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BL Herculis variables are a subclass of type II Cepheids with low luminosity and mass, that have a period of less than eight days.cite-ref-wallerstein02-2-0[2]cite-ref-so08-3-0[3] They are pulsating stars 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.cite-ref-so08-3-1[3] Like other type II Cepheids, they are very old population II stars found in the galaxy’s halo and globular clusters.cite-ref-hodson81-4-0[4] Also, compared to other type II Cepheids, BL Herculis 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 their brightest and class F when most dim.cite-ref-gcvs-5-0[5] When plotted on the Hertzsprung–Russell diagram they fall in-between W Virginis and RR Lyrae variables.cite-ref-hodson81-4-1[4]

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, with their maximum magnitudes, are:cite-ref-gcvs-5-1[5]

• VY Pyxidis, 7.7

• 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

• UY Eridani, 10.9

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:cite-ref-mcnamara-6-0[6]cite-ref-diethelm1994-7-0[7]cite-ref-diethelm1996-8-0[8]

• 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

• References

• External links

cite-note-hipcurve-11. "Light Curve". Hipparcos ESA. ESA. Retrieved 17 February 2022.

cite-note-wallerstein02-22. ↑ citerefwallerstein-george2002Wallerstein, 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.

cite-note-so08-33. ↑ citerefsoszy-ski-i-udalski-a-szyma-ski-m-k-kubiak-m-2008Soszyński, 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.

cite-note-hodson81-44. ↑ "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.

cite-note-gcvs-55. ↑ "General Catalogue of Variable Stars". GCVS. Institute of Astronomy of Russian Academy of Sciences and Sternberg State Astronomical Institute. Retrieved 31 May 2013.

cite-note-mcnamara-66. ↑ citerefmcnamarapyne1994McNamara, 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.

cite-note-diethelm1994-77. ↑ citerefdiethelm1983Diethelm, 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.

cite-note-diethelm1996-88. ↑ citerefdiethelm1996Diethelm, R. (1996). "Period changes of AHB1 variables". Astronomy and Astrophysics. 307: 803. Bibcode:1996A&A...307..803D.

• OGLE Atlas of Variable Star Light Curves - BL Herculis stars