- Cepheids redirects here. For the fictional species, see "Blind Alley".
A Cepheid (pron.: // or //) 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.
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.
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 namesake for classical Cepheids is the star 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 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 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, 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. 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 magnitudes (but still brighter than RR Lyrae stars). Initial studies of Cepheid variable distances were complicated by the inadvertent admixture of classical Cepheids and Type II Cepheids. Walter Baade's seminal discovery led to a fourfold increase in the distance to M31, and the extragalactic distance scale. RR Lyrae stars were recognized fairly early (by the 1930s) as being a separate class of variable, due in part to their short periods.
Uncertainties in Cepheid determined 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 passbands, the impact of metallicity on both the zero-point and slope of those relations, and the effects of photometric contamination (blending) and a changing (typically unknown) extinction law on Cepheid distances. All these topics are actively debated in the literature.
These unresolved matters have resulted in cited values for the Hubble constant (established from Classical Cepheids) ranging between 60 km/s/Mpc and 80 km/s/Mpc. 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.
Dynamics of the pulsation 
The accepted explanation for the pulsation of Cepheids is called the Eddington valve, or κ-mechanism, where the Greek letter κ (kappa) denotes gas opacity. Helium is the gas thought to be most active in the process. Doubly ionized helium (helium whose atoms are missing two electrons) is more opaque than singly ionized helium. The more helium is heated, the more ionized it becomes. At the dimmest part of a Cepheid's cycle, the ionized gas in the outer layers of the star is opaque, and so is heated by the star's radiation, and due to the increased temperature, begins to expand. As it expands, it cools, and so becomes less ionized and therefore more transparent, allowing the radiation to escape. Then the expansion stops, and reverses due to the star's gravitational attraction. The process then repeats.
The mechanics of the pulsation as a heat-engine was proposed in 1917 by Arthur Stanley Eddington (who wrote at length on the dynamics of Cepheids), but it was not until 1953 that S. A. Zhevakin identified ionized helium as a likely valve for the engine.
- Classical Cepheids include: Eta Aquilae, Zeta Geminorum, Beta Doradus, RT Aurigae, Polaris, as well as the namesake Delta Cephei. The North Star (Polaris) is the closest classical Cepheid, although the star exhibits many peculiarities and its distance is a topic of active debate.
- Type II Cepheids include: W Virginis and BL Herculis.
- Dwarf Cepheids include: Delta Scuti, SX Phoenicis.
See also 
- Udalski, A.; Soszynski, I.; Szymanski, M.; Kubiak, M.; Pietrzynski, G.; Wozniak, P.; Zebrun, K. (1999). "The Optical Gravitational Lensing Experiment. Cepheids in the Magellanic Clouds. IV. Catalog of Cepheids from the Large Magellanic Cloud". Acta Astronomica 49: 223. arXiv:astro-ph/9908317. Bibcode:1999AcA....49..223U.
- Soszynski, I.; Poleski, R.; Udalski, A.; Szymanski, M. K.; Kubiak, M.; Pietrzynski, G.; Wyrzykowski, L.; Szewczyk, O. et al. (2008). "The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. I. Classical Cepheids in the Large Magellanic Cloud". Acta Astronomica 58: 163. arXiv:0808.2210. Bibcode:2008AcA....58..163S.
- Freedman, Wendy L.; Madore, Barry F.; Gibson, Brad K.; Ferrarese, Laura; Kelson, Daniel D.; Sakai, Shoko; Mould, Jeremy R.; Kennicutt, Jr., Robert C. et al. (2001). "Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant". The Astrophysical Journal 553: 47–72. arXiv:astro-ph/0012376. Bibcode:2001ApJ...553...47F. doi:10.1086/320638.
- Tammann, G. A.; Sandage, A.; Reindl, B. (2008). "The expansion field: the value of H 0". The Astronomy and Astrophysics Review 15 (4): 289–331. arXiv:0806.3018. Bibcode:2008A&ARv..15..289T. doi:10.1007/s00159-008-0012-y.
- Majaess, D. J.; Turner, D. G.; Lane, D. J. (2009). "Characteristics of the Galaxy according to Cepheids". Monthly Notices of the Royal Astronomical Society 398: 263–270. arXiv:0903.4206. Bibcode:2009MNRAS.398..263M. doi:10.1111/j.1365-2966.2009.15096.x.
- Freedman, Wendy L.; Madore, Barry F. (2010). "The Hubble Constant". Annual Review of Astronomy and Astrophysics 48: 673. arXiv:1004.1856. Bibcode:2010ARA&A..48..673F. doi:10.1146/annurev-astro-082708-101829.
- de Zeeuw, P. T.; Hoogerwerf, R.; de Bruijne, J. H. J.; Brown, A. G. A.; Blaauw, A. (1999). A HIPPARCOS Census of the Nearby OB Associations, AJ
- Majaess, D.; Turner, D.; Gieren, W. (2012). New Evidence Supporting Cluster Membership for the Keystone Calibrator Delta Cephei, ApJ
- Benedict, G. Fritz; McArthur, B. E.; Fredrick, L. W.; Harrison, T. E.; Slesnick, C. L.; Rhee, J.; Patterson, R. J.; Skrutskie, M. F.; Franz, O. G.; Wasserman, L. H.; Jefferys, W. H.; Nelan, E.; van Altena, W.; Shelus, P. J.; Hemenway, P. D.; Duncombe, R. L.; Story, D.; Whipple, A. L.; Bradley, A. J. (2002). Astrometry with the Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator δ Cephei, AJ
- Turner, David G. (1996). "The Progenitors of Classical Cepheid Variables". Journal of the Royal Astronomical Society of Canada 90: 82. Bibcode:1996JRASC..90...82T.
- Turner, David G. (2010). "The PL calibration for Milky Way Cepheids and its implications for the distance scale". Astrophysics and Space Science 326 (2): 219–231. arXiv:0912.4864. Bibcode:2010Ap&SS.326..219T. doi:10.1007/s10509-009-0258-5.
- Rodgers, A. W. (1957). "Radius variation and population type of cepheid variables". Monthly Notices of the Royal Astronomical Society 117: 85. Bibcode:1957MNRAS.117...85R.
- W. Strohmeier, Variable Stars, Pergamon (1972)[page needed]
- Ngeow, C.; Kanbur, S. M. (2006). "The Hubble Constant from Type Ia Supernovae Calibrated with the Linear and Nonlinear Cepheid Period-Luminosity Relations". The Astrophysical Journal 642: L29–L32. arXiv:astro-ph/0603643. Bibcode:2006ApJ...642L..29N. doi:10.1086/504478.
- Macri, Lucas M.; Riess, Adam G.; Guzik, Joyce Ann; Bradley, Paul A. (2009). "The SH0ES Project: Observations of Cepheids in NGC 4258 and Type Ia SN Hosts". AIP Conference Proceedings. pp. 23–25. Bibcode:2009AIPC.1170...23M. doi:10.1063/1.3246452.
- Wallerstein, George (2002). "The Cepheids of Population II and Related Stars". Publications of the Astronomical Society of the Pacific 114 (797): 689–699. Bibcode:2002PASP..114..689W. doi:10.1086/341698.
- Soszyński, I.; Udalski, A.; Szymański, M. K.; Kubiak, M.; Pietrzyński, G.; Wyrzykowski, Ł.; Szewczyk, O.; Ulaczyk, K. et al. (2008). "The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. II.Type II Cepheids and Anomalous Cepheids in the Large Magellanic Cloud". Acta Astronomica 58: 293. arXiv:0811.3636. Bibcode:2008AcA....58..293S.
- Kubiak, M.; Udalski, A. (2003). "The Optical Gravitational Lensing Experiment. Population II Cepheids in the Galactic Bulge". Acta Astronomica 53: 117. arXiv:astro-ph/0306567. Bibcode:2003AcA....53..117K.
- Matsunaga, Noriyuki; Fukushi, Hinako; Nakada, Yoshikazu; Tanabé, Toshihiko; Feast, Michael W.; Menzies, John W.; Ita, Yoshifusa; Nishiyama, Shogo et al. (2006). "The period-luminosity relation for type II Cepheids in globular clusters". Monthly Notices of the Royal Astronomical Society 370 (4): 1979–1990. arXiv:astro-ph/0606609. Bibcode:2006MNRAS.370.1979M. doi:10.1111/j.1365-2966.2006.10620.x.
- Feast, Michael W.; Laney, Clifton D.; Kinman, Thomas D.; Van Leeuwen, Floor; Whitelock, Patricia A. (2008). "The luminosities and distance scales of type II Cepheid and RR Lyrae variables". Monthly Notices of the Royal Astronomical Society 386 (4): 2115–2134. arXiv:0803.0466. Bibcode:2008MNRAS.386.2115F. doi:10.1111/j.1365-2966.2008.13181.x.
- Majaess, D.; Turner, D.; Lane, D. (2009). "Type II Cepheids as Extragalactic Distance Candles". Acta Astronomica 59: 403. arXiv:0909.0181. Bibcode:2009AcA....59..403M.
- Majaess, D. J. (2010). "RR Lyrae and Type II Cepheid Variables Adhere to a Common Distance Relation". The Journal of the American Association of Variable Star Observers 38: 100. arXiv:0912.2928. Bibcode:2010JAVSO..38..100M.
- Matsunaga, Noriyuki; Feast, Michael W.; Menzies, John W. (2009). "Period-luminosity relations for type II Cepheids and their application". Monthly Notices of the Royal Astronomical Society 397 (2): 933–942. arXiv:0904.4701. Bibcode:2009MNRAS.397..933M. doi:10.1111/j.1365-2966.2009.14992.x.
- Leavitt, Henrietta S. (1908). "1777 variables in the Magellanic Clouds". Annals of Harvard College Observatory 60: 87. Bibcode:1908AnHar..60...87L.
- Leavitt, Henrietta S.; Pickering, Edward C. (1912). "Periods of 25 Variable Stars in the Small Magellanic Cloud". Harvard College Observatory Circular 173: 1. Bibcode:1912HarCi.173....1L.
- Hubble, E. P. (1925). "Cepheids in spiral nebulae". The Observatory 48: 139. Bibcode:1925Obs....48..139H.
- Lemaître, G. (1927). Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques, Annales de la Societe Scientifique de Bruxelles
- Allen, Nick. "The Cepheid Distance Scale: A History".[self-published source?]
- Benedict, G. Fritz; McArthur, Barbara E.; Feast, Michael W.; Barnes, Thomas G.; Harrison, Thomas E.; Patterson, Richard J.; Menzies, John W.; Bean, Jacob L.; Freedman, Wendy L. (2007). Hubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity Relations, AJ
- Stanek, K. Z.; Udalski, A. (1999). The Optical Gravitational Lensing Experiment. Investigating the Influence of Blending on the Cepheid Distance Scale with Cepheids in the Large Magellanic Cloud. p. 9346. arXiv:astro-ph/9909346. Bibcode:1999astro.ph..9346S.
- Udalski, A.; Wyrzykowski, L.; Pietrzynski, G.; Szewczyk, O.; Szymanski, M.; Kubiak, M.; Soszynski, I.; Zebrun, K. (2001). "The Optical Gravitational Lensing Experiment. Cepheids in the Galaxy IC1613: No Dependence of the Period-Luminosity Relation on Metallicity". Acta Astronomica 51: 221. arXiv:astro-ph/0109446. Bibcode:2001AcA....51..221U.
- Macri, L. M.; Stanek, K. Z.; Bersier, D.; Greenhill, L. J.; Reid, M. J. (2006). "A New Cepheid Distance to the Maser‐Host Galaxy NGC 4258 and Its Implications for the Hubble Constant". The Astrophysical Journal 652 (2): 1133–1149. arXiv:astro-ph/0608211. Bibcode:2006ApJ...652.1133M. doi:10.1086/508530.
- Bono, G.; Caputo, F.; Fiorentino, G.; Marconi, M.; Musella, I. (2008). "Cepheids in External Galaxies. I. The Maser‐Host Galaxy NGC 4258 and the Metallicity Dependence of Period‐Luminosity and Period‐Wesenheit Relations". The Astrophysical Journal 684: 102–117. arXiv:0805.1592. Bibcode:2008ApJ...684..102B. doi:10.1086/589965.
- Madore, Barry F.; Freedman, Wendy L. (2009). "Concerning the Slope of the Cepheid Period-Luminosity Relation". The Astrophysical Journal 696 (2): 1498–1501. arXiv:0902.3747. Bibcode:2009ApJ...696.1498M. doi:10.1088/0004-637X/696/2/1498.
- Scowcroft, V.; Bersier, D.; Mould, J. R.; Wood, P. R. (2009). "The effect of metallicity on Cepheid magnitudes and the distance to M33". Monthly Notices of the Royal Astronomical Society 396 (3): 1287–1296. Bibcode:2009MNRAS.396.1287S. doi:10.1111/j.1365-2966.2009.14822.x.
- Majaess, D. (2010). "The Cepheids of Centaurus A (NGC 5128) and Implications for H0". Acta Astronomica 60: 121. arXiv:1006.2458. Bibcode:2010AcA....60..121M.
-  Webb, Stephen, Measuring the Universe: The Cosmological Distance Ladder, Springer, (1999)
- Eddington, A. S. (1917). "The pulsation theory of Cepheid variables". The Observatory 40: 290. Bibcode:1917Obs....40..290E.
- Zhevakin, S. A., "К Теории Цефеид. I", Астрономический журнал, 30 161–179 (1953)
- Turner, D. G.; Guzik, Joyce Ann; Bradley, Paul A. (2009). "Polaris and its Kin". AIP Conference Proceedings. pp. 59–68. arXiv:0907.3245. Bibcode:2009AIPC.1170...59T. doi:10.1063/1.3246569.
- SEDS: Variable Stars
- McMaster Cepheid Photometry and Radial Velocity Data Archive
- American Association of Variable Star Observers
- Stellar pulsation theory - Regular versus irregular variability