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A new three-dimensional map of the Milky Way should give astronomers a unique insight into the formation and evolution of our galaxy.

Mapping the galaxy is tricky business because of the inherent difficulties in measuring distances to stars. The problem boils down to determining the inherent brightness of a star, a property called its absolute magnitude. Then the difference between this and its observed magnitude on Earth gives you some idea of its distance.

But how to determine absolute magnitude? One way is to use the famous Hertzprung-Russel diagram (also known as the colour magnitude diagram), which plots the relationship between a star's colour and its absolute magnitude. The process is relatively straightforward: measure a star's colour and observed magnitude, then determine its absolute magnitude from the colour magnitude diagram. A straightforward calculation should give you its distance (provided there's no dust obscuring your view).

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