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.
System
A view of Alpha Herculis in a small telescope. The components A and B are resolved with angular separation of 4.64'' (in 2020).
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.[3]
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.[11] The two fainter stars are far more distant than the triple system.[12]
Nomenclature
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).[13]
Alpha Herculis bore the traditional name Rasalgethi or Ras Algethi (Arabic: رأس الجاثي ra‘is al-jāthī 'Head of the Kneeler').[14] '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)[15] 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.[15]
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.[16]
A light curve for Alpha Herculis A, plotted from data published by Wasatonic (1997)[19]
Alpha Herculis A and B are more than 500 AU apart, with an estimated orbital period of approximately 3600 years.[citation needed] 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.[11] B's two components are a primary yellow giant star and a secondary, yellow-white dwarf star in a 51.578 day orbit.[20]
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 3,155 K (2,882 °C)
and the luminosity is of 7,200 solar luminosities, while at its maximum the temperature is of 3,365 K (3,092 °C) and the luminosity is of 9,330 solar luminosities.[3] If Alpha Herculis were at the center of the Solar System its radius would extend past the orbit of Earth at 1.23 – 1.4 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 2.175-3.250 M☉.[3]
The primary has been specified as a standard star for the spectral class M5 Ib-II.[3] 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.[20] 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.[21] The full range in brightness is from magnitude 2.7 to 4.0,[2] but it usually varies over a much smaller range of around 0.6 magnitudes.[21]
References
^ abcdefHøg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27. Bibcode:2000A&A...355L..27H. doi:10.1888/0333750888/2862. ISBN0333750888.
^ abcSamus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
^ abDucati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237. Bibcode:2002yCat.2237....0D.
^Reimers, D. (1977). "On the absolute scale of mass-loss in red giants. I - Circumstellar absorption lines in the spectrum of the visual companion of Alpha-1 HER". Astronomy and Astrophysics. 61: 217. Bibcode:1977A&A....61..217R.
^Moravveji, E.; Guinan, E. F.; Sobouti, Y. (2011). "On the Mass and Evolutionary Status of the Bright Red AGB Supergiant α1 Herculis". Why Galaxies Care About Agb Stars Ii: Shining Examples and Common Inhabitants. 445: 163. Bibcode:2011ASPC..445..163M.
^Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
