Theta Boötis

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Theta Boötis
Observation data
Epoch J2000      Equinox J2000
Constellation Boötes
Right ascension 14h 25m 11.79703s[1]
Declination +51° 51′ 02.6769″[1]
Apparent magnitude (V) 4.05[2]
Spectral type F7 V[2]
U−B color index 0.01
B−V color index +0.50[2]
R−I color index 0.25
Radial velocity (Rv) -10.9 km/s
Proper motion (μ) RA: -235.40[1] mas/yr
Dec.: -399.07[1] mas/yr
Parallax (π) 68.82 ± 0.14[1] mas
Distance 47.39 ± 0.10 ly
(14.53 ± 0.03 pc)
Mass 1.3[2] M
Radius 1.733 ± 0.011[3] R
Luminosity 4.131 ± 0.096[3] L
Surface gravity (log g) 4.12[2] cgs
Temperature 6265 ± 41[3] K
Other designations
Asellus Primus, 23 Boötis, BD +52°1804, FK5 531, Gl 549, HD 126660, HIP 70497, HR 5404, LTT 14245, SAO 29137.
Database references
Data sources:
Hipparcos Catalogue,
CCDM (2002),
Bright Star Catalogue (5th rev. ed.)
Database references

Theta Boötis (θ Boo, θ Boötis) is a star in the constellation Boötes. It has the traditional name Asellus Primus (Latin for "first donkey colt")[4] and the Flamsteed designation 23 Boötis.

This star, along with the other Aselli (ι Boo and κ Boo) and λ Boo, were Al Aulād al Dhiʼbah (ألعولد ألذعب - al aulād al dhiʼb), "the Whelps of the Hyenas".[5]

In Chinese, 天枪 (Tiān Qiāng), meaning Celestial Spear, refers to an asterism consisting of θ Boötis, κ2 Boötis and ι Boötis.[6] Consequently, θ Boötis itself is known as 天枪三 (Tiān Qiāng sān, English: the Third Star of Celestial Spear.)[7]


Theta Boötis has apparent magnitude +4.04 and belongs to the spectral class F7V. It is approximately 47 light years from Earth. From about 4300 BC until 3942 BC, it was the closest star to the celestial north pole visible to the naked eye, although it was still too far away and too dim to be regarded as a pole star.

There is a nearby 11th magnitude optical companion star about 70 arcseconds away. This is a class M2.5 dwarf that is separated by approximately 1,000 AUs. It is uncertain whether they are gravitationally bound, but they do have a common motion through space and so the two stars probably share a common origin.


  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:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 
  2. ^ a b c d e 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 
  3. ^ a b c Boyajian, Tabetha S. et al. (February 2012), "Stellar Diameters and Temperatures. I. Main-sequence A, F, and G Stars", The Astrophysical Journal 746 (1): 101, arXiv:1112.3316, Bibcode:2012ApJ...746..101B, doi:10.1088/0004-637X/746/1/101 . See Table 10.
  4. ^ Star Name - R.H.Allen p.105
  5. ^ Allen, R. H. (1963), Star Names: Their Lore and Meaning (Reprint ed.), New York, NY: Dover Publications Inc, p. 105, ISBN 0-486-21079-0, retrieved 2010-12-12 
  6. ^ (Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  7. ^ (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.

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