What is a variable star and how does it differ from other stars?
Variable stars show a change in their brightness over time. This was very
useful early in making distance measurements. We could see nearby stars and
could find the relation between the time period of the light intensity
variation and the size of the star. When we then saw stars of the same time
variation very far away as away we were able to know how bright these stars
would have been if they were close. Using simple ideas about how light
becomes dimmer with distance, we then could calculate the distance. The
stars' light varies because of the instability of the star as it burns fuel.
Samuel P Bowen
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Update: June 2012