Xi Ursae Majoris

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**Xi Ursae Majoris**
Observation data Epoch J2000
Constellation (pronunciation)	Ursa Major
Right ascension	11^h 18^m 11.0^s
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 (R_v)	-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 (M_V)	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 yr
Semimajor 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
Ksi UMa, Alula Australis, 53 Ursae Majoris, Gl 423 A/B, HR 4374/4375, BD +32°2132, HD 98230/98231, LHS 2390/2391, LTT 13045, GCTP 2625.00, SAO 62484, LFT 790, ADS 8119, CCDM 11182+3132, Σ 1523, HIP 55203.

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. 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.