Alpha Herculis, Alpha2 Herculis - Ras Algethi

Alpha Herculis is a beautiful double star in the constellation Hercules, which commemorates the Greek mythological hero. At magnitude 3.48, it is only the fifth-brightest star in the constellation, however. Alpha Herculis has the traditional name Ras Algethi or Rasalgethi (from the Arabic "ra's al-jathiyy", meaning "Head of the Kneeler"), in reference to an early name for the constellation, the figure of a man seen upside down with his head toward the south.

Components and Variability

A telescope will resolve α Her into two components, designated α1 and α2 Her, separated by about 5". Contrast effects cause them to appear orange-red and greenish. The primary (α1 Her) is a red giant semi-regular variable with a magnitude range of 3.1 to 3.9, and a period that very roughly averages 90 days. To follow the light variation of α1 Her, compare it to ι and κ Oph, which lie about 6.5 degrees to its SW and have magnitudes of 4.3 and 3.2, respectively.

The secondary (α2 Her) is a spectroscopic binary, consisting of a yellow giant and main-sequence star; it has a combined magnitude of 5.0. At a distance of 380 light years, α1 and α2 Her are more than 550 AU apart, with an estimated orbital period of approximately 3600 years.

Properties and Evolution

α1 Her, also designated α Her A, is a massive class M5 Ib-II red supergiant (or bright giant), with a surface temperature of about 3300 K. The angular diameter of α Her A has been measured with an interferometer as 0.034"; this corresponds to a spectacular physical radius of 1.9 AU, 400 times the Sun's. Visually, the star is 475 times more luminous than the Sun; factoring in the invisible infrared radiation from its cool surface, its luminosity climbs to 17,000 suns.

α2 Her is a binary star, with a yellow class G5 III giant primary and a yellow-white class F2 V secondary. These components are sometimes designated α Her Ba and α Her Bb, respectively. The giant is a four-solar-mass star with a temperature about the same as the Sun's; the somewhat hotter F2 secondary contains 2.5 solar masses. They orbit each other at an average distance of 0.4 AU, with a 52 day period.

As do many red supergiants, α Her A varies irregularly, by about a factor of two - over a periods of months to years. It is generating a powerful stellar wind, enveloping its companions in an expanding cloud of gas. Rasalgethi appears to be losing mass, at the rate of about one ten-millionth of a solar mass per year. With a birth mass of perhaps 7 or 8 solar masses, the star has probably either finished core helium fusion, or will do so shortly. It does not seem to have quite enough mass to explode as a supernova, and will die as a relatively massive white dwarf.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]