Historical view of the Hercules constellation showing Rasalgethi as the α star and "Head of the Kneeler"
Epoch J2000 Equinox J2000
|Right ascension||17h 14m 38.853s|
|Declination||+14° 23′ 25.0″|
|Apparent magnitude (V)||3.350 (2.7–4.0)|
|Right ascension||17h 14m 39.181s|
|Declination||+14° 23′ 23.98″|
|Apparent magnitude (V)||5.322|
|Spectral type||M5 Ib-II|
|U−B color index||+1.01|
|B−V color index||+1.45|
|Spectral type||G8III + A9IV-V|
|Proper motion (μ)||RA: −6.71 mas/yr
Dec.: 32.78 mas/yr
|Parallax (π)||9.07 ± 1.32 mas|
|Distance||approx. 360 ly
(approx. 110 pc)
|Absolute magnitude (MV)||−2.3 + 1.8 + 2.8|
|Radius||284 ± 60, 264–303 R☉|
|Surface gravity (log g)||±0.19−0.41 cgs|
|A: HD 156014, HR 6406, SAO 102680|
|B: HD 156015, HR 6407, SAO 102681|
Alpha Herculis (α Herculis, abbreviated Alpha Her, α Her), also designated 64 Herculis, is a multiple star system in the constellation of Hercules. When viewed through a telescope, this system is resolved into a number of components, the brightest one of which has been named Rasalgethi.
α Herculis (Latinised to Alpha Herculis) is the system's Bayer designation and 64 Herculis its Flamsteed designation. The two components are designated α¹ Herculis (the brightest of the two) and α² Herculis. The latter is itself a binary star and all three stars are sometimes designated α Herculis A, Ba and Bb, respectively.
The system bore the traditional name Rasalgethi or Ras Algethi (Arabic: رأس الجاثي ra‘is al-jāthī 'Head of the Kneeler'). 'Head' comes from the fact that in antiquity Hercules was depicted upside down on maps of the constellation. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Rasalgethi for α¹ Herculis.
In Chinese astronomy, Rasalgethi is called 帝座, Pinyin: Dìzuò, meaning 'Emperor's Seat', this star is marking itself and stands alone in the center of the Emperor's Seat asterism, Heavenly Market enclosure (see : Chinese constellations). 帝座 (Dìzuò) was westernized into Ti Tso by R.H. Allen, with the same meaning 
α¹ and α² Herculis are more than 500 astronomical units apart, with an estimated orbital period of approximately 3600 years. α¹ is a relatively massive red bright giant. α²'s two components are a primary yellow giant star and a secondary, yellow-white dwarf star in a 51.578 day orbit.
α¹ Herculis 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. The angular diameter of the star has been measured with an interferometer as 34 ± 0.8 mas, or 0.034 arcseconds. At an estimated distance of 110 parsecs from the Sun determined by Hipparcos, this corresponds to a radius of about 280 million kilometers (or 170 million miles), which is roughly 400 R☉ or 1.87 AU. If Rasalgethi 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☉. Like most type M stars near the end of their lives, Rasalgethi is experiencing a high degree of stellar mass loss creating a sparse, gaseous envelope that extends at least 90 astronomical units.
- List of largest known stars
- Lists of stars in the constellation Hercules
- Betelgeuse mass loss
- Spectral types F, G & M
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- To determine Rasalgethi's radius in terms of solar units, the calculations begin with the formula for angular diameter as follows:
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- An Atlas of the Universe: Multiple Star Orbits
- Upside down Hercules showing Rasalgethi as the head: Hercules