Alpha Sagittarii

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

Location of α Sagittarii (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension 19h 23m 53.17483s[1]
Declination −40° 36′ 57.3705″[1]
Apparent magnitude (V) +3.97[2]
Spectral type B8 V[3]
U−B color index −0.33[2]
B−V color index −0.10[2]
Radial velocity (Rv) −0.7[4] km/s
Proper motion (μ) RA: +30.49[1] mas/yr
Dec.: −119.21[1] mas/yr
Parallax (π) 17.94 ± 0.22[1] mas
Distance 182 ± 2 ly
(55.7 ± 0.7 pc)
Radius 2.49 R
Luminosity 60 L
Temperature 12,370 ± 500[5] K
Metallicity [Fe/H] -0.02[6] dex
Other designations
Rukbat, Alrami, α Sagittarii, α Sgr, Alpha Sgr, CPD-40  8955, FK5 728, GC 26737, HD 181869, HIP95347, HR 7348, PPM 353699, SAO 325060.

Alpha Sagittarii (Alpha Sgr, α Sagittarii, α Sgr) is a star in the Sagittarius constellation. It has the traditional names Alrami and Rukbat.

Alpha Sagittarii is a blue, class B dwarf star. It does not appear particularly bright in the sky to the naked eye, with a visual apparent magnitude of +3.97. However, this is due to its distance; in reality, the star is twice as hot as the Sun and considerably more massive, with a luminosity in visible wavelengths about 40 times greater than that of the Sun. Based on an excess emission of infrared radiation, it may have a debris disk, much like Vega.[6] It is unclear why Bayer designated this star as the alpha, rather than ε Sgr or σ Sgr. This led some old star charts to occasionally depict Alpha and Beta Sgr as much brighter than they are in reality, as they are invisible from northern Europe, being too far south to see there.

This is a single-lined spectroscopic binary system. The ROSAT All Sky Survey discovered that Alpha Sagittarii is emitting an excess flux of X-rays, which is not expected to originate from a star of this spectral class. The most likely explanation is that the companion is an active pre-main sequence star or else a star that has just reached the main sequence.[7]

Name and etymology[edit]

  • The names Alrami and Rukbat, derived from the Arabic rukbat al-rāmī = the knee of the archer. It is not to be confused with Delta Cassiopeiae, which also is called Ruchbah or Rukbat, from the Arabic word ركبة rukbah meaning "knee".
  • In Chinese, 天淵 (Tiān Yuān), meaning Celestial Spring, refers to an asterism consisting of α Sagittarii, β1 Sagittarii and β2 Sagittarii. Consequently, α Sagittarii itself is known as 天淵三 (Tiān Yuān sān, English: the Third Star of Celestial Spring.)[8]
  • This star, together with β1 Sgr and β2 Sgr, were Al Ṣuradain (ألسردين), the two Surad, desert birds.[9]

See also[edit]


  1. ^ a b c d e van Leeuwen, Floor (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752v1free to read, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357  Note: see VizieR catalogue I/311.
  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), Bibcode:1966CoLPL...4...99J 
  3. ^ Buscombe, W. (1962), "Spectral classification of Southern fundamental stars", Mount Stromlo Observatory Mimeogram, 4, Bibcode:1962MtSOM...4....1B 
  4. ^ Evans, D. S. (June 20–24, 1966), Batten, Alan Henry; Heard, John Frederick, eds., The Revision of the General Catalogue of Radial Velocities, University of Toronto: International Astronomical Union, Bibcode:1967IAUS...30...57E 
  5. ^ Sokolov, N. A. (May 1995), "The determination of T_eff_ of B, A and F main sequence stars from the continuum between 3200 A and 3600 A", Astronomy and Astrophysics Supplement, 110: 553–564, Bibcode:1995A&AS..110..553S 
  6. ^ a b Saffe, C.; et al. (October 2008), "Spectroscopic metallicities of Vega-like stars", Astronomy and Astrophysics, 490 (1): 297–305, arXiv:0805.3936free to read, Bibcode:2008A&A...490..297S, doi:10.1051/0004-6361:200810260 
  7. ^ Hubrig, S.; et al. (June 2001), "Search for low-mass PMS companions around X-ray selected late B stars", Astronomy and Astrophysics, 372: 152–164, arXiv:astro-ph/0103201free to read, Bibcode:2001A&A...372..152H, doi:10.1051/0004-6361:20010452 
  8. ^ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 2 日
  9. ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 357. ISBN 0-486-21079-0. Retrieved 2012-09-04.