RR Lyrae variables are variable stars often used as standard candles.
An RR Lyrae is a pulsating Horizontal_branch star, 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 an 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.
There are three main types of RR Lyrae stars: RRab, RRc and RRd. The RRc stars are of shorter period and the RRd stars are double-mode pulsators.
The relationship between an 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.
See also: RR Lyrae