Rotating Variable Stars
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In 1612 Galileo Galilei discovered that the Sun rotates on its axis. He worked this out by watching the movement of sunspots across the solar disk and calculated the solar rotation period to be approximately 25 days on average. For the first time, a body such as a star was found to be imperfect and blemished, and this led over the next two centuries to speculation regarding the nature of some variable stars. Did they vary due to star spots? Was their rotation similar to that of the Sun? Indeed, do other stars have spots, and if so, are the mechanisms that cause them similar to those of our own Sun?