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Often simply called Cepheids , these variables are massive, high-luminosity stars with short periods of one to 70 days and light variations of 0.1 to 2 magnitudes. Astronomers use Cepheids as a “standard” for distances because of the direct relationship between a Cepheid’s luminosity and variable pulsation period. Once you determine a specific Cepheid’s luminosity and variable pulsation period, you can determine how distant it is because of the inverse square law. This makes Cepheids a major galactic distance indicator.