Editing Cepheid variable (section)

== Classes ==
Cepheid variables are divided into two subclasses which exhibit markedly different masses, ages, and evolutionary histories: [[Classical Cepheid variable|classical Cepheids]] and [[type II Cepheid]]s. [[Delta Scuti variable]]s are A-type stars on or near the main sequence at the lower end of the [[instability strip]] and were originally referred to as dwarf Cepheids. [[RR Lyrae variable]]s have short periods and lie on the instability strip where it crosses the [[horizontal branch]]. Delta Scuti variables and RR Lyrae variables are not generally treated with Cepheid variables although their pulsations originate with the same helium ionisation [[kappa mechanism]].

=== Classical Cepheids ===
{{Main|Classical Cepheid variable}}
[[File:Delta Cephei lightcurve.jpg|thumb|[[Light curve]] of [[Delta Cephei]], the prototype of classical cepheids, showing the regular variations produced by intrinsic stellar pulsations]]
Classical Cepheids (also known as Population I Cepheids, type I Cepheids, or Delta Cepheid variables) undergo pulsations with very regular periods on the order of days to months. Classical Cepheids are [[Population I]] [[variable star]]s which are 4–20 times more massive than the Sun,<ref name=turner96>{{cite journal |bibcode=1996JRASC..90...82T |title=The Progenitors of Classical Cepheid Variables |author1=Turner, David G. |volume=90 |date=1996 |pages=82 |journal=[[Journal of the Royal Astronomical Society of Canada]]}}</ref> and up to 100,000 times more luminous.<ref name=turner10>{{cite journal |arxiv=0912.4864 |bibcode=2010Ap&SS.326..219T |doi=10.1007/s10509-009-0258-5 |title=The PL calibration for Milky Way Cepheids and its implications for the distance scale |date=2010 |last1=Turner |first1=David G. |journal=[[Astrophysics and Space Science]] |volume=326 |issue=2 |pages=219–231 |s2cid=119264970 }}</ref> These Cepheids are yellow bright giants and supergiants of [[spectral classification|spectral class]] F6 – K2 and their radii change by (~25% for the longer-period [[HD 84810|I Carinae]]) millions of kilometers during a pulsation cycle.<ref>{{cite journal |bibcode=1957MNRAS.117...85R |title=Radius variation and population type of cepheid variables |author1=Rodgers, A. W. |volume=117 |date=1957 |pages=85–94 |journal=[[Monthly Notices of the Royal Astronomical Society]] |doi=10.1093/mnras/117.1.85|doi-access=free }}</ref>

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.<ref name=freedman2001>{{cite journal |arxiv=astro.ph/0012376 |bibcode=2001ApJ...553...47F |doi=10.1086/320638 |title=Final Results from the ''Hubble Space Telescope'' Key Project to Measure the Hubble Constant |date=2001 |last1=Freedman |first1=Wendy L. |last2=Madore |first2=Barry F. |last3=Gibson |first3=Brad K. |last4=Ferrarese |first4=Laura |last5=Kelson |first5=Daniel D. |last6=Sakai |first6=Shoko |last7=Mould |first7=Jeremy R. |last8=Kennicutt, Jr. |first8=Robert C. |last9=Ford |first9=Holland C. |last10=Graham |first10=John A. |last11=Huchra |first11=John P. |last12=Hughes |first12=Shaun M. G. |last13=Illingworth |first13=Garth D. |last14=Macri |first14=Lucas M. |last15=Stetson |first15=Peter B. |journal=[[The Astrophysical Journal]] |volume=553 |issue=1 |pages=47–72 |s2cid=119097691 }}</ref><ref name="tammannsandage2008" /><ref name=freedman2010>{{cite journal |arxiv=1004.1856 |bibcode=2010ARA&A..48..673F |title=The Hubble Constant |author1=Freedman, Wendy L. |author2=Madore, Barry F. |volume=48 |date=2010 |pages=673–710 |journal=[[Annual Review of Astronomy and Astrophysics]] |doi=10.1146/annurev-astro-082708-101829 |s2cid=13909389 }}</ref><ref name="ngeow2006" /><ref name="macririess2009" /> Classical Cepheids have also been used to clarify many characteristics of the Milky Way galaxy, such as the Sun's height above the galactic plane and the Galaxy's local spiral structure.<ref name=majaess2009>{{cite journal |arxiv=0903.4206 |bibcode=2009MNRAS.398..263M |doi=10.1111/j.1365-2966.2009.15096.x |title=Characteristics of the Galaxy according to Cepheids |date=2009 |last1=Majaess |first1=D. J. |last2=Turner |first2=D. G. |last3=Lane |first3=D. J. |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=398 |issue=1 |pages=263–270 |doi-access=free |s2cid=14316644 }}</ref>

A group of classical Cepheids with small amplitudes and [[sinusoidal]] light curves are often separated out as Small Amplitude Cepheids or s-Cepheids, many of them pulsating in the first overtone.

=== Type II Cepheids ===
{{Main|Type II Cepheid}}
[[File:Kappa Pavonis TESS lightcurve.png|thumb|Light curve of [[κ Pavonis]], a Type II cepheid, recorded by NASA's [[Transiting Exoplanet Survey Satellite]] (TESS)]]
Type II Cepheids (also termed Population II Cepheids) are [[population II]] variable stars which pulsate with periods typically between 1 and 50 days.<ref name=wallerstein02>{{cite journal |bibcode=2002PASP..114..689W |doi=10.1086/341698 |title=The Cepheids of Population II and Related Stars |date=2002 |last1=Wallerstein |first1=George |journal=[[Publications of the Astronomical Society of the Pacific]] |volume=114 |issue=797 |pages=689–699|s2cid=122225966 }}</ref><ref name=so08>{{cite journal |arxiv=0811.3636 |bibcode=2008AcA....58..293S |title=The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. II.Type II Cepheids and Anomalous Cepheids in the Large Magellanic Cloud |author1=Soszyński, I. |author2=Udalski, A. |author3=Szymański, M. K. |author4=Kubiak, M. |author5=Pietrzyński, G. |author6=Wyrzykowski, Ł. |author7=Szewczyk, O. |author8=Ulaczyk, K. |author9=Poleski, R. |volume=58 |date=2008 |pages=293 |journal=[[Acta Astronomica]] }}</ref> Type II Cepheids are typically [[metallicity|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 Herculis variable|BL Her subclass]], 10–20 days belong to the [[W Virginis variable|W Virginis subclass]], and stars with periods greater than 20 days belong to the [[RV Tauri variable|RV Tauri subclass]].<ref name="wallerstein02" /><ref name="so08" />

Type II Cepheids are used to establish the distance to the [[Galactic Center]], [[globular clusters]], and [[galaxies]].<ref name="majaess2009" /><ref name=ku02>{{cite journal |arxiv=astro.ph/0306567 |bibcode=2003AcA....53..117K |title=The Optical Gravitational Lensing Experiment. Population II Cepheids in the Galactic Bulge |author1=Kubiak, M. |author2=Udalski, A. |volume=53 |date=2003 |pages=117 |journal=[[Acta Astronomica]]}}</ref><ref name=mat06>{{cite journal |arxiv=astro.ph/0606609 |bibcode=2006MNRAS.370.1979M |doi=10.1111/j.1365-2966.2006.10620.x |title=The period-luminosity relation for type II Cepheids in globular clusters |date=2006 |last1=Matsunaga |first1=Noriyuki |last2=Fukushi |first2=Hinako |last3=Nakada |first3=Yoshikazu |last4=Tanabé |first4=Toshihiko |last5=Feast |first5=Michael W. |last6=Menzies |first6=John W. |last7=Ita |first7=Yoshifusa |last8=Nishiyama |first8=Shogo |last9=Baba |first9=Daisuke |last10=Naoi |first10=Takahiro |last11=Nakaya |first11=Hidehiko |last12=Kawadu |first12=Takahiro |last13=Ishihara |first13=Akika |last14=Kato |first14=Daisuke |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=370 |issue=4 |pages=1979–1990 |doi-access=free |s2cid=25991504 |display-authors=8 }}</ref><ref name=feast08>{{cite journal |arxiv=0803.0466 |bibcode=2008MNRAS.386.2115F |doi=10.1111/j.1365-2966.2008.13181.x |title=The luminosities and distance scales of type II Cepheid and RR Lyrae variables |date=2008 |last1=Feast |first1=Michael W. |last2=Laney |first2=Clifton D. |last3=Kinman |first3=Thomas D. |last4=Van Leeuwen |first4=Floor |last5=Whitelock |first5=Patricia A. |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=386 |issue=4 |pages=2115–2134 |doi-access=free |s2cid=14459638 }}</ref><ref name="majaess2009b" /><ref name=majaess09c>{{cite journal |arxiv=0912.2928 |bibcode=2010JAVSO..38..100M |title=RR Lyrae and Type II Cepheid Variables Adhere to a Common Distance Relation |author1=Majaess, D. J. |volume=38 |issue=1 |date=2010 |pages=100–112 |journal=[[The Journal of the American Association of Variable Star Observers]]}}</ref><ref name=mat09>{{cite journal |arxiv=0904.4701 |bibcode=2009MNRAS.397..933M |doi=10.1111/j.1365-2966.2009.14992.x |title=Period-luminosity relations for type II Cepheids and their application |date=2009 |last1=Matsunaga |first1=Noriyuki |last2=Feast |first2=Michael W. |last3=Menzies |first3=John W. |journal=[[Monthly Notices of the Royal Astronomical Society ]]|volume=397 |issue=2 |pages=933–942 |doi-access=free |s2cid=13912466 }}</ref>

=== Anomalous Cepheids ===
A group of pulsating stars on the instability strip have periods of less than 2 days, similar to RR Lyrae variables but with higher luminosities. Anomalous Cepheid variables have masses higher than type II Cepheids, RR Lyrae variables, and the Sun. It is unclear whether they are young stars on a "turned-back" horizontal branch, [[blue straggler]]s formed through [[mass transfer]] in binary systems, or a mix of both.<ref name=caputo>{{cite journal|bibcode=2004A&A...424..927C|title=Bright metal-poor variables: Why ''Anomalous'' Cepheids?|journal=[[Astronomy and Astrophysics]]|volume=424|issue=3|pages=927–934 |last1=Caputo|first1=F.|last2=Castellani|first2=V.|last3=Degl'Innocenti|first3=S.|last4=Fiorentino|first4=G.|last5=Marconi|first5=M. |year=2004|doi=10.1051/0004-6361:20040307|arxiv=astro.ph/0405395 |s2cid=45306570}}</ref><ref name=sosznytski>{{cite journal|bibcode=2008AcA....58..293S|title=The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. II.Type II Cepheids and Anomalous Cepheids in the Large Magellanic Cloud|journal=[[Acta Astronomica]]|volume=58|pages=293|last1=Soszyński|first1=I.|last2=Udalski|first2=A.|last3=Szymański|first3=M. K.|last4=Kubiak|first4=M.|last5=Pietrzyński|first5=G.|last6=Wyrzykowski|first6=Ł.|last7=Szewczyk|first7=O.|last8=Ulaczyk|first8=K.|last9=Poleski |first9=R.|year=2008|arxiv=0811.3636 }}</ref>

=== Double-mode Cepheids ===
A small proportion of Cepheid variables have been observed to pulsate in two modes at the same time, usually the fundamental and first overtone, occasionally the second overtone.<ref name=smolec>{{cite journal|bibcode=2008AcA....58..233S|arxiv=0809.1986|title=Double-Mode Classical Cepheid Models, Revisited|journal=[[Acta Astronomica]]|volume=58|pages=233|last1=Smolec|first1=R.|last2=Moskalik|first2=P.|year=2008}}</ref> A very small number pulsate in three modes, or an unusual combination of modes including higher overtones.<ref>{{cite journal|bibcode=2008AcA....58..153S|arxiv=0807.4182|title=The Optical Gravitational Lensing Experiment. Triple-Mode and 1O/3O Double-Mode Cepheids in the Large Magellanic Cloud|journal=[[Acta Astronomica]]|volume=58|pages=153 |last1=Soszynski|first1=I.|last2=Poleski|first2=R.|last3=Udalski|first3=A.|last4=Kubiak|first4=M.|last5=Szymanski|first5=M. K.|last6=Pietrzynski|first6=G.|last7=Wyrzykowski|first7=L.|last8=Szewczyk|first8=O.|last9=Ulaczyk|first9=K.|year=2008}}</ref>