BL Herculis is a variable star in the northern constellation of Hercules. Its apparent visual magnitude ranges from 9.70 to 10.62,[3] so it is never bright enough to be seen with the naked eye, even with ideal observing conditions. Its distance from the Sun is about 3,850 light-years,[2] and it is moving away from us at 18 km/sec.[10] It is the prototype of the BL Herculis class of variable star, a short-period subset of the pulsating Cepheid variables.
The variability of BL Herculis was discovered by Cuno Hoffmeister, and announced in 1929.[11] Early observations of the star produced a very inaccurate period of 4.2 days, which resulted in peculiar light and radial velocity curves.[12] The first accurate period, 1.3 days, was published by Pavel Parenago in 1940.[13] and a far more precise period of 1.30744185 days was derived from photometric observations in 1983.[6] The descending portion of the star's light curve shows a "bump" (near phase=0.3, with peak brightness phase defined as 0), which models suggest arises from a 2:1 resonance between the fundamental and second overtone pulsation modes.[14] This bump is considered the primary characteristic of BL Her stars, although its position relative to peak brightness varies as a function of the star's period.[6][15]
The mass of BL Herculis is estimated to be about 0.75 solar masses, just slightly greater than the mass of a typical RR Lyrae variable.[6]
References
^"Light Curve". Hipparcos ESA. ESA. Retrieved 17 February 2022.
^ abcd"BL Her". The International Variable Star Index. AAVSO. Retrieved 23 August 2022.
^Diethelm, R. (November 1990). "Physical parameters of pulsating variables with periods between one and three days. II. Fundamental parameters". Astronomy and Astrophysics. 239: 186. Bibcode:1990A&A...239..186D.
^Galazutdinov, G. A. (1996). "A Spectroscopic Study of the Pulsating Star BL Her". Astronomy Letters. 22 (3): 364–371. Bibcode:1996AstL...22..326G.
^ abMaas, Thomas; Giridhar, Sunetra; Lambert, David L. (September 2007). "The Chemical Compositions of the Type II Cepheids-The BL Herculis and W Virginis Variables". The Astrophysical Journal. 666 (1): 378–392. arXiv:0706.2029. Bibcode:2007ApJ...666..378M. doi:10.1086/520081. S2CID18862838.
^ abKovtyukh, V.; Wallerstein, G.; Yegorova, I.; Andrievsky, S.; Korotin, S.; Saviane, I.; Belik, S.; Davis, C. E.; Farrell, E. M. (May 2018). "Metal-poor Type II Cepheids with Periods Less Than Three Days". Publications of the Astronomical Society of the Pacific. 130 (987): 054201. arXiv:1803.05041. Bibcode:2018PASP..130e4201K. doi:10.1088/1538-3873/aaacf7. S2CID59486927.
^Wilson, Ralph Elmer (1953). "General catalogue of stellar radial velocities". Carnegie Institute Washington D.C. Publication. Bibcode:1953GCRV..C......0W.
