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Alpha Herculis (α Herculis, abbreviated Alpha Her, α Her), also designated Rasalgethi and 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 is also close to another bright star Rasalhague in the vicinity.
SystemEdit
Alpha Herculis is a triple star system. The primary (brightest) of the three stars, designated α1 Herculis or α Herculis A, is a pulsating variable star on the asymptotic giant branch (AGB). The primary star forms a visual binary pair with a second star, which is itself a spectroscopic binary.<ref name=moravveji/>
Alpha Herculis also forms the A and B components of a wider system designated WDS J17146+1423, with two additional faint visual companions designated WDS J17146+1423C and D.<ref name="WDS"/> The two fainter stars are far more distant than the triple system.<ref name=dr2cd/>
NomenclatureEdit
α Herculis (Latinised to Alpha Herculis) is the system's Bayer designation; α1 and α2 Herculis, those of its two visible 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' main components as Alpha Herculis A and B and the wider system's four components as WDS J17146+1423A, B, C and D, together with the spectroscopic pair - Alpha Herculis 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).<ref name="planetnaming"/>
Alpha Herculis bore the traditional name Rasalgethi or Ras Algethi (Template:Langx 'Head of the Kneeler').<ref name="BBC1"/> '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)<ref name="WGSN"/> to catalog and standardize proper names for stars. The WGSN approved the name Rasalgethi for the component Alpha Herculis A (α1) on 30 June 2016 and it is now so included in the List of IAU-approved Star Names.<ref name="WGSN"/>
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.<ref name=Knobel/>
In Chinese astronomy, Alpha Herculis is called 帝座, Pinyin: Dìzuò, meaning 'Emperor's Seat'. The star is seen as marking itself, and stands alone in the center of the Emperor's Seat asterism, Heavenly Market enclosure (see: Chinese constellations).<ref name=aeea/> 帝座 (Dìzuò) was westernized into Ti Tso by R.H. Allen, with the same meaning <ref name=RHA/>
PropertiesEdit
Alpha Herculis A and B are more than 500 AU apart, with an estimated orbital period of approximately 3600 years.Template:Cn 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.<ref name="WDS"/> B's two components are a primary yellow giant star and a secondary, yellow-white dwarf star in a 51.578 day orbit.<ref name=deutsch/>
Alpha Herculis A 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. Its radius pulsates between 264 and 303 solar radii. At its minimum, the effective temperature is of Template:Convert and the luminosity is of 7,200 solar luminosities, while at its maximum the temperature is of Template:Convert and the luminosity is of 9,330 solar luminosities.<ref name="moravveji"/> If Alpha Herculis were at the center of the Solar System its radius would extend past the orbit of Earth at 1.23Template:Snd1.4 AU but not quite as far as the orbit of Mars or the asteroid belt. The red giant is estimated to have started its life with about Template:Solar mass.<ref name="moravveji"/>
The primary has been specified as a standard star for the spectral class M5 Ib-II.<ref name="moravveji"/> Like most type M stars near the end of their lives, Alpha Herculis is experiencing a high degree of stellar mass loss creating a sparse, gaseous envelope that extends at least 930 AU.<ref name=deutsch/> 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.<ref name=percy/> The full range in brightness is from magnitude 2.7 to 4.0,<ref name=gcvs/> but it usually varies over a much smaller range of around 0.6 magnitudes.<ref name=percy/> Template:Clear left
ReferencesEdit
External linksEdit
- An Atlas of the Universe: Multiple Star Orbits
- Upside down Hercules showing Alpha Herculisethi as the head: Hercules