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 relationship between the st ...

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 of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...

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 During 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations. In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926: Baade noticed ...

stars found in the galaxy’s halo and

globular cluster A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...

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,Wallerstein, G."The Cepheids of Population II and Related Stars" ''Publications of the Astronomical Society of the Pacific'', 114 p.689– ...

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 variables. The prototype star,

BL Herculis BL Herculis is a variable star in the northern constellation of Hercules. Its apparent visual magnitude ranges from 9.70 to 10.62, so it is never bright enough to be seen with the naked eye, even with ideal observing conditions. Its dis ...

, 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 RT Trianguli Australis, or RT TrA, is a BL Herculis variable ( type II Cepheid) in the constellation of Triangulum Australe. Variable RT TrA varies between apparent magnitudes 9.4 and 10.2 over a period of 1.95 days. It was first discov ...

, 9.4 * V351 Cephei, 9.5 *

. 9.7 *

BD Cassiopeiae BD, Bd or bd may refer to: In arts and entertainment * B. D. (Doonesbury), a major character in the ''Doonesbury'' comic strip * '' Bande dessinée'' (or "bédé"), a French term for comics * Bass drum, in sheet music notation * Brahe Djäknar ...

, 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 * CW stars (AHB2),

W Virginis variables W Virginis variables are a subclass of Type II Cepheids which exhibit pulsation periods between 10–20 days,Wallerstein, G."The Cepheids of Population II and Related Stars" ''Publications of the Astronomical Society of the Pacific'', 114 p.689– ...

, 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