Xi Ursae Majoris
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| Observation data 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 |
| Visual binary orbit | |
| Companion | Xi Ursae Majoris Bb |
| Period (P) | 59.84 a |
| Semimajor axis (a) | 2.53" |
| Eccentricity (e) | 0.414 |
| Inclination (i) | 122.65° |
| 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.
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 and is separated from the primary by 0.05 arcseconds. The orbital period of the two stars is 1.83 years and they are 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. The A binary star has the same orbital period, 1.83 years, as the AB system, due to tidal interactions. 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.
[edit] External links
- Xi Ursae Majoris system at SolStation.com
- Alula Australis by Dr. Jim Kaler.
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