Cepheid
A Cepheid variable () is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period (typically 1–100 days) and amplitude. Cepheids are important cosmic benchmarks for scaling galactic and extragalactic distances; a strong direct relationship exists between a Cepheid variable's luminosity and its pulsation period.
This characteristic of classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt when she studied thousands of variable stars in the Magellanic Clouds. The discovery establishes the true luminosity of a Cepheid by observing its pulsation period. This in turn gives the distance to the star by comparing its known luminosity to its observed brightness, calibrated by direct observation of the parallax distance to the closest Cepheids such as RS Puppis and Polaris.
Cepheids change brightness due to the κ–mechanism, which occurs when the opacity of a star's atmosphere increases with temperature rather than decreasing. The main gas involved is thought to be helium. On this analysis, the Cepheid cycle is driven by the fact that doubly ionized helium, the form found at high temperatures, is more opaque than singly ionized helium. As a result the outer layer of the star cycles between compression and expansion: Compression heats the helium until it becomes doubly ionized. Its opacity when doubly ionized causes the helium to absorb sufficient heat to expand, whereupon it cools until it becomes singly ionized again and, because of its transparency when singly ionized, cools until it collapses. Cepheid variables become dimmest during the part of the cycle when the helium is doubly ionized.
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