cepheid

Cepheid variables are pulsating stars whose brightness changes in a regular cycle due to periodic expansion and contraction of their outer layers. The pulsations cause simultaneous changes in the star’s radius, temperature, and luminosity, producing characteristic light curves that range from roughly one to several tens of days, though some long-period Cepheids have even longer cycles. They are among the most luminous stars visible in the Milky Way and nearby galaxies, making them important standard candles for distance measurement.

There are two main classes: Classical Cepheids (Type I) and Type II Cepheids. Classical Cepheids are young, metal-rich supergiants associated with star-forming regions, while Type II Cepheids are older, lower-mass, metal-poor stars found in halo and globular clusters. A third group, anomalous Cepheids, shares some properties but is distinct in origin. Delta Cephei is the eponymous prototype of Classical Cepheids.

A key property of Cepheids is the period-luminosity relation, discovered by Henrietta Leavitt. The longer the pulsation period, the greater the intrinsic luminosity. This Leavitt law allows astronomers to determine absolute magnitudes from observed periods and, by comparing with apparent magnitudes, to estimate distances to Cepheids and their host galaxies. The relation is tighter in the near-infrared and is affected by metallicity.

Cepheids are central to the cosmic distance ladder and have played a crucial role in calibrating the Hubble constant. They illuminate stellar evolution pathways and continue to be studied across multiple wavelengths to refine distance measurements.