Xi Ursae Majoris
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Ursa Major |
| Right ascension | 11h 18m 11.0s |
| Declination | +31° 31′ 45″ |
| Apparent magnitude (V) | 3.79 (4.32/4.84) |
| Characteristics | |
| Spectral type | G0 Ve/G0 Ve |
| U−B color index | 0.04 |
| B−V color index | 0.59 |
| Variable type | ? |
| Astrometry | |
| Radial velocity (Rv) | -15.0 km/s |
| Proper motion (μ) | RA: -429 mas/yr Dec.: -587 mas/yr |
| Parallax (π) | 119.51 ± 0.79 mas |
| Distance | 27.3 ± 0.2 ly (8.37 ± 0.06 pc) |
| Absolute magnitude (MV) | 4.71/5.23 |
| Details | |
| Mass | 1.05/0.90 M☉ |
| Radius | 1.01/0.78 R☉ |
| Luminosity | 1.1/0.67 L☉ |
| Temperature | ~5,900/5,900 K |
| Metallicity | 0.98/0.76 |
| Rotation | 3 km/s |
| Age | 6 × 109 years |
| Orbit | |
| Companion | Xi Ursae Majoris Bb |
| Period (P) | 59.84 yr |
| Semi-major axis (a) | 2.53″ |
| Eccentricity (e) | 0.414 |
| Inclination (i) | 122.65° |
| Longitude of the node (Ω) | 101.59 (ascending)° |
| Periastron epoch (T) | 1935.17 |
| Other designations | |
Xi Ursae Majoris (ξ UMa / ξ Ursae Majoris) is a star system in the constellation Ursa Major. It also has the traditional name Alula Australis (The word Alula comes from an Arabic phrase meaning ‘first leap’; the distinctions ‘northern’ (Borealis) and ‘southern’ (Australis) are added in Latin).
On May 2, 1780, Sir William Herschel discovered that this was a binary star system, making it the first such system ever discovered. It was also the first binary system to have its orbit calculated, in 1828.
The system is composed of a double star whose two components are yellow G-type main sequence dwarfs. The brighter component, Xi Ursae Majoris A, has a mean apparent magnitude of +4.41. It is classified as an RS Canum Venaticorum type variable star and its brightness varies by 0.01 magnitudes. The companion star, Xi Ursae Majoris B has an apparent magnitude of +4.87. The orbital period of the two stars is 59.84 years, and they are currently separated by 1.2 arcseconds, or at least 10 Astronomical Units. Xi Ursae Majoris was the first visual double star for which an orbit was calculated, when it was computed by Félix Savary in 1828.
Each component of this double star is itself a spectroscopic binary. B's binary companion, denoted Xi Ursae Majoris Bb, is unresolved, but the binary star is known to have an orbital period of 3.98 days.
External links
- Xi Ursae Majoris system at SolStation.com
- Alula Australis by Dr. Jim Kaler.
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