this by measuring the annual parallax angle then using trigonometry to find the distance.

• Angles are measured in degrees. There are 360 degrees in a circle.
• For small angles we use minutes also. A minute is a 60th of a degree.
• For very small angles we use seconds also. A second is a 60th of a minute.

This may seem a very tiny angle but it is measurable with the sensitive instruments that astronomers use.

Imagine that a star has parallax angle of 1 second in 6 months. (i.e. a 3600th of a degree) We say that such a star would be a distance of one parsec away. So how far is a parsec in metres?

This seems like such a huge number it is hard to imagine it. Consider this though;

A light year is the distance that light travels in a year ( at 3 x 10⁸ m/s)

So 1 parsec = 3.08 x 10¹⁶ / 9.47 x 10¹⁵ = 3.26 light years. (useful to remember 1 parsec is about 3.2 light years). Other useful distances include the megaparsec Mpc and the kiloparsec kpc.