BL Herculis variables are a subclass of type II Cepheids with low luminosity and mass, that have a period of less than eight days.[2][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.[3] Like other type II Cepheids, they are very old population II stars found in the galaxy’s halo and globular clusters.[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.[5] When plotted on the Hertzsprung–Russell diagram they fall in-between W Virginis and RR 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]
- 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:[6][7][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