RR Lyrae variable

An RR Lyrae variable (or RR Lyrae star or just RR Lyrae or RRL or just RR) is a type of pulsating variable star with a predictable luminosity such that it can serve as a standard candle. They are named after the prototype star, RR Lyrae. They are A-type or F-type stars within the horizontal branch (HB), that are in the instability strip. Their pulsation period shows a relation to their absolute magnitude, making them useful for determining the distance to Milky-Way globular clusters and distances within the Local Group and its environs but they are not sufficiently bright to determine distances to other galaxy clusters. The Baily classification of RR Lyrae variables:

• RRab or ab - the usual RR Lyrae light curve, which has a steeper rise than fall.
• RRc or c - has shorter period than usual and the light curve is more sinusoidal.
• RRd or d - rare type with a more complex light curve combining long and short period components.

The mass of RR Lyrae variables is less than that of the Sun: their hotter temperature and spectral class stems from the non-main-sequence fusion of HB stars. They are thought to be former Sun-like stars (e.g., G-type) but which had less metallicity, i.e., Population II stars.