RR Lyrae variables¶
The RR Lyrae variables are metal-poor (population II) stars of about 80% the mass of the Sun that are fusing helium into carbon in their cores. They pulsate in either the radial fundamental mode (RRab variables), the first overtone (RRc variables) or both (RRd variables). The pulsations are driven by helium going back and forth between its first and second ionization state.
The fundamental period is almost always about half a day.
An enormous number of RR Lyrae variables are known. Like the Cepheid variables, they follow period-luminosity relations but, though fainter, are more common and still useful as distance indicators, especially for older stellar populations.
Many RR Lyrae variables, including RR Lyrae itself, show a modulation over long timescales (months or years) known as the Blazhko effect. Its mechanism is still not clearly understood.