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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 ξ UMa A
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
Alula Australis, ξ UMa, 53 UMa, Gl 423, 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 proper name Alula Australis (former Alula australis[1], and erroneously Alula Australe[2]) meaning "the Southern (star) of Alula." The words Alula, El Acola[3][4], and el-awla[5] (with ν) come from the Arabic phrase (al-Qafzah) al-Ūlā meaning "the first (leap)" (the distinctions "southern" (australis) is added in Latin). With Nu Ursae Majoris, they were Hea Tae (下台), the Lower Dignitary, in Chinese astronomy.[6]

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 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.

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) (deduced by the sum total mass of the system minus the likely masses of Aa and Ba determined by their class) 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. [7]

Orbit of Xi Ursae Majoris.

Notes

  1. ^ Piazzi, G., The Palermo Catalogue, Palermo, 1814.
  2. ^ Bečvář, A., Atlas Coeli (Atlas of the Heavens) II - Catalogue, Plague, 1964.
  3. ^ Burritt, E. H., Atlas, Designed to Illustrate the Geography of the Heavens, New York, 1835.
  4. ^ Funck & Wagenall, New Standard Dictionary of the English Language, New York & London, 1947.
  5. ^ Bode J., Uranographia, Berlin, 1801.
  6. ^ Allen, Richard Hinckley (1899). Star-Names and Their Meanings. New York: G. E. Stechert.  , p.443.
  7. ^ http://www.astro.illinois.edu/~jkaler/sow/alulaaus.html

External links

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