Alpha Herculis

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.

α Herculis

Location of α Herculis (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Hercules
A
Right ascension	17h 14m 38.853s[1]
Declination	+14° 23′ 25.34″[1]
Apparent magnitude (V)	3.350[1] (2.7–4.0[2])
B
Right ascension	17h 14m 39.181s[1]
Declination	+14° 23′ 23.98″[1]
Apparent magnitude (V)	5.322[1]
Characteristics
A
Evolutionary stage	AGB[3]
Spectral type	M5 Ib-II[3]
U−B color index	+1.01[4]
B−V color index	+1.45[4]
Variable type	SRc[2]
B
Spectral type	G8III + A9IV-V[3]
Astrometry
Proper motion (μ)	RA: −7.32[5] mas/yr Dec.: 36.07[5] mas/yr
Parallax (π)	9.07±1.32 mas[5]
Distance	approx. 360 ly (approx. 110 pc)
Absolute magnitude (MV)	−2.3[6] + 1.8 + 2.8[7]
Details
A
Mass	2.5+1.6 −1.1[8] M☉
Radius	284 ± 60, 264–303[3] R☉
Luminosity	7,244–9,333[3] L☉
Surface gravity (log g)	−0.41±0.19[9] cgs
Temperature	3,155–3,365[3] K
Ba
Mass	~2.5[3] M☉
Luminosity	126[3] L☉
Temperature	4,900[3] K
Bb
Mass	~2[3] M☉
Luminosity	26[3] L☉
Temperature	7,350[3] K
Age	0.41–1.25[3] Gyr
Other designations
Ras Algethi, Rasalgethi, α Her, 64 Her, BD+14°3207, HIP 84345, CCDM J17146+1424, AAVSO 1710+14, WDS J17146+1423[10]
A: HD 156014, HR 6406, SAO 102680
B: HD 156015, HR 6407, SAO 102681
Database references
SIMBAD	α Her
	A
	B

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]

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).^[17] 帝座 (Dìzuò) was westernized into Ti Tso by R.H. Allen, with the same meaning ^[18]

Properties

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]