Cepheids change in brightness over a time that depends on just how luminous they are. A Cepheid as luminous as the Sun would have a shorter period of brightness change than a Cepheid that was 10 times as luminous. Now, from nearby Cepheid stars like the ones observed by the Hipparchos satellite, you can determine how far away a set of standard Cepheids are. This, then 'calibrates' once and for all the Cepheid period-luminosity curve .Then, all you have to do is identify a Cepheid star in a distant galaxy, measure its period of brightness change ( 1 - 50 days or so), and this establishes just how luminous it is from you calibrated Period- Luminosity curve. From its apparent brightness and absolute luminosity, you then can easily compute its distance.
Copyright 1997 Dr. Sten Odenwald
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