Sigma2 Ursae Majoris
Binary star in the constellation Ursa Major
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Sigma2 Ursae Majoris (Ï2 Ursae Majoris, Ï2 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).
  Location of Ï2 Ursae Majoris (circled) | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Ursa Major |
| Ï2 UMa A | |
| Right ascension | 09h 10m 23.538s[1] |
| Declination | +67° 08â² 02.44â³[1] |
| Apparent magnitude (V) | +4.813[1] |
| Ï2 UMa B | |
| Right ascension | 09h 10m 23.508s[1] |
| Declination | +67° 08â² 06.58â³[1] |
| Apparent magnitude (V) | +10.26[1] |
| Characteristics | |
| Spectral type | F6IV-V / K2V[2] |
| UâB color index | +0.01[3] |
| BâV color index | +0.48[3] |
| Variable type | Suspected[4] |
| Astrometry | |
| Ï2 UMa A | |
| Radial velocity (Rv) | â2.92 ± 0.12[5] km/s |
| Proper motion (μ) | RA: 7.1[1] mas/yr Dec.: â95.1[1] mas/yr |
| Parallax (Ï) | 49.07±0.37 mas[6] |
| Distance | 66.5 ± 0.5 ly (20.4 ± 0.2 pc) |
| Absolute bolometric magnitude (Mbol) | 3.18[7] |
| Ï2 UMa B | |
| Proper motion (μ) | RA: 4.1[1] mas/yr Dec.: â30.0[1] mas/yr |
| Absolute magnitude (MV) | 7.16[7] |
| Orbit[8] | |
| Period (P) | 970±118 yr |
| Semi-major axis (a) | 5.80±0.14Ⳡ|
| 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 (Ï) (secondary) | 332.4±1.9° |
| Details[9] | |
| Ï2 UMa A | |
| Mass | 1.31+0.04 â0.05 Mâ |
| Radius | 1.70±0.03 Râ |
| Luminosity | 4.31+0.25 â0.23 Lâ |
| Surface gravity (log g) | 4.070±0.070 cgs |
| Temperature | 6381+69 â66 K |
| Metallicity [Fe/H] | â0.03±0.07[7] dex |
| Rotational velocity (v sin i) | 4.1±0.8[7] km/s |
| Ï2 UMa B | |
| Mass | ~0.73[7] Mâ |
| Temperature | ~4600[7] K |
| Other designations | |
| BD+67°577, Gl 335, HD 78154, HIP 45038, HR 3616, SAO 14788[4] | |
| Ï2 UMa A: TYC 4141-1496-1 | |
| Ï2 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 Ï1, Ï2, Ï1, Ï, A and d, it composed the Arabic asterism اÙظÙÙبÙاء aáº-áºibÄʾ 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 ṬhibÄ were the title for seven stars : A as Althiba I, Ï1 as Althiba II, Ï2 as Althiba III, Ï as Althiba IV, Ï1 as Althiba V, this star (Ï2) as Althiba VI, and d as Althiba VII.[12]
- In Chinese, ä¸å¸« (SÄn ShÄ«), meaning Three Top Instructors, refers to an asterism consisting of Ï2 Ursae Majoris and Ï Ursae Majoris. Consequently, Ï2 Ursae Majoris itself is known as ä¸å¸«ä¸ (SÄn ShÄ« sÄn, English: the Third Star of Three Top Instructors.).[13]