Alpha Herculis

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Alpha Herculis
Hercules Historical View.png
Historical view of the Hercules constellation showing Rasalgethi as the α star and "Head of the Kneeler"
Observation data
Epoch J2000      Equinox J2000
Constellation Hercules
Right ascension 17h 14m 38.80s[1][2][3]
Declination +14° 23′ 25.0″[1][2][3]
Apparent magnitude (V) 2.9137 / 5.4[1][2]
Characteristics
Spectral type M5IIvar / (G5III+F2V)
U−B color index 1.01[4]
B−V color index 1.45[4]
Variable type Semiregular
Astrometry
Proper motion (μ) RA: -6.71[1][2][3] mas/yr
Dec.: 32.78[1][2][3] mas/yr
Parallax (π) 9.07 ± 1.32[1][2] mas
Distance approx. 360 ly
(approx. 110 pc)
Absolute magnitude (MV) -2.30
Details
Mass 2.15 ± 0.22[5] M☉
Radius 387[5] R☉
Luminosity 8,300[5] L☉
Surface gravity (log g) −0.41 ± 0.19[5] cgs
Temperature 2,800[5] K
Other designations
Ras Algethi, Rasalgethi, α Her, 64 Her, HR 6406, BD +14° 3207, HD 156014J, HIP 84345, SAO 102680, CCDM J17146+1424AB, AAVSO 1710+14
Database references
SIMBAD data

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.[6] The traditional name "Head" comes from the fact that in antiquity Hercules was depicted upside down on the constellation maps.

The term ra's al-jaθiyy or Ras al Djathi was 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, 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 constellations).[8] 帝座 (Dìzuò) was westernized into Ti Tso by R.H. Allen, with the same meaning [9]

Properties

The angular diameter of the red giant, α1 Her, has been measured with an interferometer as 34 ± 0.8 mas, or 0.034 arcseconds.[10] At an estimated distance of 110 parsecs from Hipparcos,[1][2][11] 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.[12] 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.[13]

Star system

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.)

See also

Notes

  1. ↑ To determine the Rasalgheti's radius in terms of solar units, the calculations begin with the formula for angular diameter as follows:
    \begin{smallmatrix} {\delta} = \frac{d_R}{D_R}\end{smallmatrix}
    where δ equals Rasalgheti's angular diameter in arcseconds, dR the star's diameter in AU, and DR the Distance from Earth in parsecs. If we know the angular diameter and the Distance, then we can solve for dR as follows:
    \begin{smallmatrix}d_R = \delta \cdot D_R = {0.034} \cdot 110.0 = 3.740 AU \end{smallmatrix}.
    To obtain Rasalgheti's radius:
    \begin{smallmatrix}R_R = {\left ( {\frac {d_R}{2}} \right )} = {\left ( {\frac {3.740}{2}} \right )} = 1.870 AU \end{smallmatrix}.
    Converting into Solar units, 1 AU = 149,597,871 km and the mean radius of the Sun = 696,000 km, hence the calculation:
    \begin{smallmatrix}d_B = {\left ( 1.87 AU \right )} {\left ( {\frac {149,597,871 km}{696,000 km}} \right )} = 280,000,000 km = 402 R_{\odot} \end{smallmatrix} (rounded).

References

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External links

Coordinates: Sky map 17h 14m 38.858s, +14° 23′ 25.20″

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  8. ↑ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 25 日
  9. ↑ Richard Hinckley Allen: Star Names — Their Lore and Meaning: Hercules
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.
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  13. ↑ Lua error in package.lua at line 80: module 'strict' not found.
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