Alpha Ophiuchi

Coordinates: Sky map 17h 34m 56.06945s, +12° 33′ 36.1346″
Source: Wikipedia, the free encyclopedia.
α Ophiuchi
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]
Characteristics
Spectral type A5IVnn[3] / K5–7 V[4]
U−B color index +0.10[2]
B−V color index +0.15[2]
Distance
48.6 ± 0.8 ly
(14.9 ± 0.2 pc)
Absolute magnitude (MV)+1.248[6]
Semi-amplitude
(K1)
(primary)		12.7±0.2 km/s
Semi-amplitude (K2)
(secondary)		33.74±0.35 km/s
Details
α Oph A
Gyr
α Oph B
Mass0.824±0.023[7] M☉
HR 6556, SAO 102932[11]
Database references
SIMBADdata

Alpha Ophiuchi (α Ophiuchi, abbreviated Alpha Oph, α Oph), also named Rasalhague /ˈræsəlheɪɡ/,[12][13] is a binary star and the brightest star in the constellation of Ophiuchus.

Nomenclature

The name Alpha Ophiuchi is a

Romanisation of the star's Bayer designation, α Ophiuchi. It is also known by the traditional name Rasalhague, from the Arabic رأس الحواء raʼs al-ḥawwāʼ "the head of the serpent collector".[13] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[14] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[15] included a table of the first two collections of approved names, formally listing Alpha Ophiuchi as Rasalhague.[16]

Properties

Alpha Ophiuchi is a

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 A5IVnn indicates that the primary is a bluish-white subgiant star that has evolved away from the main sequence after consuming the hydrogen at its core.[3] 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.[17][18]

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.[19]

Rotation

oblate spheroid
bulging at the equator and hotter at the poles.

Alpha Ophiuchi A is a rapidly rotating star with a

oblate spheroid shape with an equatorial bulge about 20% larger than the polar radius.[10] The polar radius is calculated to be 2.388 R☉ and the equatorial radius 2.858 R☉.[8]

Because of the oblateness and rapid spin, the surface gravity at the pole is higher than at the equator. An effect known as gravity darkening means that the temperature at the poles is also higher than at the equator. The polar temperature is calculated to be 9,384 K and the equatorial temperature 7,569 K.[8]

The energy radiated by an oblate star is higher along its axis of rotation because of the larger projected area and the

bolometric luminosity seen at this angle is 25.6 L☉, but the true luminosity of the star is thought to be 31.3 L☉. Since a star's effective temperature is simply the temperature which would produce its total energy output from a black body, the true effective temperature of 8,336 K is different from the apparent effective temperature of 8,047 K.[8]

See also

References

  1. ^
    S2CID 18759600
  2. ^
    Bibcode:1966CoLPL...4...99J
  3. ^ a b "alf Oph". SIMBAD. Retrieved December 23, 2023.
  4. ^
    S2CID 50830196
  5. Bibcode:1967IAUS...30...57E
  6. ISSN 0004-6256
    .
  7. ^
    ISSN 0004-637X
    .
  8. ^
    S2CID 51105576
    .
  9. Bibcode:1990A&AS...85.1015M
  10. ^
    S2CID 554145
  11. ^ "RASALHAGUE -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2011-12-25
  12. ^ also /-ˈheɪɡjuː/ or /-ˈheɪɡwiː/.
    "Rasalhague". Merriam-Webster.com Dictionary. Merriam-Webster.
  13. ^
    ISBN 978-1-931559-44-7
    .
  14. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  15. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  16. ^ "Stars Pronunciation Guide". Space.com. Retrieved 2017-06-25.
  17. ^
    S2CID 118433889
  18. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on March 18, 2012, retrieved 2012-01-16
  19. S2CID 42241365
  20. ^
    Bibcode:2010RMxAC..38..117Z


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