Cepheid Variable
From Slackerpedia Galactica
A Cepheid variable star is a type of variable star whose magnitude, luminosity, and period can be used to determine astronomical distances. Cepheids pulsate, meaning their surfaces move inwards and outwards. The time Cepheids take to undergo one pulsation cycle is partly dependent upon their physical size -- larger star, longer period. But if the star is larger, it is also more luminous. Longer period, more luminous. Because both of these change in a predictable way for Cepheids (and some other kinds of pulsating star), these variables can be used as "standard candles".
