Theta Andromedae

Theta Andromedae is a binary star^[11] system in the northern constellation of Andromeda. Theta Andromedae, Latinized from θ Andromedae, is its Bayer designation. It is located at a distance of approximately 173 light-years (53 parsecs) from the Sun,^[1] and has an apparent visual magnitude of 4.6.^[2] On the Bortle Dark-Sky Scale, this makes it visible to the naked eye from outside urban regions. Based on its motion through space, this system appears to be a member of the Sirius supercluster.^[12]

Theta Andromedae

Location of θ Andromedae (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Andromeda
Right ascension	00h 17m 05.50236s[1]
Declination	+38° 40′ 53.8886″[1]
Apparent magnitude (V)	4.61[2]
Characteristics
Evolutionary stage	main sequence[3]
Spectral type	A2 V[4]
U−B color index	+0.05[2]
B−V color index	+0.06[2]
Variable type	Constant[5]
Astrometry
Radial velocity (Rv)	0.9[6] km/s
Proper motion (μ)	RA: −57.551(307)[1] mas/yr Dec.: −14.795(155)[1] mas/yr
Parallax (π)	18.8747±0.4251 mas[1]
Distance	173 ± 4 ly (53 ± 1 pc)
Absolute magnitude (MV)	−0.25[7]
Orbit[8]
Period (P)	1033+91 −77 days
Semi-major axis (a)	25+32 −13 mas
Eccentricity (e)	0.95+0.04 −0.40
Inclination (i)	69+8 −22°
Longitude of the node (Ω)	263+50 −156°
Periastron epoch (T)	238+343 −143
Argument of periastron (ω) (secondary)	89+175 −29°
Details
A
Mass	2.83±0.08[3] M☉
Luminosity	113[7] L☉
Surface gravity (log g)	3.95[9] cgs
Temperature	8,960[9] K
Metallicity [Fe/H]	+0.14[9] dex
Rotational velocity (v sin i)	102[3] km/s
Other designations
θ And, 24 Andromedae, BD+37°34, HD 1280, HIP 1366, HR 63, SAO 53777, PPM 65154[10]
Database references
SIMBAD	data

The brighter component is a white hued A-type main-sequence star with a stellar classification of A2 V.^[4] It is one of the least photometrically variable stars known.^[5] The star shows a high rate of rotation with a projected rotational velocity of 102 km/s.^[3] It has an estimated 2.8^[3] times the mass of the Sun and is radiating 113^[7] times the Sun's luminosity from its photosphere at an effective temperature of 8,960 K.^[9] The relatively high chemical abundances of iron and heavier elements suggests it may be a fast rotating Am star.^[5]

A stellar companion was detected in 1986 and reported in 1989.^[13] This fainter companion is separated from Theta Andromedae by 0.06 arcseconds.^[11] The secondary appears to be a massive, possibly A-type, star orbiting at a distance of around one astronomical unit with a period of 2.83 years and a large orbital eccentricity (ovalness) of 0.95.^[8]

Naming

In Chinese, 天廄 (Tiān Jiù), meaning Celestial Stable, refers to an asterism consisting of θ Andromedae, ρ Andromedae and σ Andromedae. Consequently, the Chinese name for θ Andromedae itself is known as 天廄一 (Tiān Jiù yī, English: the First Star of Celestial Stable.)^[14]