10 HER (10 Herculis). Just over the border from Serpens Caput, the Head of the Serpent, and five degrees north-northwest of Beta Herculis (the luminary of Hercules, who needs no introduction), 10 Herculis shines at a fairly dim 5th (nominally 5.70) magnitude. Alpha Herculis, in Hercules' head, is actually the fifth brightest in the constellation, showing that Bayer also had position in mind when in 1603 he lettered the stars. 10 Herculis was too dim for anyone at the time to take note of; the number comes from the later work of Flamsteed. But we digress. As a cool (3455 Kelvin) class M (M4.5) giant, the star is readily noticeable in binoculars by its reddish color. While 10 Her at first was classed to be an irregular variable that wanders between magnitudes 5.58 and 5.83 (which gives it the variable star name LQ Herculis), there seems to be an uncertain (and probably variable) period of around 27 days. From a distance of 682 light years (give or take 44) and after factoring in a lot of infrared radiation, the star is seen to shine with a luminosity of 1750 times that of the Sun. In spite of the distance, dimming by interstellar dust does not seem to be a problem. Luminosity and temperature then conspire to give a radius 117 times solar, 0.54 Astronomical Units, or 1.4 times the radius of Mercury's orbit. Interferometric measures of the angular diameter coupled with distance give 130 solar radii, 11 percent greater. The difference is not all that surprising, since for cool, large red giants stars like this one, a "surface" is hard to define, as such stars have rather fuzzy boundaries and the radius depends on the wavelength of observation and on the presence of absorption features within the observed wavelength band, particularly those of titanium oxide. Moreover, variability in brightness implies change in radius, so the timing of observation becomes a significant factor. Given such problems, the agreement between the two values of radius is actually rather good. Given its coolness, 10 Herculis is probably brightening as a red giant for the second time, now with a dead carbon/oxygen core, the result of helium fusion (the first time was with a dead helium core, the result of hydrogen fusion), and may be on its way to becoming a long-period variable like Mira. The theory of stellar structure and evolution suggests a star with a mass of around 1.5 times that of the Sun or perhaps a bit greater, but it's hard to tell as stars in this state over a wide range of mass have similar parameters. There seems to be no companion to watch the star lose its outer envelope and generate a planetary nebula from the expanding leavings and exposed hot core, which will then turn into a white dwarf with a mass around 60 percent that of the Sun.

Written byJim Kaler 7/31/15. Return to STARS.