RR Lyrae variable

An old, low-mass pulsating star whose brightness varies regularly, usually over hours. Its predictable luminosity makes it a standard candle for measuring distances, especially within the Milky Way.

Radial pulsation: A star-wide expansion and contraction in which the stellar radius changes coherently. RR Lyrae stars brighten and dim as radial pulsations repeatedly change their radius and surface temperature.
Standard candle: An astronomical object whose intrinsic luminosity can be inferred well enough to estimate its distance. RR Lyrae stars serve as standard candles once their luminosities are calibrated.
Stellar evolution: The changes in a star’s structure and observable properties over its lifetime. It explains how old, low-mass stars reach the horizontal branch and become RR Lyrae variables.
Classical Cepheid variable: A young, massive pulsating star whose period is strongly related to its luminosity. Cepheids are generally brighter and younger than RR Lyrae stars, extending distance measurements farther.
κ-mechanism: A stellar pulsation-driving process in which opacity changes trap and release heat during a cycle. It supplies the energy that sustains pulsations in RR Lyrae stars.
Period–luminosity relation: A relation linking a pulsating star’s variability period to its intrinsic luminosity in a specified wavelength band. In infrared bands, RR Lyrae periods help estimate luminosities and distances.
Pulsating variable star: A star whose brightness changes as recurring internal oscillations alter its radius and surface conditions. RR Lyrae variables belong to this broader class of stars.
Type II Cepheid: An old, low-mass pulsating star with longer periods than typical RR Lyrae variables. Its greater period and distinct evolutionary status separate it from RR Lyrae stars.
Instability strip: A region of the Hertzsprung–Russell diagram where stars become unstable to pulsation. RR Lyrae stars occupy this region, where their envelopes can pulsate.
Baade–Wesselink method: A technique for estimating a pulsating star’s radius and distance by combining angular and physical changes. It provides a route to calibrating the luminosities of pulsating stars, including RR Lyrae variables.

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