RR Lyrae variable

RR Lyrae variables are variable stars often used as standard candles.

A RR Lyrae is pulsating red giant, with a mass about half of our Sun. It pulses in a manner similar to Cepheid variables, but some important differences exist.

RR Lyrae are old, relatively low mass stars. Therefore, they are much more common than Cepheids, but less luminous. The average absolute magnitude of a RR Lyrae is 0.75, only 40 or 50 times our Sun. Their period is shorter, typically less than one day, and sometimes down to eight or nine hours.

The relationship between a RR Lyrae's variability period and absolute magnitude makes them good standard candles for relatively near objects. They are extensively used in globular clusters studies, but difficult to observe in external galaxies.

This type of variable is named after the prototype, the variable RR in the Lyra constellation.