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Theta Boötis, Latinized from θ Boötis, is a star in the northern constellation of Boötes the herdsman, forming a corner of the upraised left hand of this asterism.[15] It has the traditional name Asellus Primus (/əˈsɛləs ˈpraɪməs/; Latin for "first donkey colt")[15] and the Flamsteed designation 23 Boötis. Faintly visible to the naked eye, this star has a yellow-white hue with an apparent visual magnitude of 4.05.[2] It is located at a distance of 47.2 light years from the Sun based on parallax,[1] but is drifting closer with a radial velocity of −10.6 km/s.[5]
 θ Boötis in optical light | |
  | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Boötes |
| Right ascension | 14h 25m 11.79695s[1] |
| Declination | +51° 51′ 02.6772″[1] |
| Apparent magnitude (V) | 4.05[2] + 13.23[3] |
| Characteristics | |
| Spectral type | F7 V[2] + M2.5V[3] |
| U−B color index | −0.02[4] |
| B−V color index | +0.50[2] |
| Astrometry | |
| Radial velocity (Rv) | −10.627±0.0065[5] km/s |
| Proper motion (μ) | RA: −235.97 mas/yr[1] Dec.: −399.696 mas/yr[1] |
| Parallax (π) | 69.0686 ± 0.1579 mas[1] |
| Distance | 47.2 ± 0.1 ly (14.48 ± 0.03 pc) |
| Absolute magnitude (MV) | 3.25[6] |
| Details | |
| A | |
| Mass | 1.41[7] 1.24[8] M☉ |
| Radius | 1.4±0.15[9] R☉ |
| Luminosity | 4.131±0.096[10] L☉ |
| Surface gravity (log g) | 4.07[8] cgs |
| Temperature | 6,294±40[8] K |
| Metallicity [Fe/H] | 0.03[8] dex |
| Rotational velocity (v sin i) | 29.2[11] km/s |
| Age | 3.1 Gyr[12] 3.83[8] Gyr |
| B | |
| Mass | 0.4–0.48[13] M☉ |
| Radius | 0.4013±0.012[13] R☉ |
| Other designations | |
| Database references | |
| SIMBAD | data |
| ARICNS | data |
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The stellar classification of Theta Boötis is F7 V,[2] matching an F-type main-sequence star. It is a solar-type star that may be near the end of its main sequence lifetime based on a high luminosity for a star of its type.[12] Theta Boötis is a suspected variable star[16] and a source of X-ray emission.[17] There is evidence for low amplitude radial velocity variation of about 5 km/s.[12] The star has a 24/41% greater mass and a 40% larger radius than the Sun.[7][8][9] It is about 3–4 billion years old and is spinning with a projected rotational velocity of 29 km/s.[11] The star is radiating 4.1[10] times the luminosity of the Sun from its photosphere at an effective temperature of 6,294 K.[8]
There is a nearby 11th magnitude optical companion star about 70 arcseconds away. This is a class M2.5 red dwarf that is separated by a minimum of 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.[18][3]
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θ Boötis, along with the other Aselli (ι Boo and κ Boo) and λ Boo, were Aulād al Dhiʼbah (أولاد الضّباع - awlād al-ḍibā‘), "the Whelps of the Hyenas".[15]
In Chinese, 天枪 (Tiān Qiāng), meaning Celestial Spear, refers to an asterism consisting of θ Boötis, κ2 Boötis and ι Boötis.[19] Consequently, the Chinese name for θ Boötis itself is 天枪三 (Tiān Qiāng sān, English: the Third Star of Celestial Spear.)[20]
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