Cepheid variable

type of variable star

Cepheids are a type of very luminous variable stars. There is a strong direct relationship between a Cepheid's luminosity and pulsation period.[1][2] This makes Cepheids important standard candles for the galactic and extragalactic distance scales.[3]

RS Puppis. It is one of the brightest known Cepheid variable stars in the Milky Way galaxy; image, Hubble Space Telescope

Cepheid variables are divided into several subclasses which exhibit clearly different masses, ages, and evolutionary histories:

  1. Classical Cepheids
  2. Type II Cepheids
  3. Anomalous Cepheids
  4. Dwarf Cepheids

The first cepheid known was Delta Cephei in the 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,[4][5] and the precise Hubble Space Telescope/Hipparcos parallaxes.[6]


Classical CepheidsEdit

Classical Cepheids (also known as Population I Cepheids, Type I Cepheids, or Delta Cephei variables) pulsate with very regular periods of the order of days to months. Classical Cepheids are population I young variable stars which are 4–20 times more massive than the Sun,[7] and up to 100,000 times more luminous.[8] Cepheids are yellow supergiants of spectral class F6 – K2. When they pulsate, their radii change by ~25%. For the longer-period I Carinae this means millions of kilometers for a pulsation cycle.[9][10]

Type II CepheidsEdit

Type II Cepheids (also termed Population II Cepheids) are population II variable stars which pulsate with periods between 1 and 50 days.[11][12] Type II Cepheids are typically metal-poor, old (~10 giga years), low mass objects (~half the mass of the Sun). Type II Cepheids are divided into several subgroups by period.

Type II Cepheids are used to fix the distance to the galactic center of the Milky Way, globular clusters, and galaxies.[13][14][15][16][17]

Anomalous CepheidsEdit

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 our sun. It is unclear whether they are young stars on a "turned-back" horizontal branch, blue stragglers formed through mass transfer in binary systems, or a mix of both.[18][19]

Double-mode CepheidsEdit

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.[20] A very small number pulsate in three modes, or an unusual combination of modes including higher overtones.[19]


  1. Udalski A.; et al. (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.
  2. Soszynski I.; 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.
  3. 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.CS1 maint: multiple names: authors list (link)
  4. de Zeeuw P.T. et al 1999. A HIPPARCOS census of the nearby OB associations, AJ
  5. Majaess D.; Turner D & Gieren W. 201). New evidence supporting cluster membership for the keystone calibrator Delta Cephei, ApJ
  6. Benedict, G. Fritz et al 2002. Astrometry with the Hubble Space Telescope: a parallax of the fundamental distance calibrator δ Cephei, AJ
  7. Turner, David G. (1996). "The progenitors of classical Cepheid variables". Journal of the Royal Astronomical Society of Canada. 90: 82. Bibcode:1996JRASC..90...82T.
  8. 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.
  9. 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.
  10. Strohmeier W. 1972. Variable stars'. Pergamon.
  11. 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.
  12. Soszyński I.; 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.
  13. 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.
  14. Matsunaga, Noriyuki; 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.
  15. Feast, Michael W.; et al. (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.
  16. 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.
  17. 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.
  18. Caputo, F.; Castellani, V.; Degl'Innocenti, S.; Fiorentino, G.; Marconi, M. (2004-09). "Bright metal-poor variables: Why "Anomalous" Cepheids?". Astronomy & Astrophysics. 424 (3): 927–934. doi:10.1051/0004-6361:20040307. ISSN 0004-6361. Check date values in: |date= (help)
  19. 19.0 19.1 Pietrukowicz, P.; Udalski, A.; Soszyński, I.; Nataf, D. M.; Wyrzykowski, Ł.; Poleski, R.; Kozłowski, S.; Szymański, M. K.; Kubiak, M. (2012-04-26). "THE OPTICAL GRAVITATIONAL LENSING EXPERIMENT: ANALYSIS OF THE BULGE RR LYRAE POPULATION FROM THE OGLE-III DATA". The Astrophysical Journal. 750 (2): 169. doi:10.1088/0004-637x/750/2/169. ISSN 0004-637X.
  20. Smolec, R.; Moskalik, P.; Guzik, Joyce Ann; Bradley, Paul A. (2009). "On Resonant and Non-Resonant Origin of Double-Mode Cepheid Pulsation". AIP Conference Proceedings. AIP. doi:10.1063/1.3246578.