Editing Cepheid variable (section)

== History ==
[[File:Period-Luminosity Relation for Cepheids.png|thumb|left|The period-luminosity curves of [[classical Cepheid variable|classic]] and [[type II Cepheid]]s]]
On September 10, 1784, [[Edward Pigott]] detected the variability of [[Eta Aquilae]], the first known representative of the class of classical Cepheid variables.<ref>{{Cite journal|last=Pigott|first=Edward|title=Observations of a new variable star|journal=[[Philosophical Transactions of the Royal Society]] |volume=75 |pages=127–136 |bibcode=1785RSPT...75..127P |year=1785 |doi=10.1098/rstl.1785.0007|s2cid=186212958}}</ref> The eponymous star for classical Cepheids, [[Delta Cephei]], was discovered to be variable by [[John Goodricke]] a few months later.<ref>{{cite journal | author = Goodricke, John | title = A series of observations on, and a discovery of, the period of the variation of the light of the star marked δ by Bayer, near the head of Cepheus. In a letter from John Goodricke, Esq. to Nevil Maskelyne, D.D.F.R.S. and Astronomer Royal | journal = [[Philosophical Transactions of the Royal Society of London]] |year = 1786 | volume = 76 | pages = 48–61 | doi = 10.1098/rstl.1786.0002 | url = https://books.google.com/books?id=0YMFAAAAQAAJ&pg=PA48 | bibcode = 1786RSPT...76...48G | doi-access = free | url-access = subscription }}</ref> 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.<ref name=clerke>{{cite book |last1=Clarke |first1=Agnes Mary |title=Problems in Astrophysics |date=1903 |publisher=[[Adam & Charles Black]] |location=London, England |page=319 |isbn=978-0-403-01478-1 |url=https://books.google.com/books?id=8pQAAAAAMAAJ&pg=PA319}}</ref> 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]].<ref name=engle>{{cite thesis |bibcode=2015PhDT........45E |title=The Secret Lives of Cepheids: A Multi-Wavelength Study of the Atmospheres and Real-Time Evolution of Classical Cepheids |last1=Engle |first1=Scott |year=2015 |arxiv=1504.02713 |doi=10.5281/zenodo.45252 }}</ref>

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]].<ref>{{cite journal |bibcode=1908AnHar..60...87L |title=1777 variables in the Magellanic Clouds |author1=Leavitt, Henrietta S. |volume=60 |issue=4 |date=1908 |pages=87–108 |journal=[[Annals of the Astronomical Observatory of Harvard College]] |url=https://babel.hathitrust.org/cgi/pt?id=coo.31924056092228&view=1up&seq=103 }}</ref> She published it in 1912 with further evidence.<ref>{{cite journal |bibcode=1912HarCi.173....1L |title=Periods of 25 variable stars in the Small Magellanic Cloud |author1=Leavitt, Henrietta S. |author2=Pickering, Edward C. |volume=173 |date=1912 |pages=1–3 |journal=[[Harvard College Observatory Circular]] }}</ref> Cepheid variables were found to show [[radial velocity]] variation with the same period as the luminosity variation, and initially this was interpreted as evidence that these stars were part of a [[binary system]]. However, in 1914, [[Harlow Shapley]] demonstrated that this idea should be abandoned.<ref>{{cite journal | title=On the Nature and Cause of Cepheid Variation | last=Shapley | first=Harlow | journal=Astrophysical Journal | volume=40 | page=448 | date=December 1914 | doi=10.1086/142137 | bibcode=1914ApJ....40..448S | doi-access=free }}</ref> Two years later, Shapley and others had discovered that Cepheid variables changed their [[spectral type]]s over the course of a cycle.<ref name=Shapley_1916>{{citation | postscript=. | title=The variations in spectral type of twenty Cepheid variables | last=Shapley | first=H. | journal=Astrophysical Journal | volume=44 | pages=273 | year=1916 | bibcode=1916ApJ....44..273S | doi=10.1086/142295 | doi-access=free }}</ref>

In 1913, [[Ejnar Hertzsprung]] attempted to find distances to 13 Cepheids using their motion through the sky.<ref>{{cite journal |last1=Hertzsprung |first1=E. |title=Über die räumliche Verteilung der Veränderlichen vom δ Cephei-Typus |journal=[[Astronomische Nachrichten]] |date=1913 |volume=196 |issue=4692 |pages=201–208 |trans-title=On the spatial distribution of variable [stars] of the δ Cephei type |language=de|bibcode=1913AN....196..201H }}</ref> (His results would later require revision.) In 1918, Harlow Shapley used Cepheids to place initial constraints on the size and shape of the [[Milky Way]] and of the placement of the Sun within it.<ref>{{cite journal |bibcode=1918PASP...30...42S |title=Globular Clusters and the Structure of the Galactic System |last1=Shapley |first1=H. |journal=[[Publications of the Astronomical Society of the Pacific]] |year=1918 |volume=30 |issue=173 |page=42 |doi=10.1086/122686 |doi-access=free }}</ref> In 1924, [[Edwin Hubble]] established the distance to classical Cepheid variables in the [[Andromeda Galaxy]], until then known as the "Andromeda [[Spiral galaxy#Spiral nebula|Nebula]]" and showed that those variables were not members of the Milky Way. Hubble's finding settled the question raised in the "[[Great Debate (astronomy)|Great Debate]]" of whether the Milky Way represented the entire Universe or was merely one of many [[Galaxy|galaxies]] in the Universe.<ref name=hubble1925>{{cite journal |bibcode=1925Obs....48..139H |title=Cepheids in spiral nebulae |author1=Hubble, E. P. |volume=48 |date=1925 |pages=139 |journal=[[The Observatory (journal)|The Observatory]] }}</ref>

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 [[Metric expansion of space|the Universe is expanding]], confirming the theories of [[Georges Lemaître]].<ref name=lemaitre1927>{{cite journal|bibcode=1927ASSB...47...49L|title=Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques|journal=[[Annales de la Société Scientifique de Bruxelles]]|volume=47|pages=49|last1=Lemaître|first1=G.|year=1927}}</ref>

[[File:VISTA finds hidden feature of Milky Way.jpg|thumb|Illustration of Cepheid variables (red dots) at the center of the Milky Way<ref>{{cite web|title=VISTA Discovers New Component of Milky Way|url=http://www.eso.org/public/news/eso1542/|access-date=29 October 2015}}</ref>]]
In the mid 20th century, significant problems with the astronomical distance scale were resolved by dividing the Cepheids into different classes with very different properties. In the 1940s, [[Wilhelm Heinrich Walter Baade|Walter Baade]] recognized two separate populations of Cepheids (classical and type II). Classical Cepheids are younger and more massive population I stars, whereas type II Cepheids are older, fainter Population II stars.<ref name="wallerstein02" /> Classical Cepheids and type II Cepheids follow different period-luminosity relationships. The luminosity of type II Cepheids is, on average, less than classical Cepheids by about 1.5 [[Absolute magnitude|magnitudes]] (but still brighter than RR Lyrae stars). Baade's seminal discovery led to a twofold increase in the distance to M31, and the extragalactic distance scale.<ref name=baade>{{cite journal|bibcode=1958AJ.....63..207B|title=Problems in the determination of the distance of galaxies|journal=[[Astronomical Journal]]|volume=63|pages=207|last1=Baade|first1=W.|year=1958|doi=10.1086/107726}}</ref><ref>{{cite web|last=Allen|first=Nick|title=Section 2: The Great Debate and the Great Mistake: Shapley, Hubble, Baade|url=http://www.institute-of-brilliant-failures.com/section2.htm|archive-url=https://web.archive.org/web/20071210105344/http://www.institute-of-brilliant-failures.com/section2.htm|archive-date=Dec 10, 2007|work=The Cepheid Distance Scale: A History}}</ref> RR Lyrae stars, then known as Cluster Variables, were recognized fairly early as being a separate class of variable, due in part to their short periods.<ref>{{cite journal|bibcode=1918CMWCI.153....1S|title=No. 153. Studies based on the colors and magnitudes in stellar clusters. Eighth paper: The luminosities and distances of 139 Cepheid variables|journal=[[Contributions from the Mount Wilson Observatory]]|volume=153|pages=1|last1=Shapley|first1=Harlow.|year=1918}}</ref><ref>{{cite journal |last1=Shapley |first1=Harlow |title=Studies based on the colors and magnitudes in stellar clusters. Eighth paper: The luminosities and distances of 139 Cepheid variables |journal=[[Astrophysical Journal]] |date=1918 |volume=48 |pages=279–294 |bibcode=1918ApJ....48..279S |doi=10.1086/142435 }}</ref>

The mechanics of [[stellar pulsation]] as a heat-engine was proposed in 1917 by [[Arthur Stanley Eddington]]<ref>{{cite journal |bibcode=1917Obs....40..290E |title=The pulsation theory of Cepheid variables |author1=Eddington, A. S. |volume=40 |date=1917 |pages=290 |journal=[[The Observatory (journal)|The Observatory]]}}</ref> (who wrote at length on the dynamics of Cepheids), but it was not until 1953 that [[Sergei Alexandrovich Zhevakin|S. A. Zhevakin]] identified ionized helium as a [[Kappa–mechanism|likely valve]] for the engine.<ref>{{cite journal |author1=Zhevakin, S. A. |title=К Теории Цефеид. I |journal=[[Астрономический журнал]] |volume=30 |pages=161–179 |year=1953 }}</ref>