BL Herculis variable

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BL Herculis variables are a type of variable star with low luminosity and mass that have a period of less than eight days.[1][2] They are pulsating stars making up a subclass of Type II Cepheids with light curves that frequently show a bump on the descending side for stars of the shortest periods and on the ascending side for longer period stars.[2] Like other Type II Cepheids, they are very old population II stars found in the galaxy’s halo and globular clusters.[3] Also, compared to other Type II Cepheids, BL Her variables have shorter periods and are fainter than W Virginis variables. Pulsating stars vary in spectral class as they vary in brightness and BL Herculis variables are normally class A at brightest and class F when most dim.[4] When plotted on the Hertzsprung–Russell diagram they fall in-between W Virginis and RR Lyrae variables.[3]

The prototype star, BL Herculis, varies between magnitude 9.7 and 10.6 in a period of 1.3 days. The brightest BL Herculis variables are: VY Pyxidis (7.7 mag max), V553 Centauri (8.2), SW Tauri (9.3), RT Trianguli Australis (9.4), V351 Cephei (9.5), BL Herculis (9.7), BD Cassiopeiae (10.8), and UY Eridani (10.9).[4]

The BL Herculis stars show a wide variety of light curves, temperatures, and luminosity, and three subdivisions of the class have been defined, with the acronym AHB referring to above horizontal branch[5][6][7]

  • XX Virginis stars (AHB1), with very fast rises to maximum and low metallicity;
  • CW stars (AHB2), W Virginis variables, longer periods, the bump on the ascending leg
  • BL Herculis stars (AHB3), shorter periods, the bump on the descending leg


  1. ^ Wallerstein, George (2002). "The Cepheids of Population II and Related Stars". The Publications of the Astronomical Society of the Pacific. 114 (797): 689–699. Bibcode:2002PASP..114..689W. doi:10.1086/341698.
  2. ^ a b Soszyński, I.; Udalski, A.; Szymański, M. K.; Kubiak, M.; Pietrzyński, G.; Wyrzykowski, Ł.; Szewczyk, O.; Ulaczyk, K.; Poleski, R. (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.
  3. ^ a b "The Masses and Pulsations of BL Herculis Variables" (PDF). Information Bridge. US Department of Energy, Office of Scientific and Technical Information. Retrieved 30 May 2013.
  4. ^ a b "General Catalogue of Variable Stars". GCVS. Institute of Astronomy of Russian Academy of Sciences and Sternberg State Astronomical Institute. Retrieved 31 May 2013.
  5. ^ McNamara, D. H.; Pyne, M. D. (1994). "A Photometric Study of XX Virginis and V716 Ophiuchi". Publications of the Astronomical Society of the Pacific. 106: 472. Bibcode:1994PASP..106..472M. doi:10.1086/133402.
  6. ^ Diethelm, R. (1983). "A photometric classification of pulsating variables with periods between one and three days". Astronomy and Astrophysics. 124: 108. Bibcode:1983A&A...124..108D.
  7. ^ Diethelm, R. (1996). "Period changes of AHB1 variables". Astronomy and Astrophysics. 307: 803. Bibcode:1996A&A...307..803D.

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