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. It has the traditional name Asellus Primus (; Latin for "first donkey colt") 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. It is located at a distance of 47.2 light years from the Sun based on parallax, but is drifting closer with a radial velocity of −10.6 km/s.
The stellar classification of Theta Boötis is F7 V, 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. Theta Boötis is a suspected variable star and a source of X-ray emission. There is evidence for low amplitude radial velocity variation of about 5 km/s. The star has a 24/41% greater mass and a 40% larger radius than the Sun. It is about 3–4 billion years old and is spinning with a projected rotational velocity of 29 km/s. The star is radiating 4.1 times the luminosity of the Sun from its photosphere at an effective temperature of 6,294 K.
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.
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".
In Chinese, Chinese: 天枪 (Chinese: Tiān Qiāng), meaning Celestial Spear, refers to an asterism consisting of θ Boötis, κ2 Boötis and ι Boötis.[1] Consequently, the Chinese name for θ Boötis itself is Chinese: 天枪三 (Chinese: Tiān Qiāng sān, English: the Third Star of Celestial Spear.)