BL Herculis variables are a subclass of

type II Cepheid Type II Cepheids are variable stars which pulsate with periods typically between 1 and 50 days. They are population II stars: old, typically metal-poor, low mass objects. Like all Cepheid variables, Type IIs exhibit a period-luminosity relationshi ...

s with low luminosity and mass, that have a period of less than eight days. They are pulsating stars with

light curve In astronomy, a light curve is a graph (discrete mathematics), graph of the Radiance, light intensity of a celestial object or region as a function of time, typically with the magnitude (astronomy), magnitude of light received on the ''y''-axis ...

s that frequently show a bump on the descending side for stars of the shortest periods and on the ascending side for longer period stars. Like other type II Cepheids, they are very old population II stars found in the galaxy’s halo and

globular cluster A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting ...

s. Also, compared to other type II Cepheids, BL Herculis variables have shorter periods and are fainter than

W Virginis variable W Virginis variables are a subclass of Type II Cepheids which exhibit pulsation periods between 10–20 days, and are of spectral class F6 – K2. They were first recognized as being distinct from classical Cepheids by Walter Baade in 1942, in ...

s. Pulsating stars vary in

spectral class In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

as they vary in brightness and BL Herculis variables are normally class A at their brightest and class F when most dim. When plotted on the Hertzsprung–Russell diagram they fall in-between W Virginis and

RR Lyrae variable RR Lyrae variables are periodic variable stars, commonly found in globular clusters. They are used as standard candles to measure (extra) galactic distances, assisting with the cosmic distance ladder. This class is named after the prototype a ...

s. 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: * 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'': * XX Virginis stars (AHB1), with very fast rises to maximum and low

metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...

* CW stars (AHB2),

W Virginis variables W Virginis variables are a subclass of Type II Cepheids which exhibit pulsation periods between 10–20 days, and are of spectral class F6 – K2. They were first recognized as being distinct from classical Cepheids by Walter Baade in 1942, in a ...

, longer periods, the bump on the ascending leg * BL Herculis stars (AHB3), shorter periods, the bump on the descending leg

References

External links

OGLE Atlas of Variable Star Light Curves - BL Herculis stars
{{Variable star topics Astrometry Standard candles Variable stars BL Herculis variables