The answer to this question lies in the application of trigonometry. An astronomer can look at a star at a given point in time and then look at it several months later, after the Earth has moved a substantial distance in its orbit around the Sun. This allows the astronomer to view the star from two different angles. By comparing the images from the two different angles, it is possible to figure out how far away it is.

If a star happens to be too far away the first method described will not be accurate, but fortunately there is an alternative. If an astronomer measures the visible light spectrum of the star, it turns out that one can get a good idea of its actual brightness (by actual, we mean how bright the star is if you are right up close to it). This relationship isn’t entirely straightforward and has only been established after looking at data from thousands of stars. Once the astronomer knows the actual brightness, its brightness can be compared by its apparent brightness as viewed from Earth to determine how far away it is.