BL Herculis variables are a subclass oftype II Cepheids with low luminosity and mass, that have a period of less than eight days.[2][3] They are pulsating stars withlight 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.[3] Like other type II Cepheids, they are very oldpopulation II stars found in thegalaxy’s halo andglobular clusters.[4] Also, compared to other type II Cepheids, BL Herculis variables have shorter periods and are fainter thanW Virginis variables. Pulsating stars vary inspectral class as they vary in brightness and BL Herculis variables are normally class A at their brightest and class F when most dim.[5] When plotted on theHertzsprung–Russell diagram they fall in-between W Virginis andRR Lyrae variables.[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:[5]
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 toabove horizontal branch:[6][7][8]
^"Light Curve".Hipparcos ESA. ESA. Retrieved17 February 2022.
^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.
^abSoszyński, 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.
^ab"General Catalogue of Variable Stars".GCVS. Institute of Astronomy of Russian Academy of Sciences and Sternberg State Astronomical Institute. Retrieved31 May 2013.
^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.
^Diethelm, R. (1996). "Period changes of AHB1 variables".Astronomy and Astrophysics.307: 803.Bibcode:1996A&A...307..803D.
