BL Herculis

BL Herculis

A light curve for BL Herculis, plotted from Hipparcos data[1]
Observation data
Epoch J2000      Equinox J2000
Constellation Hercules
Right ascension 18h 01m 09.224s[2]
Declination 19° 14′ 56.696″[2]
Apparent magnitude (V) 9.70 to 10.62[3]
Characteristics
Evolutionary stage F0-F6II-III[3]
B−V color index 0.05[4]
Variable type BL Her[3]
Astrometry
Proper motion (μ) RA: −4.051 mas/yr[2]
Dec.: −11.847 mas/yr[2]
Parallax (π)0.8469 ± 0.0179 mas[2]
Distance3,850 ± 80 ly
(1,180 ± 20 pc)
Absolute magnitude (MV)−0.3[5]
Details
Mass0.75[6] M☉
Radius8.6[2] R☉
Luminosity101[2] L☉
Surface gravity (log g)2.53[6] cgs
Temperature6,500 - 7,000[7][8] K
Metallicity [Fe/H]−0.1 - −0.2[7][8] dex
Age377[2] Myr
Other designations
ER Vul, HD 347827, HIP 88242, 2MASS J18010922+1914567[9]
Database references
SIMBADdata

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]

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  11. ^ Cite error: The named reference Hoffmeister was invoked but never defined (see the help page).
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  15. ^ Cite error: The named reference Carson was invoked but never defined (see the help page).