Xi Ursae Majoris
| Xi Ursae Majoris A | |
| Stellar characteristics | |
|---|---|
| Constellation | Ursa Major |
| Right ascension (Epoch J2000) | 11h 18m 11.0s |
| Declination (Epoch J2000) | +31° 31′ 45″ |
| Spectral type | G0 Ve |
| Distance from Sol | 27.3 ± 0.2 ly |
| Other designations | Alula Australis, ξ UMa, 53 UMa, Gl 423, HR 4374, BD +32°2132, HD 98230, LHS 2390, LTT 13045, GCTP 2625.00, SAO 62484, LFT 790, ADS 8119, CCDM 11182+3132, Σ 1523, HIP 55203. |
| Xi Ursae Majoris B | |
| Stellar characteristics | |
|---|---|
| Constellation | Ursa Major |
| Right ascension (Epoch J2000) | 11h 18m 11.0s |
| Declination (Epoch J2000) | +31° 31′ 45″ |
| Spectral type | G0 Ve |
| Distance from Sol | 27.3 ± 0.2 ly |
| Other designations | Alula Australis, HR 4375, HD 98231, LHS 2391. |
Xi Ursae Majoris is a star system in the constellation Ursa Major. It also has the proper name Alula Australis meaning "the Southern (star) of Alula." 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 the first visual double star for which an orbit was calculated, when it was computed by Félix Savary in 1828.
The system is composed of a double star whose two components are yellow G-type main sequence dwarfs. The orbital period of the two stars is 59.84 years, and they are currently separated by at least 10 AUs.
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. The masses of both A and B's companions (Ab and Bb) indicate that they are likely MV stars (red dwarfs), Bb being on the cool end of the M spectrum, not much hotter than a brown dwarf.
Xi Ursae Majoris A
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.
Xi Ursae Majoris B
The companion star, Xi Ursae Majoris B, has an apparent magnitude of +4.87.
