RASALGETHI (Alpha Herculis). Just to the west of the bright star Vega lies the constellation Hercules, which commemorates the great and strong mythological Greek hero. Though the brightest of its stars is only middling second magnitude, the constellation as a whole is fairly prominent. Rather oddly, Rasalgethi, the Alpha star, is the fifth brightest, perhaps because it is a bit south of the main constellation pattern and (from Arabic) refers to "the Kneeler's Head," in reference to an early name for the constellation, the figure of the man seen upside down, his head toward the south. Though of the third magnitude (3.48, nearly fourth) as seen from Earth, the star itself is magnificent, a red class M (M5) supergiant (or bright giant) with a surface temperature of about 3300 degrees Kelvin. At a distance of 380 light years, Rasalgethi to the human eye is 475 times more luminous than the Sun, though in fact it varies irregularly in its brightness by about a magnitude over a periods of months to years.
Rasalgethi (Alpha, sometimes Alpha-1, Herculis) varies by almost a magnitude over the nearly 14 years of observation, the principal period 128 or so days. The scale on the bottom is the "Julian Date" of 2440000 plus the number that appears, where the Julian Date is the number of days since January 1, 4713 BC of the Julian Calendar and is commonly used for variable phenomena in astronomy. JD 2446500 corresponds to March 11, 1986. The left-hand scale expresses the difference between the apparent visual magnitude of Rasalgethi and a nearby comparison star. (From an article in the Publications of the Astronomical Society of the Pacific by J. R. Percy, J. B. Wilson, and G. W. Henry.)
Factoring in the infrared radiation that we cannot see from the cool star makes the star much brighter. From the measured (and quite large) angular diameter of 0.032 seconds of arc we find a spectacular physical radius of 1.9 Astronomical Units, 400 times that of the Sun. If placed here, the star would extend well out past the orbit of Mars and into the inner part of the asteroid belt. Radius and temperature then lead to a luminosity of 17,000 times that of the Sun. Rasalgethi has a fifth magnitude companion five seconds of arc away (Alpha-2, rendering Rasalgethi proper Alpha-1). This secondary is itself a double that consists of a four-solar-mass class G (G5) giant star with a temperature about that of the Sun and a 2.5-solar-mass F2 dwarf star (somewhat hotter than the Sun) in orbit around each other separated by 0.4 AU with a 52 day period. The bright supergiant is blowing a powerful wind that is so strong that the cloud of gas expanding around it has enveloped the companion, which is at least 550 astronomical units away from the supergiant, 275 times the supergiant's size and 14 times the dimension of our whole planetary system. The small double must thus take over 3000 years to make a full orbit of the supergiant. Rasalgethi appears to be losing mass at a rate of about a ten-millionth of a solar mass per year, hardly a record, but substantial enough, consistent with its being a highly distended supergiant. With a birth mass of perhaps 7 or 8 times that of the Sun, the star has probably either finished core helium fusion or will do so shortly and will die as a relatively massive white dwarf. It does not seem to have quite enough mass to explode as a supernova, though only time will tell.