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 ...

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 th ...

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 memb ...

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.689–6 ...

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 th ...

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 The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their stellar classifications or effective tempe ...

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 ...

s. 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 VY Pyxidis is a BL Herculis variable (type II Cepheid) in the constellation of Pyxis. It ranges between apparent magnitudes 7.13 and 7.40 over a period of 1.23995 days. Located around 826 light-years distant, it shines with a luminosit ...

, 7.7 * V553 Centauri, 8.2 * SW Tauri, 9.3 * RT Trianguli Australis, 9.4 * V351 Cephei, 9.5 *

. 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 elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as ...

* 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

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