Cepheid variable

A Cepheid variable is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute stellar luminosity.

Because of this correlation, Cepheid variables can be used as a standard candle to determine the distance to their host globular clusters or galaxies. Since the period-luminosity relation can be calibrated with great precision using the nearest Cepheid stars, the distances found with this method are among the most accurate available.

Usually giant yellow stars, they pulse regularly expanding and contracting their bodies, resulting in a regular oscillation of their luminosity.

Some Cepheid stars (for example the Polaris), have shown a decrease in their oscillation over a period of few tens of years, and now are virtually constant.