Some 90 years ago, Edwin Hubble used pulsating stars called cepheids to demonstrate that the universe extends far beyond our own galaxy . He proved that these stars were too far away to be part of our Milky Way galaxy. The circle in the box, top right, marks the location of one of the cepheids he used, in the galaxy Andromeda . At first he thought it was an exploding star (labelling it N for nova) but then realised it was a cepheid variable, crossing out the N and writing 'VAR!'