Sigma2 Ursae Majoris

Sigma² Ursae Majoris (σ² Ursae Majoris, σ² UMa) is a binary star in the constellation of Ursa Major. Parallax measurements made by the Hipparcos spacecraft put it at a distance of about 66.5 light years (20.4 parsecs) from Earth, making this a fairly nearby system. The primary component has an apparent magnitude of about 4.8,^[1] meaning it can be seen with the naked eye (see Bortle scale).

For other stars with this Bayer designation, see σ Ursae Majoris.

Sigma² Ursae Majoris

Location of σ2 Ursae Majoris (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Ursa Major
σ2 UMa A
Right ascension	09h 10m 23.538s[1]
Declination	+67° 08′ 02.44″[1]
Apparent magnitude (V)	+4.813[1]
σ2 UMa B
Right ascension	09h 10m 23.508s[1]
Declination	+67° 08′ 06.58″[1]
Apparent magnitude (V)	+10.26[1]
Characteristics
Spectral type	F6IV-V / K2V[2]
U−B color index	+0.01[3]
B−V color index	+0.48[3]
Variable type	Suspected[4]
Astrometry
σ2 UMa A
Radial velocity (Rv)	−2.92 ± 0.12[5] km/s
Proper motion (μ)	RA: 7.1[1] mas/yr Dec.: −95.1[1] mas/yr
Parallax (π)	49.07±0.37 mas[6]
Distance	66.5 ± 0.5 ly (20.4 ± 0.2 pc)
Absolute bolometric magnitude (Mbol)	3.18[7]
σ2 UMa B
Proper motion (μ)	RA: 4.1[1] mas/yr Dec.: −30.0[1] mas/yr
Absolute magnitude (MV)	7.16[7]
Orbit[8]
Period (P)	970±118 yr
Semi-major axis (a)	5.80±0.14″
Eccentricity (e)	0.801±0.017
Inclination (i)	145.4±1.5°
Longitude of the node (Ω)	102.1±1.9°
Periastron epoch (T)	B 1917.39±0.12
Argument of periastron (ω) (secondary)	332.4±1.9°
Details[9]
σ2 UMa A
Mass	1.31+0.04 −0.05 M☉
Radius	1.70±0.03 R☉
Luminosity	4.31+0.25 −0.23 L☉
Surface gravity (log g)	4.070±0.070 cgs
Temperature	6381+69 −66 K
Metallicity [Fe/H]	−0.03±0.07[7] dex
Rotational velocity (v sin i)	4.1±0.8[7] km/s
σ2 UMa B
Mass	~0.73[7] M☉
Temperature	~4600[7] K
Other designations
BD+67°577, Gl 335, HD 78154, HIP 45038, HR 3616, SAO 14788[4]
σ2 UMa A: TYC 4141-1496-1
σ2 UMa B: TYC 4141-1496-2
Database references
SIMBAD	AB
	A
	B

This is a visual binary, meaning that the two components can be resolved, and the orbit is derived from the positions of the two stars. The primary component Sigma² Ursae Majoris A, is a white-colored F-type subgiant. Its radius is about 1.70 times that of the Sun, and it is 31% more massive.^[9] The companion is an orange K-type main-sequence star that is much fainter. The two stars are separated about 4 arcseconds away, and because of their slow orbital motion the orbit is poorly known: estimates of the orbital period range from 970 years^[8] to over 1,500 years.^[10] There is a third component, designated Sigma² Ursae Majoris C. Located 205 arcseconds from the primary, it is thought to be a line-of-sight coincidence, and is not related to the system.

Naming

• With π¹, π², σ¹, ρ, A and d, it composed the Arabic asterism الظِّبَاء aẓ-Ẓibāʾ meaning the Gazelles.^[11] According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Al Ṭhibā were the title for seven stars : A as Althiba I, π¹ as Althiba II, π² as Althiba III, ρ as Althiba IV, σ¹ as Althiba V, this star (σ²) as Althiba VI, and d as Althiba VII.^[12]
• In Chinese, 三師 (Sān Shī), meaning Three Top Instructors, refers to an asterism consisting of σ² Ursae Majoris and ρ Ursae Majoris. Consequently, σ² Ursae Majoris itself is known as 三師三 (Sān Shī sān, English: the Third Star of Three Top Instructors.).^[13]