A Cepheid is a member of a class of very luminous variable stars. The strong direct relationship between a Cepheid variable's luminosity and pulsation period secures for Cepheids their status as important standard candles for establishing the Galactic and extragalactic distance scales. Cepheid variables are divided into several subclasses which exhibit markedly different masses, ages, and evolutionary histories: Classical Cepheids, Type II Cepheids, Anomalous Cepheids, and Dwarf Cepheids. The term cepheid originates from Delta Cephei in the constellation Cepheus, the first star of this type identified, by John Goodricke in 1784. Delta Cephei is also of particular importance as a calibrator of the Cepheid period-luminosity relation since its distance is among the most precisely established for a Cepheid, thanks in part to its membership in a star cluster and the availability of precise Hubble Space Telescope/Hipparcos parallaxes. Classes Classical Cepheids Classical Cepheids (also known as Population I Cepheids, Type I Cepheids, or Delta Cephei variables) undergo pulsations with very regular periods on the order of days to months. Classical Cepheids are population I variable stars which are 4--20 times more massive than the Sun, and up to 100,000 times more luminous. Cepheids are yellow supergiants of spectral class F6 -- K2 and their radii change by (~25% for the longer-period I Carinae) millions of kilometers during a pulsation cycle. Classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, and are a means by which the Hubble constant can be established. Classical Cepheids have also been used to clarify many characteristics of our galaxy, such as the Sun's height above the galactic plane and the Galaxy's local spiral structure. Type II Cepheids Type II Cepheids (also termed Population II Cepheids) are population II variable stars which pulsate with periods typically between 1 and 50 days. Type II Cepheids are typically metal-poor, old (~10 Gyr), low mass objects (~half the mass of the Sun). Type II Cepheids are divided into several subgroups by period. Stars with periods between 1 and 4 days are of the BL Her subclass, 10--20 days belong to the W Virginis subclass, and stars with periods greater than 20 days belong to the RV Tauri subclass. Type II Cepheids are used to establish the distance to the Galactic center, globular clusters, and galaxies. History On September 10, 1784, Edward Pigott detected the variability of Eta Aquilae, the first known representative of the class of Classical Cepheid variables. However, the eponymous star for classical Cepheids is Delta Cephei, discovered to be variable by John Goodricke a few months later. 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. In 1913, Ejnar Hertzsprung did some work on Cepheids, which needed some revision. In 1915, Harlow Shapley used Cepheids to place initial constraints on the size and shape of the Milky Way, and of the placement of our Sun within it. In 1924, Edwin Hubble established the distance to Classical Cepheid variables in the Andromeda Galaxy, and showed that the variables were not members of the Milky Way. That settled the Island Universe debate which was concerned with whether the Milky Way and the Universe were synonymous, or was the Milky Way merely one in a plethora of galaxies that constitute the Universe. In 1929, Hubble and Milton L. Humason formulated what is now known as Hubble's Law by combining Cepheid distances to several galaxies with Vesto Slipher's measurements of the speed at which those galaxies recede from us. They discovered that the Universe is expanding (see the expansion of the Universe). However, the expansion of the Universe was posited several years before by Georges Lemaître. In theThis is a synthesized speech reading of the Wikipedia article "Cepheid variable" and is intended primarily for blind and visually impaired individuals who can not view Wikipedia. This video can also be used for mobile users who wish to listen to Wikipedia articles on the go, or by those who wish to learn a second language by reading the captions in that language while listening in English. Original article available at http://en.wikipedia.org/wiki/Cepheid_variable All text from Wikipedia is licensed under CC-BY-SA: https://en.wikipedia.org/wiki/Wikipedia:Reusing_Wikipedia_content "You are free ... to make commercial use of the work": http://creativecommons.org/licenses/by-sa/3.0/ Text-to-speech synthesized from FestVox, which permits commercial use: "no restrictions on its use (commercial or otherwise)" http://festvox.org/festival/downloads.html Note: all these licenses permit commercial use, and therefore we are permitted to monetize videos derived from these contents.

• published: 25 Aug 2013
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