Theta Boötis

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**Theta Boötis A**
Observation data Epoch J2000
Constellation	Boötes
Right ascension	14^h 25^m 11.8^s
Declination	+51° 51′ 03″
Apparent magnitude (V)	4.04
Spectral Characteristics
Spectral type	F7 V
U-B color index	0.00
B-V color index	0.50
Variable type	Suspected
Astrometry
Radial velocity (R_v)	-10.9 km/s
Proper motion (μ)	RA: -236.06 mas/yr Dec.: -399.07 mas/yr
Parallax (π)	68.63 ± 0.56 mas
Distance	47.5 ± 0.4 ly (14.6 ± 0.1 pc)
Absolute magnitude (M_V)	3.22
Physical Characteristics
Mass	1.25-1.5 M_☉
Radius	? R_☉
Luminosity	? L_☉
Temperature	6300 K
Metallicity	85%
Rotation	34 km/s.
Age	~3 × 10⁹ years
Other designations
Asellus Primus, 23 Boötis, Gl 549, HR 5404, BD +52°1804, HD 126660, LTT 14245, GCTP 3274.00, SAO 29137, FK5 531, HIP 70497, GC 19467, CCDM 14252+5151

Theta Boötis (θ Boo / θ Boötis) is a star in the constellation Boötes. It also has the traditional name Asellus Primus and the Flamsteed designation 23 Boötis.

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 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 likely share a common origin.