BL Herculis is a

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as e ...

in the northern constellation of

Hercules Hercules (, ) is the Roman equivalent of the Greek divine hero Heracles, son of Jupiter and the mortal Alcmena. In classical mythology, Hercules is famous for his strength and for his numerous far-ranging adventures. The Romans adapted the ...

. Its

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's lig ...

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 distance from the Sun is about 3,850 light-years, and it is moving away from us at 18 km/sec. It is the prototype of the BL Herculis class of variable star, a short-period subset of the pulsating

Cepheid variable A Cepheid variable () is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid vari ...

s. The variability of BL Herculis was discovered by

Cuno Hoffmeister Cuno Hoffmeister (2 February 1892 – 2 January 1968) was a German astronomer, observer and discoverer of variable stars, comets and minor planets, and founder of Sonneberg Observatory. Born in Sonneberg in 1892 to Carl and Marie Hoffmeister, C ...

, and announced in 1929. Early observations of the star produced a very inaccurate period of 4.2 days, which resulted in peculiar light and radial velocity curves. The first accurate period, 1.3 days, was published by Pavel Parenago in 1940. and a far more precise period of 1.30744185 days was derived from photometric observations in 1983. 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. 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. 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.

References

{{DEFAULTSORT:BL Herculis Hercules (constellation) Herculis, BL 347827 BL Herculis variables