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, meaning it can be seen with the naked eye (see Bortle scale).
Location of σ2 Ursae Majoris (circled) | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Ursa Major |
| σ2 UMa A | |
| Right ascension | 09h 10m 23.538s |
| Declination | +67° 08′ 02.44″ |
| Apparent magnitude (V) | +4.813 |
| σ2 UMa B | |
| Right ascension | 09h 10m 23.508s |
| Declination | +67° 08′ 06.58″ |
| Apparent magnitude (V) | +10.26 |
| Characteristics | |
| Spectral type | F6IV-V / K2V |
| U−B color index | +0.01 |
| B−V color index | +0.48 |
| Variable type | Suspected |
| Astrometry | |
| σ2 UMa A | |
| Radial velocity (Rv) | −2.92 ± 0.12 km/s |
| Proper motion (μ) | RA: 7.1 mas/yr Dec.: −95.1 mas/yr |
| Parallax (π) | 49.07 ± 0.37 mas |
| Distance | 66.5 ± 0.5 ly (20.4 ± 0.2 pc) |
| Absolute bolometric magnitude (Mbol) | 3.18 |
| σ2 UMa B | |
| Proper motion (μ) | RA: 4.1 mas/yr Dec.: −30.0 mas/yr |
| Absolute magnitude (MV) | 7.16 |
| Orbit | |
| 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 | |
| σ2 UMa A | |
| Mass | 1.31 M☉ |
| Radius | 1.75 ± 0.07 R☉ |
| Surface gravity (log g) | 4.00 ± 0.10 cgs |
| Temperature | 6276 ± 80 K |
| Metallicity [Fe/H] | −0.03 ± 0.07 dex |
| Rotational velocity (v sin i) | 4.1 ± 0.8 km/s |
| σ2 UMa B | |
| Mass | ~0.73 M☉ |
| Temperature | ~4600 K |
| Other designations | |
| 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 1.75 times that of the Sun, and it is 31% more massive. 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 to over 1,500 years. 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.