Ask A Scientist© Astronomy Archive

Question:  What is a variable star and how does it differ from other
stars?
kathy

Answer:  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 an d the size of the star.  When we then saw stars 
of the same time variation very far was as away we were able to know how bright
these stars would have been if they were close.  Using simple ideas about how li ght becomes
dimmer with distance we then could calculate the distance.  The stars light
output varies because of the instability of the star as it burns fuel.
samuel p bowen