What is Cepheid variable?
[[File:Heic1323a -1243686232.jpg|thumb|right|320px|<center>[[RS Puppis]]. It is one of the brightest known Cepheid variable stars in the [[Milky Way]] galaxy; image, [[Hubble Space Telescope]]</center>]] '''Cepheids''' are a type of very luminous [[variable star]]s. There is a strong direct relationship between a Cepheid's [[luminosity]] and its [[periodic function|pulsation period]].<ref name=udalski99>{{cite journal |arxiv=astro-ph/9908317 |bibcode=1999AcA....49..223U |title=The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. IV. Catalog of Cepheids from the Large Magellanic Cloud |author=Udalski A. |display-authors = etal |volume=49 |year=1999 |pages=223–317 |journal=Acta Astronomica}}</ref><ref name=sos08>{{cite journal |arxiv=0808.2210 |bibcode=2008AcA....58..163S |title=The Optical gravitational lensing experiment. The OGLE-III Catalog of Variable Stars. I. Classical Cepheids in the Large Magellanic Cloud |author=Soszynski I. |display-authors = etal |volume=58 |year=2008 |pages=163 |journal=Acta Astronomica}}</ref> This makes Cepheids important [[standard candle]]s for the galactic and [[extragalactic distance scale]]s.<ref name=freedman2010>{{cite journal |arxiv=1004.1856 |bibcode=2010ARA&A..48..673F |title=The Hubble Constant |author=Freedman, Wendy L. & Madore, Barry F. |volume=48 |year=2010 |pages=673–710 |journal=Annual Review of Astronomy and Astrophysics |doi=10.1146/annurev-astro-082708-101829|s2cid=13909389 }}</ref> Cepheid variables are divided into several subclasses which have clearly different masses, ages, and evolutionary histories: #Classical Cepheids #Type II Cepheids #Anomalous Cepheids #Dwarf Cepheids The first cepheid known was [[Delta Cephei]] in the constellation [[Cepheus (constellation)|Cepheus]], found by John Goodricke in 1784. Delta Cephei is of great importance because its distance is extremely well known, thanks in part to it being in a [[star cluster]],<ref name=dezeeuw1999>de Zeeuw P.T. ''et al'' 1999. [http://adsabs.harvard.edu/abs/1999AJ..