X HER (X Herculis). While obscure -- hardly anyone studies the star -- and just over the line into seventh magnitude (6.58 on a sort-of average), X Herculis still has a couple stories to tell. First, if one does not pay attention, it is easy to mix it up with brighter, rather sun-like Chi Herculis, the Greek "Chi" written much like Roman "X." The confusion is not helped by X and Chi being close together in far northwestern Hercules. They are close only in direction, though, as Chi Her is a mere 52 light years away, while X Her, at 450 light years, is almost nine times farther. Curiously, they share high speed relative to the Sun, Chi moving at 80 kilometers per second, X Her at a very substantial 110, seven times average, both stars visitors from different parts of the Galaxy. More importantly, the "X" tells us not that the star is necessarily mysterious, but is a variable star, the seventh one found in Hercules that already did not have a name (the discovery-progression of variables in a constellation starting with "R"). Called a "semi-regular" variable (in the trade, an "SRb" star), this deep red class M giant (the class probably variable as well, given as M7 or M8) varies with an average 95 day period between magnitudes 6 and 7 with a longer two- year additional variation superimposed on it.
X Herculis The visual light curve of the semi-regular variable X Herculis is plotted against Julian Date. JD 2453750 is January 14, 2006, JD 2454500 February 3, 2008. The black points are visual estimates, the green ones more-accurate photoelectric measures. The star varies between magnitudes 6 and 7 with a rough 95 day period. Note the superimposed longer-period variation. Courtesy of the American Association of Variable Star Observers (AAVSO).
Typical of its class, it is one of the cooler stars in the sky, its temperature hovering at a measured 3300 Kelvin. Such extreme red giants are entering the last stages of their lives with dead carbon-oxygen cores, and are -- like the red giant variable Mira -- preparing to slough off their outer envelopes in the process of becoming white dwarfs. Distance and temperature (to account for a LOT of infrared radiation) give a luminosity of 680 times that of the Sun and a radius of 80 Suns, 0.38 Astronomical Units, about the size of Mercury's orbit. Oddly, the radius from direct measure of angular diameter gives a wildly different number, 180 solar, 0.84 AU. When compared to theory, the luminosity is too low for the temperature. An M7 giant should have a temperature closer to 3000 Kelvin, which would give a luminosity of 3200 Suns and a radius of 200 solar (nearly the size of Earth's orbit), closer to the mark. The birth mass is then around one to 1.5 times that of the Sun or so. The problem may also be compounded by a surrounding disk-like structure observed in radio radiation that is caused by windy mass loss. A low metal content implied by high velocity might also confuse theory. Clearly, this much neglected star (which shows us something of what will happen to the Sun) has some problems and needs additional work to sort out its parameters, in this sense "X" really denoting some mystery. Best appreciate it now, as it is dying fast, at least on an astronomical time scale.
Written by Jim Kaler 8/15/08. Return to STARS.