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Concept
Cepheid variable
Summary
A Cepheid variable (ˈsɛfi.ɪd,_ˈsiːfi-) 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 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 after studying thousands of variable stars in the Magellanic Clouds. The discovery allows one to know the true luminosity of a Cepheid by just observing its pulsation period. This tells one the distance to the star, by comparing its known luminosity to its observed brightness.
The term Cepheid originates from Delta Cephei in the constellation Cepheus, identified by John Goodricke in 1784. It was the first of its type to be identified.
The mechanics of stellar pulsation as a heat-engine was proposed in 1917 by Arthur Stanley Eddington (who wrote at length on the dynamics of Cepheids). It was not until 1953 that S. A. Zhevakin identified ionized helium as a likely valve for the engine.
On September 10, 1784, Edward Pigott detected the variability of Eta Aquilae, the first known representative of the class of classical Cepheid variables. The eponymous star for classical Cepheids, Delta Cephei, was discovered to be variable by John Goodricke a few months later. The number of similar variables grew to several dozen by the end of the 19th century, and they were referred to as a class as Cepheids. Most of the Cepheids were known from the distinctive light curve shapes with the rapid increase in brightness and a hump, but some with more symmetrical light curves were known as Geminids after the prototype ζ Geminorum.
A relationship between the period and luminosity for classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt in an investigation of thousands of variable stars in the Magellanic Clouds. She published it in 1912 with further evidence.
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