Alpha Ophiuchi

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Alpha Ophiuchi
Diagram showing star positions and boundaries of the Ophiuchus constellation and its surroundings
Cercle rouge 100%.svg

Location of α Ophiuchi (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Ophiuchus
Right ascension 17h 34m 56.06945s[1]
Declination +12° 33′ 36.1346″[1]
Apparent magnitude (V) 2.07[2]
Spectral type A5 III[3] / K5–7 V[4]
U−B color index +0.10[2]
B−V color index +0.15[2]
Radial velocity (Rv) +12.6[5] km/s
Proper motion (μ) RA: 108.07[1] mas/yr
Dec.: -221.57[1] mas/yr
Parallax (π) 67.13 ± 1.06[1] mas
Distance 48.6 ± 0.8 ly
(14.9 ± 0.2 pc)
Absolute magnitude (MV) +1.248[6]
Period (P) 3148.4 days
Semi-major axis (a) 427+20
Eccentricity (e) 0.92±0.03
Inclination (i) 125+6
Longitude of the node (Ω) 232±9°
Periastron epoch (T) 2452888±53 JD
Argument of periastron (ω)
α Oph A
Mass 2.40+0.23
[4] M
Radius 2.6[7] R
Luminosity 25.1–25.6[8] L
Surface gravity (log g) 3.91[7] cgs
Temperature 78808050[8] K
Rotational velocity (v sin i) 240[8] km/s
Age 0.77±0.03[9] Gyr
α Oph B
Mass 0.85+0.06
[4] M
Other designations
Ras Alhague, Rasalhague, 55 Ophiuchi, BD +12° 3252, FK5 656, HD 159561, HIP 86032, HR 6556, SAO 102932.[10]

Alpha Ophiuchi (α Oph, α Ophiuchi) is the brightest star in the constellation Ophiuchus. It has the traditional name Ras Alhague, often condensed to Rasalhague.

Alpha Ophiuchi is a binary star system with an orbital period of about 8.62 years. The orbital parameters were only poorly known until 2011 when observations using adaptive optics produced a better orbital fit, allowing the individual masses of the two components to be determined. The primary component, Alpha Ophiuchi A, has a mass of about 2.4 times the mass of the Sun, while the secondary, Alpha Ophiuchi B, has 0.85 solar masses.[4] Estimates of the mass of the primary by other means range from a low of 1.92 to 2.10 solar masses, up to 2.84 or even 4.8 solar masses.[9] The mass of the secondary suggests that it has a stellar classification in the range K5V to K7V, which indicates it is a main sequence star that is still generating energy by the thermonuclear fusion of hydrogen at its core. The pair reached periastron passage, or closest approach, around April 19, 2012, when they had an angular separation of 50 milliarcseconds.[4]

This star system has a combined apparent magnitude of +2.08 and is located at a distance of about 48.6 light-years (14.9 parsecs) from the Earth. The stellar classification of A5 III indicates that the primary is a giant star that has evolved away from the main sequence after consuming the hydrogen at its core. It is radiating about 25 times the luminosity of the Sun and has an effective temperature of about 8,000 K, giving it the characteristic white hue of an A-type star.[8][11]

Alpha Ophiuchi A is a rapidly rotating star with a projected rotational velocity of 240 km s−1.[8] It is spinning at a rate of about 88.5% of the velocity that would cause the star to break up.[12] The resulting equatorial bulge is about 20% larger than the polar radius, giving the star the shape of an oblate spheroid. Because of this distorted shape, the poles have an effective temperature about 1,840 K greater than along the equator.[4] The axis of rotation of the star is inclined about 87.°7 ± 0°4 to the line of sight from the Earth, so that it is being observed from nearly equator-on.[12]

The spectrum of Alpha Ophiuchi shows an anomalously high level of absorption of the lines for singly-ionized calcium (Ca II). However, this is likely the result of interstellar matter between the Earth and the star, rather than a property of the star or circumstellar dust.[13]

Name and etymology[edit]

  • The name Ras Alhague is from the Arabic رأس الحواء (raʾis al-ḥawwāʾ), meaning "the Head of the Serpent collector".[14]
  • The Chinese name 候 (Mandarin: Hòu) meaning Astrologer, because this star is marking itself and stand alone in Astrologer asterism, Heavenly Market enclosure (see : Chinese constellation).[15] 候 (Hòu) westernized into How in R.H. Allen's work, meaning "the Duke".[14]


  1. ^ a b c d e van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752free to read, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 
  2. ^ a b c Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99): 99, Bibcode:1966CoLPL...4...99J 
  3. ^ Cowley, A.; et al. (April 1969), "A study of the bright A stars. I. A catalogue of spectral classifications", Astronomical Journal, 74: 375–406, Bibcode:1969AJ.....74..375C, doi:10.1086/110819 
  4. ^ a b c d e f g Hinkley, Sasha; et al. (January 2011), "Establishing α Oph as a Prototype Rotator: Improved Astrometric Orbit", The Astrophysical Journal, 726 (2): 104, arXiv:1010.4028free to read, Bibcode:2011ApJ...726..104H, doi:10.1088/0004-637X/726/2/104 
  5. ^ Evans, D. S. (June 20–24, 1966), Batten, Alan Henry; Heard, John Frederick, eds., "The Revision of the General Catalogue of Radial Velocities", Determination of Radial Velocities and their Applications, University of Toronto: International Astronomical Union, 30: 57, Bibcode:1967IAUS...30...57E 
  6. ^ Gatewood, George (2005). "An Astrometric Study of the Binary Star α Ophiuchi". The Astronomical Journal. 130 (2): 809–814. Bibcode:2005AJ....130..809G. doi:10.1086/431723. ISSN 0004-6256. 
  7. ^ a b Malagnini, M. L.; Morossi, C. (November 1990), "Accurate absolute luminosities, effective temperatures, radii, masses and surface gravities for a selected sample of field stars", Astronomy and Astrophysics Supplement Series, 85 (3): 1015–1019, Bibcode:1990A&AS...85.1015M 
  8. ^ a b c d e Deupree, Robert G. (November 2011), "Theoretical p-Mode Oscillation Frequencies for the Rapidly Rotating δ Scuti Star α Ophiuchi", The Astrophysical Journal, 742 (1): 9, arXiv:1110.1345free to read, Bibcode:2011ApJ...742....9D, doi:10.1088/0004-637X/742/1/9 
  9. ^ a b Zhao, M.; et al. (August 2009), "Imaging and Modeling Rapidly Rotating Stars: α Cephei and α Ophiuchi", The Astrophysical Journal, 701 (1): 209–224, arXiv:0906.2241free to read, Bibcode:2009ApJ...701..209Z, doi:10.1088/0004-637X/701/1/209 
  10. ^ "RASALHAGUE -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2011-12-25 
  11. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, retrieved 2012-01-16 
  12. ^ a b Zhao, M.; et al. (February 2010), Rivinius, Th.; Curé, M., eds., "Imaging and Modeling Rapid Rotators: α Cep and α Oph", The Interferometric View on Hot Stars, Revista Mexicana de Astronomía y Astrofísica (Serie de Conferencias), 38: 117–118, Bibcode:2010RMxAC..38..117Z 
  13. ^ Redfield, Seth; Kessler-Silacci, Jacqueline E.; Cieza, Lucas A. (June 2007), "Spitzer Limits on Dust Emission and Optical Gas Absorption Variability around Nearby Stars with Edge-on Circumstellar Disk Signatures", The Astrophysical Journal, 661 (2): 944–971, arXiv:astro-ph/0703089free to read, Bibcode:2007ApJ...661..944R, doi:10.1086/517516 
  14. ^ a b Allen, Richard Hinckley (1899), "Star-names and their meanings", New York, G. E. Stechert: 300,, retrieved 2011-12-25 
  15. ^ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 25 日

External links[edit]