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Diagram depicting the light curve of a Cepheid variable in the Andromeda galaxy (M31). Plotted against a background image of the spiral Andromeda galaxy, a blue curve continues through seven cycles, each time rising sharply and falling rather more slowly. Red dots on the first five cycles from the left indicate individual measurements made by amateur astronomers, and blue/yellow stars on the two cycles at the right indicate predicted values used to guide Hubble telescope observations made by NASA. The vertical scale on the left is labelled as brightness, with the horizontal axis labelled with the dates of observation from July 2010 on the left, to January 2011 on the right.

 2.1 Pulsating variables and their light curves