Alpha Herculis

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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. Five degrees east-south-east of Rasalgethi lies Rasalhague, the brightest star in Ophiuchus. These stars may be among the closest-lying pair of alpha stars in the entire sky. Despite the star designated as "alpha", it is actually the 5th brightest star in Hercules.

System

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File:Double star Alpha Hercules.jpg
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.[1]

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.[2] The two fainter stars are far more distant than the triple system.[3]

Nomenclature

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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).[4]

Alpha Herculis bore the traditional name Rasalgethi or Ras Algethi (Template:Langx 'Head of the Kneeler').[5] '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)[6] 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.[6]

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.[7]

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).[8] 帝座 (Dìzuò) was westernized into Ti Tso by R.H. Allen, with the same meaning [9]

Properties

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File:AlphaHerLightCurve.png
A light curve for Alpha Herculis A, plotted from data published by Wasatonic (1997)[10]

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.[2] B's two components are a primary yellow giant star and a secondary, yellow-white dwarf star in a 51.578 day orbit.[11]

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.[1] 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 Template:Solar mass.[1]

The primary has been specified as a standard star for the spectral class M5 Ib-II.[1] 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.[11] 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.[12] The full range in brightness is from magnitude 2.7 to 4.0,[13] but it usually varies over a much smaller range of around 0.6 magnitudes.[12]

References

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  1. ↑ 1.0 1.1 1.2 1.3 Moravveji, Ehsan; Guinan, Edward F.; Khosroshahi, Habib; Wasatonic, Rick (2013). "The Age and Mass of the α Herculis Triple-star System from a MESA Grid of Rotating Stars with 1.3 <= M/M ⊙ <= 8.0". The Astronomical Journal. 146 (6): 148. arXiv:1308.1632. Bibcode:2013AJ....146..148M. doi:10.1088/0004-6256/146/6/148. S2CID 117872505.
  2. ↑ 2.0 2.1 "Washington Double Star Catalog". United States Naval Observatory. Archived from the original on 2011-02-14. Retrieved 2018-07-10.
  3. ↑ Template:Cite DR2
  4. ↑ 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].
  5. ↑ Kurt Vonnegut. "Constellations: Hercules 'the Strongman'". The BBC (British Broadcasting Corporation). Retrieved 2010-11-14.
  6. ↑ 6.0 6.1 "IAU Working Group on Star Names (WGSN)". Archived from the original on 10 June 2016. Retrieved 22 May 2016.
  7. ↑ Knobel, E. B. (June 1895). "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket". Monthly Notices of the Royal Astronomical Society. 55 (8): 429. Bibcode:1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429.
  8. ↑ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 25 日 Archived 2021-04-21 at the Wayback Machine
  9. ↑ Richard Hinckley Allen: Star Names — Their Lore and Meaning: Hercules
  10. ↑ Wasatonic, Richard P. (January 1997). "Photoelectric Photometry of TX Psc, Alpha Her A, Omicron Cet, and RT Cyg". The Journal of the American Association of Variable Star Observers. 26 (1): 1–13. Bibcode:1997JAVSO..26....1W. Retrieved 18 December 2022.
  11. ↑ 11.0 11.1 Deutsch, Armin J. (March 1956). "The Circumstellar Envelope of Alpha Herculis". Astrophysical Journal. 123: 210–227. Bibcode:1956ApJ...123..210D. doi:10.1086/146152.
  12. ↑ 12.0 12.1 Percy, John R; Wilson, Joseph B; Henry, Gregory W (2001). "Long-TermVRIPhotometry of Small-Amplitude Red Variables. I. Light Curves and Periods". Publications of the Astronomical Society of the Pacific. 113 (786): 983. Bibcode:2001PASP..113..983P. doi:10.1086/322153.
  13. ↑ Samus, 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.

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