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 38.80s|
|Declination||+14° 23′ 25.0″|
|Apparent magnitude (V)||2.9137 / 5.4|
|Spectral type||M5IIvar / (G5III+F2V)|
|U−B color index||1.01|
|B−V color index||1.45|
|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)||-2.30|
|Mass||2.15 ± 0.22 M☉|
|Surface gravity (log g)||−0.41 ± 0.19 cgs|
Alpha Herculis (α Her, α Herculis) is a multiple star system in the constellation Hercules. It has the traditional name Rasalgethi or Ras Algethi (Arabic: رأس الجاثي ra's al-jaθiyy Head of the Kneeler), and the Flamsteed designation 64 Herculis. The traditional name "Head" comes from the fact that in antiquity Hercules was depicted upside down on the constellation maps.
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 Emperor's Seat asterism, Heavenly Market enclosure (see : Chinese constellation). 帝座 (Dìzuò) was westernized into Ti Tso by R.H. Allen, with the same meaning 
The angular diameter of the red giant, α1 Her, has been measured with an interferometer as 34 ± 0.8 mas, or 0.034 arcseconds. At an estimated distance of 110 parsecs from Hipparcos, this corresponds to a radius of about 280 million kilometers (or 170 million miles), which is roughly 400 R☉ or 1.87 AU.[note 1] If Rasalgheti were at the center of our 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 7-8 solar masses. 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.
When viewed through a telescope, this system is resolved into two components designated α1 and α2. These two components 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 is actually a double star system with a primary yellow giant star and a secondary, yellow-white dwarf star. (These components are sometimes designated α Herculis A, Ba and Bb, respectively.)
- List of largest known stars
- Lists of stars in the constellation Hercules
- Betelgeuse mass loss
- Spectral types F, G & M
- To determine the Rasalgheti's radius in terms of solar units, the calculations begin with the formula for angular diameter as follows:
- Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J. et al. (July 1997), "The Hipparcos Catalogue", Astronomy and Astrophysics 323: L49–L52, Bibcode:1997A&A...323L..49P
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- Kurt Vonnegut. "Constellations: Hercules 'the Strongman'". The BBC (British Broadcasting Corporation). Retrieved 2010-11-14.
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- (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 25 日
- Richard Hinckley Allen: Star Names — Their Lore and Meaning: Hercules
- Benson, J. A.; Dyck, H. M.; Mason, W. L.; Howell, R. R.; Ridgway, S. T. et al. (December 1991). "The infrared angular diameter of Alpha Herculis measured with a Michelson interferometer". Astronomical Journal 102: 2091–2097. Bibcode:1991AJ....102.2091B. doi:10.1086/116033.
- Perryman, Michael (2010), The Making of History's Greatest Star Map, Heidelberg: Springer-Verlag, doi:10.1007/978-3-642-11602-5
- Kaler, James B. "RASALGETHI (Alpha Herculis)". University of Illinois. Retrieved 2007-03-07.
- Deutsch, Armin J. (March 1956). "The Circumstellar Envelope of Alpha Herculis". Astrophysical Journal 123: 210–227. Bibcode:1956ApJ...123..210D. doi:10.1086/146152.
- An Atlas of the Universe: Multiple Star Orbits
- Upside down Hercules showing Rasalgethi as the head: Hercules