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BL Herculis variable
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<p>BL Herculis variables are a subclass of <a href="/facts/Type_II_Cepheid/VEd29tsL">type II Cepheids</a> with low luminosity and mass, that have a period of less than eight days. They are pulsating stars with <a href="/facts/Light_curve/zN0wdtaR">light curves</a> 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 <a href="/facts/Population_II/8ur1qDRK">population II</a> stars found in the <a href="/facts/Galactic_halo/A6jsautY">galaxy’s halo</a> and <a href="/facts/Globular_cluster/FL0EERJU">globular clusters</a>. Also, compared to other type II Cepheids, BL Herculis variables have shorter periods and are fainter than <a href="/facts/W_Virginis_variable/KDvLcEGo">W Virginis variables</a>. Pulsating stars vary in <a href="/facts/Spectral_class/dYrIuK9H">spectral class</a> 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 <a href="/facts/Hertzsprung%E2%80%93Russell_diagram/53YWnCvR">Hertzsprung–Russell diagram</a> they fall in-between W Virginis and <a href="/facts/RR_Lyrae_variable/vjoHuKUe">RR Lyrae variables</a>. </p><p>The prototype star, <a href="/facts/BL_Herculis/KMRFw1oG">BL Herculis</a>, 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: </p> <ul><li><a href="/facts/VY_Pyxidis/h3wknDJU">VY Pyxidis</a>, 7.7</li> <li><a href="/facts/V553_Centauri/4efSbcng">V553 Centauri</a>, 8.2</li> <li>SW Tauri, 9.3</li> <li><a href="/facts/RT_Trianguli_Australis/SjG42N9j">RT Trianguli Australis</a>, 9.4</li> <li>V351 Cephei, 9.5</li> <li><a href="/facts/BL_Herculis/KMRFw1oG">BL Herculis</a>. 9.7</li> <li>BD Cassiopeiae, 10.8</li> <li>UY Eridani, 10.9</li></ul> <p>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 <i>above horizontal branch</i>: </p> <ul><li>XX Virginis stars (AHB1), with very fast rises to maximum and low <a href="/facts/Metallicity/nhhPqbYF">metallicity</a></li> <li>CW stars (AHB2), <a href="/facts/W_Virginis_variables/KDvLcEGo">W Virginis variables</a>, longer periods, the bump on the ascending leg</li> <li>BL Herculis stars (AHB3), shorter periods, the bump on the descending leg</li></ul>