Editing Cepheid variable (section)

== Uncertain distances ==
Chief among the uncertainties tied to the classical and type II Cepheid distance scale are: the nature of the period-luminosity relation in various [[passband]]s, the impact of [[metallicity]] on both the zero-point and slope of those relations, and the effects of photometric contamination (blending with other stars) and a changing (typically unknown) [[Extinction (astronomy)|extinction]] law on Cepheid distances. All these topics are actively debated in the literature.<ref name=tammannsandage2008>{{cite journal |arxiv=0806.3018 |bibcode=2008A&ARv..15..289T |doi=10.1007/s00159-008-0012-y |title=The expansion field: the value of H 0 |date=2008 |last1=Tammann |first1=G. A. |last2=Sandage |first2=A. |last3=Reindl |first3=B. |journal=[[The Astronomy and Astrophysics Review]] |volume=15 |issue=4 |pages=289–331 |s2cid=18463474 }}</ref><ref name="turner10" /><ref name=ngeow2006>{{cite journal |arxiv=astro.ph/0603643 |bibcode=2006ApJ...642L..29N |doi=10.1086/504478 |title=The Hubble Constant from Type Ia Supernovae Calibrated with the Linear and Nonlinear Cepheid Period-Luminosity Relations |date=2006 |last1=Ngeow |first1=C. |last2=Kanbur |first2=S. M. |journal=[[The Astrophysical Journal]] |volume=642 |issue=1 |pages=L29–L32 |s2cid=17860528 }}</ref><ref name=majaess2009b>{{cite journal |arxiv=0909.0181 |bibcode=2009AcA....59..403M |title=Type II Cepheids as Extragalactic Distance Candles |author1=Majaess, D. |author2=Turner, D. |author3=Lane, D. |volume=59 |issue=4 |date=2009 |pages=403 |journal=[[Acta Astronomica]]}}</ref><ref name=benedict2007>{{cite journal|bibcode=2007AJ....133.1810B|title=Hubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity Relations|journal=[[The Astronomical Journal]]|volume=133|issue=4|pages=1810|last1=Benedict|first1=G. Fritz|last2=McArthur|first2=Barbara E.|last3=Feast|first3=Michael W.|last4=Barnes|first4=Thomas G.|last5=Harrison|first5=Thomas E.|last6=Patterson|first6=Richard J.|last7=Menzies|first7=John W.|last8=Bean|first8=Jacob L.|last9=Freedman|first9=Wendy L.|year=2007|doi=10.1086/511980|arxiv=astro.ph/0612465 |s2cid=16384267 }}</ref><ref name=stanekudalski1999>{{cite arXiv |eprint=astro-ph/9909346 |title=The Optical Gravitational Lensing Experiment. Investigating the Influence of Blending on the Cepheid Distance Scale with Cepheids in the Large Magellanic Cloud |author1=Stanek, K. Z. |author2=Udalski, A. |date=1999}}</ref><ref name=udalski2001>{{cite journal |arxiv=astro.ph/0109446 |bibcode=2001AcA....51..221U |title=The Optical Gravitational Lensing Experiment. Cepheids in the Galaxy IC1613: No Dependence of the Period-Luminosity Relation on Metallicity |author1=Udalski, A. |author2=Wyrzykowski, L. |author3=Pietrzynski, G. |author4=Szewczyk, O. |author5=Szymanski, M. |author6=Kubiak, M. |author7=Soszynski, I. |author8=Zebrun, K. |volume=51 |date=2001 |pages=221 |journal=[[Acta Astronomica]]}}</ref><ref name=macri2006>{{cite journal |arxiv=astro.ph/0608211 |bibcode=2006ApJ...652.1133M |doi=10.1086/508530 |title=A New Cepheid Distance to the Maser-Host Galaxy NGC 4258 and Its Implications for the Hubble Constant |date=2006 |last1=Macri |first1=L. M. |last2=Stanek |first2=K. Z. |last3=Bersier |first3=D. |last4=Greenhill |first4=L. J. |last5=Reid |first5=M. J. |journal=[[The Astrophysical Journal]] |volume=652 |issue=2 |pages=1133–1149 |s2cid=15728812 }}</ref><ref name=bono2008>{{cite journal |arxiv=0805.1592 |bibcode=2008ApJ...684..102B |doi=10.1086/589965 |title=Cepheids in External Galaxies. I. The Maser-Host Galaxy NGC 4258 and the Metallicity Dependence of Period-Luminosity and Period-Wesenheit Relations |date=2008 |last1=Bono |first1=G. |last2=Caputo |first2=F. |last3=Fiorentino |first3=G. |last4=Marconi |first4=M. |last5=Musella |first5=I. |journal=[[The Astrophysical Journal]] |volume=684 |issue=1 |pages=102–117 |s2cid=6275274 }}</ref><ref name=madore2009>{{cite journal |arxiv=0902.3747 |bibcode=2009ApJ...696.1498M |doi=10.1088/0004-637X/696/2/1498 |title=Concerning the Slope of the Cepheid Period-Luminosity Relation |date=2009 |last1=Madore |first1=Barry F. |last2=Freedman |first2=Wendy L. |journal=[[The Astrophysical Journal]] |volume=696 |issue=2 |pages=1498–1501 |s2cid=16325249 }}</ref><ref name=scowcroft2009>{{cite journal |bibcode=2009MNRAS.396.1287S |doi=10.1111/j.1365-2966.2009.14822.x |title=The effect of metallicity on Cepheid magnitudes and the distance to M33 |date=2009 |last1=Scowcroft |first1=V. |last2=Bersier |first2=D. |last3=Mould |first3=J. R. |last4=Wood |first4=P. R. |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=396 |issue=3 |pages=1287–1296|doi-access=free |arxiv=0903.4088 }}</ref><ref name=majaess2010>{{cite journal |arxiv=1006.2458 |bibcode=2010AcA....60..121M |title=The Cepheids of Centaurus A (NGC 5128) and Implications for H0 |author1=Majaess, D. |volume=60 |issue=2 |date=2010 |pages=121 |journal=[[Acta Astronomica]]}}</ref>

These unresolved matters have resulted in cited values for the [[Hubble constant]] (established from Classical Cepheids) ranging between 60&nbsp;km/s/Mpc and 80&nbsp;km/s/Mpc.<ref name="freedman2001" /><ref name="tammannsandage2008" /><ref name="freedman2010" /><ref name="ngeow2006" /><ref name="macririess2009" /> Resolving this discrepancy is one of the foremost problems in astronomy since the cosmological parameters of the Universe may be constrained by supplying a precise value of the Hubble constant.<ref name="freedman2010" /><ref name=macririess2009>{{cite conference |bibcode=2009AIPC.1170...23M |doi=10.1063/1.3246452 |chapter=The SH0ES Project: Observations of Cepheids in NGC 4258 and Type Ia SN Hosts |title=AIP Conference Proceedings |conference=Stellar Pulsation: Challenges for Theory and Observation: Proceedings of the International Conference. AIP Conference Proceedings |volume=1170 |date=2009 |last1=Macri |first1=Lucas M. |last2=Riess |first2=Adam G. |last3=Guzik |first3=Joyce Ann |last4=Bradley |first4=Paul A. |pages=23–25}}</ref> Uncertainties have diminished over the years, due in part to discoveries such as [[RS Puppis]].

Delta Cephei is also of particular importance as a [[calibrator star|calibrator]] of the Cepheid period-luminosity relation since its distance is among the most precisely established for a Cepheid, partly because it is a member of a [[star cluster]]<ref name=dezeeuw1999>{{cite journal|bibcode=1999AJ....117..354D|title=A HIPPARCOS Census of the Nearby OB Associations|journal=[[The Astronomical Journal]]|volume=117|issue=1|pages=354–399|last1=De Zeeuw|first1=P. T.|last2=Hoogerwerf|first2=R.|last3=De Bruijne|first3=J. H. J.|last4=Brown|first4=A. G. A.|last5=Blaauw|first5=A.|year=1999|doi=10.1086/300682|arxiv=astro.ph/9809227 |s2cid=16098861}}</ref><ref name=majaess2012>{{cite journal|bibcode=2012ApJ...747..145M|title=New Evidence Supporting Cluster Membership for the Keystone Calibrator Delta Cephei|journal=[[The Astrophysical Journal]]|volume=747|issue=2|pages=145|last1=Majaess|first1=D.|last2=Turner|first2=D.|last3=Gieren|first3=W.|year=2012|doi=10.1088/0004-637X/747/2/145|arxiv=1201.0993|s2cid=118672744}}</ref> and the availability of precise parallaxes observed by the [[Hubble Space Telescope|Hubble]], [[Hipparcos]], and [[Gaia (spacecraft)|Gaia]] space telescopes.<ref name="benedict02">{{cite journal|bibcode=2002AJ....124.1695B|title=Astrometry with the Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator δ Cephei|journal=[[The Astronomical Journal]]|volume=124|issue=3|pages=1695|last1=Benedict|first1=G. Fritz|last2=McArthur|first2=B. E.|last3=Fredrick|first3=L. W.|last4=Harrison|first4=T. E.|last5=Slesnick|first5=C. L.|last6=Rhee|first6=J.|last7=Patterson|first7=R. J.|last8=Skrutskie|first8=M. F.|last9=Franz|first9=O. G.|last10=Wasserman|first10=L. H.|last11=Jefferys|first11=W. H.|last12=Nelan|first12=E.|last13=Van Altena|first13=W.|last14=Shelus|first14=P. J.|last15=Hemenway|first15=P. D.|last16=Duncombe|first16=R. L.|last17=Story|first17=D.|last18=Whipple|first18=A. L.|last19=Bradley|first19=A. J.|year=2002|doi=10.1086/342014|arxiv=astro.ph/0206214 |s2cid=42655824}}</ref> The accuracy of [[parallax]] distance measurements to Cepheid variables and other bodies within 7,500 light-years is vastly improved by comparing images from Hubble taken six months apart, from opposite points in the Earth's orbit. (Between two such observations 2 [[Astronomical unit|AU]] apart, a star at a distance of 7500 light-years = 2300 [[parsec]]s would appear to move an angle of <sup>2</sup>/<sub>2300</sub> arc-seconds = 2 x 10<sup>−7</sup> degrees, the [[Angular resolution|resolution]] limit of the available telescopes.)<ref>{{cite journal|bibcode=2014ApJ...785..161R|title=Parallax beyond a Kiloparsec from Spatially Scanning the Wide Field Camera 3 on the Hubble Space Telescope|journal=[[The Astrophysical Journal]]|volume=785|issue=2|pages=161|last1=Riess|first1=Adam G.|last2=Casertano|first2=Stefano|last3=Anderson|first3=Jay|last4=MacKenty|first4=John|last5=Filippenko|first5=Alexei V.|year=2014|doi=10.1088/0004-637X/785/2/161|arxiv=1401.0484 |s2cid=55928992}}</ref>