Cepheids

Cepheids are a class of pulsating variable stars whose regular brightness variations arise from radial pulsations. Named after the prototype Delta Cephei, these stars lie in the instability strip of the Hertzsprung–Russell diagram and are typically young, massive stars found in the disks of spiral and irregular galaxies.

A defining property is the period–luminosity relation, known as the Leavitt Law: for Cepheids, the longer the pulsation period, the brighter the star intrinsically. After correcting for extinction and metallicity, the observed period and brightness yield a distance. This makes classical Cepheids among the most important standard candles for calibrating the cosmic distance scale.

Two main classes are recognized. Classical or Type I Cepheids are young, metal-rich population I stars with masses roughly 3–12 solar, pulsating with periods from about 1 to 70 days (some longer). Type II Cepheids are older, lower-mass population II stars with shorter periods, including subclasses such as BL Herculis, W Virginis, and RV Tauri. There are also anomalous Cepheids in some systems.

Physically, Cepheids pulsate in radial modes, driven by the kappa mechanism in partially ionized helium. They show characteristic light curves with a rapid rise to maximum brightness followed by a slower decline, and their spectra and color vary with the cycle. They are giant or supergiant stars with temperatures roughly 5,000–6,500 K.

Cepheids are used to measure distances to nearby galaxies and to calibrate distances to farther galaxies, including Type Ia supernova hosts. Gaia parallaxes and other calibrations help set the zero point of the period–luminosity relation, contributing to measurements of the Hubble constant.