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 is also close to another bright star
Rasalhague in the vicinity.
System
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), and is the second nearest AGB star after
Mira. 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]
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.[3] The angular diameter of the star has been measured with an
interferometer as
34 ± 0.8
mas, or 0.034 arcseconds.[21] At its estimated distance of 110
parsecs this corresponds to a radius of about 280 million kilometers (or 170 million miles), which is roughly 400
R☉ or 1.87
AU.[a] If Alpha Herculis 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 2.175-3.250 M☉.[3]
Alpha Herculis A 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.[22] 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.[22]
Notes
^To determine Rasalgethi's radius in terms of solar units, the calculations begin with the formula for
angular diameter as follows:
where δ equals Rasalgethi's angular diameter in
arcseconds, dR the star's diameter in
AU, and DR the Distance from Earth in parsecs. If one knows the angular diameter and the Distance, then one can solve for dR as follows:
.
To obtain Rasalgheti's radius:
.
Converting into Solar units, 1
AU = 149,597,871 km and the mean radius of the
Sun = 696,000 km, hence the calculation:
(rounded).
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].