On a clear, dark night we may be able to see a few thousand stars in the sky, a tiny proportion of the billions of stars that are thought to exist in the Milky Way alone. Although the stars we see with the naked eye look similar in size, they vary enormously in their distance from the Earth. Furthermore, how bright a star appears is ultimately no indication as to how close it is to us. Astronomers use many different ways to determine just how far away a star is. Almost all are based on parallax.

Parallax

If you hold one finger at arm's length in front of your face and close each eye in turn, you will see that the finger appears to move compared to distant objects behind it. This apparent movement is known as parallax. Astronomers use this effect to measure the distance to stars by determining the angle between the lines of sight of a star from two different positions of the observer.

ESA's Hipparcos Mission

Launched in 1989 the ESA Hipparcos mission used the parallax method to observe positions of stars within the galaxy. As a result of the mission two catalogues of observations were produced:

• The Hipparcos Catalogue - 120 000 stars to a precision 200 times better than any previous observations.
• The Tycho Catalogue - detailed distribution and data map of a further 1.2 million stars.

The parsec