Alpha Herculis

Alpha Herculis (α Herculis, abbreviated Alpha Her, α Her), also designated 64 Herculis, is a multiple star system in the constellation of Hercules. Appearing as a single point of light to the naked eye, it is resolvable into a number of components through a telescope. It has a combined apparent magnitude of 3.08, although the brightest component is variable in brightness. Based on parallax measurements obtained during the Hipparcos mission, it is approximately 360 light-years (110 parsecs) distant from the Sun.

It consists of two binary pairs in mutual orbit designated α¹ Herculis or α Herculis A (the brightest of the two) and α² Herculis or α Herculis B. A's two components are themselves designated α Herculis Aa (officially named Rasalgethi , from the traditional name of the system) and Ab; B's as α Herculis Ba and Bb.

Alpha Herculis also forms the A and B components of a wider system designated WDS J17146+1423 with two additional visual companions designated WDS J17146+1423C and D.


α Herculis (Latinised to Alpha Herculis) is the system's Bayer designation; α¹ and α² Herculis, those of its two components. 64 Herculis is the system's Flamsteed designation. WDS J17146+1423 is the wider system's designation in the Washington Double Star Catalog. The designations of Alpha Herculis' two components as Alpha Herculis A and B and the wider system's four components as WDS J17146+1423A, B, C and D, together with those of A's and B's components - Alpha Herculis Aa, Ab, Ba and Bb - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).

Alpha Herculis bore the traditional name Rasalgethi or Ras Algethi ( رأس الجاثي ra‘is al-jāthī 'Head of the Kneeler'). 'Head' comes from the fact that in antiquity Hercules was depicted upside down on maps of the constellation. In 2016, the IAU organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN approved the name Rasalgethi for the component Alpha Herculis Aa on 30 June 2016 and it is now so included in the List of IAU-approved Star Names.

The term ra's al-jaθiyy or Ras al Djathi appeared in the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, which was translated into Latin as Caput Ingeniculi.

In Chinese astronomy, Alpha Herculis is called 帝座, Pinyin: Dìzuò, meaning 'Emperor's Seat', this star is marking itself and stands alone in the center of the Emperor's Seat asterism, Heavenly Market enclosure (see : Chinese constellations). 帝座 (Dìzuò) was westernized into Ti Tso by R.H. Allen, with the same meaning


Alpha Herculis A and B are more than 500 AU apart, with an estimated orbital period of approximately 3600 years. A presents as a relatively massive red bright giant, but radial velocity measurements suggest a companion with a period of the order of a decade. B's two components are a primary yellow giant star and a secondary, yellow-white dwarf star in a 51.578 day orbit.

Alpha Herculis Aa is an asymptotic giant branch (AGB) star, a luminous red giant that has both hydrogen and helium shells around a degenerate carbon-oxygen core. It is the second nearest AGB star to the Sun. The angular diameter of the star has been measured with an interferometer as 34 ± 0.8 mas, or 0.034 arcseconds. At its estimated distance of 110 parsecs this corresponds to a radius of about 280 million kilometers (or 170 million miles), which is roughly or 1.87 AU. If Rasalgethi were at the center of the Solar System its radius would extend past the orbit of Mars at 1.5 AU but not quite as far as the asteroid belt. The red giant is estimated to have started its life with about .

Like most type M stars near the end of their lives, Rasalgethi is experiencing a high degree of stellar mass loss creating a sparse, gaseous envelope that extends at least 90 AU. It is a semiregular variable with complex changes in brightness with periods ranging from a few weeks to many years. The most noticeable variations occur at timescales of 80–140 days and at 1,000 - 3,000 days. The strongest detectable period is 128 days. The full range in brightness is from magnitude 2.7 to 4.0, but it usually varies over a much smaller range of around 0.6 magnitudes.