Theta1 Sagittarii

Theta¹ Sagittarii (θ¹ Sagittarii) is a close binary star^[10] system in the zodiac constellation of Sagittarius. It is visible to the naked eye with an apparent visual magnitude of 4.37.^[2] Based upon an annual parallax shift of 6.29 mas as seen from Earth,^[1] this star is located around 520 light years from the Sun. At that distance, the visual magnitude is diminished by an extinction factor of 0.24 due to interstellar dust.^[11]

For other stars with this Bayer designation, see θ Sagittarii.

Theta¹ Sagittarii

Location of θ1 Sagittarii (circled)
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Sagittarius
Right ascension	19h 59m 44.17834s[1]
Declination	−35° 16′ 34.7049″[1]
Apparent magnitude (V)	4.37[2]
Characteristics
Spectral type	B3 IVp[3]
U−B color index	−0.67[2]
B−V color index	−0.15[2]
Astrometry
Proper motion (μ)	RA: +5.60[1] mas/yr Dec.: −25.81[1] mas/yr
Parallax (π)	6.29±0.21 mas[1]
Distance	520 ± 20 ly (159 ± 5 pc)
Absolute magnitude (MV)	−1.63[4]
Orbit[5]
Period (P)	2.1051 d
Eccentricity (e)	0.00
Periastron epoch (T)	2411140.645 JD
Argument of periastron (ω) (secondary)	0.00°
Semi-amplitude (K1) (primary)	15.9 km/s
Details
θ1 Sgr A
Mass	6.6±0.1[6] M☉
Radius	5.6[7] R☉
Luminosity (bolometric)	2,271[8] L☉
Temperature	17,900[8] K
Rotational velocity (v sin i)	73[3] km/s
Age	32.8±5.0[6] Myr
Other designations
θ1 Sgr, CD−35°13831, HD 189103, HIP 98412, HR 7623, SAO 211716, WDS J19597-3517[9]
Database references
SIMBAD	data

This is a single-lined spectroscopic binary with an orbital period of just 2.1 days in a circular orbit.^[5] The visible member, component A, is a B-type subgiant star with a stellar classification of B3 IVp.^[3] It is around 33^[6] million years old and is spinning with a projected rotational velocity of 73 km/s.^[3] The primary has 6.6^[6] times the mass of the Sun and about 5.6^[7] times the Sun's radius. It is radiating 2,271^[8] times the solar luminosity from its photosphere at an effective temperature of 17,900 K.^[8]