Xi Ursae Majoris
Epoch J2000 Equinox J2000
|ξ UMa A|
|Right ascension||11h 18m 10.902s|
|Declination||+31° 31′ 44.98″|
|Apparent magnitude (V)||4.264|
|ξ UMa B|
|Right ascension||11h 18m 10.950s|
|Declination||+31° 31′ 45.74″|
|Apparent magnitude (V)||4.729|
|Spectral type||F8.5:V / G2V|
|U−B color index||0.04|
|B−V color index||0.59|
|Variable type||RS CVn|
|Radial velocity (Rv)||−18.2 ± 2.7 km/s|
|Proper motion (μ)|| RA: −453.7 mas/yr |
Dec.: −591.4 mas/yr
|Parallax (π)||113.2 ± 4.6 mas|
|Distance||29 ± 1 ly |
(8.8 ± 0.4 pc)
|ξ UMa Aa|
|Absolute magnitude (MV)||4.66[note 1]|
|4.54 ± 0.06|
|ξ UMa Ba|
|Absolute magnitude (MV)||5.16[note 2]|
|5.00 ± 0.06|
|Primary||ξ UMa A|
|Companion||ξ UMa B|
|Period (P)||59.878 yr|
|Semi-major axis (a)||2.536″|
|Longitude of the node (Ω)||101.85 (ascending)°|
|Periastron epoch (T)||1935.195|
|Primary||ξ UMa B|
|Period (P)||1.832 yr|
|Semi-major axis (a)||0.057″|
|ξ UMa Aa|
|Radius||1.02 ± 0.04 R☉|
|Surface gravity (log g)||4.39 ± 0.10 cgs|
|Temperature||6005 ± 80 K|
|Rotational velocity (v sin i)||1.0 ± 1.0 km/s|
|ξ UMa Ab|
|Mass||0.38 ± 0.02 M☉|
|Temperature||∼3700[note 3] K|
|ξ UMa Ba|
|Radius||0.92 ± 0.04 R☉|
|Surface gravity (log g)||4.46 ± 0.10 cgs|
|Temperature||5692 ± 90 K|
|Metallicity [Fe/H]||−0.35 ± 0.08 dex|
|Rotational velocity (v sin i)||3.0 ± 1.0 km/s|
|ξ UMa Bb|
|A: HD 98231, HR 4375|
|B: HD 98230, HR 4374|
|ξ UMa AB|
|ξ UMa A|
|ξ UMa B|
|ξ UMa Bb|
Xi Ursae Majoris (ξ Ursae Majoris, abbreviated Xi UMa, ξ UMa), also named Alula Australis, is a star system in the constellation of Ursa Major. 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. It is also a variable star with a small amplitude. Xi Ursae Majoris is found in the left hind paw of the Great Bear.
The two components are yellow main sequence stars. The brighter component (designated Xi Ursae Majoris A), has a mean apparent magnitude of +4.41. 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 AU.
Each component of this double star is itself a single-lined spectroscopic binary. The orbit of the A pair has been determined from spectroscopy and speckle interferometry, giving a period of 669 days and an eccentricity of 0.53. B's binary companion (Xi Ursae Majoris Bb)has not been detected visually or spectroscopically, but the radial velocity variations of the spectral lines show a circular orbit with a 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 probably red dwarfs, Bb being on the cool end of the M spectrum, not much hotter than a brown dwarf.
In 2012 Wright et al. discovered the fifth component and the second brown dwarf (if Bb is also a brown dwarf) of the system using Wide-field Infrared Survey Explorer (WISE) data—a T8.5 brown dwarf WISE J111838.70+312537.9 with angular separation 8.5 arc-min, and the projected physical separation about 4000 AU.
ξ Ursae Majoris is classified as an RS Canum Venaticorum variable and its brightness varies by 0.01 magnitudes. Component B is believed to be the variable star, showing characteristic emission lines in its spectrum that are not present for component A.
It also bore the traditional names Alula Australis (and erroneously Alula Australe). Alula (shared with Nu Ursae Majoris) comes from the Arabic phrase Al Ḳafzah al Ūla 'the First Spring' and Australis is Latin for 'the south side'. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Alula Australis for this star.
In Chinese, 三台 (Sān Tái), meaning Three Steps, refers to an asterism consisting of Xi Ursae Majoris, Iota Ursae Majoris, Kappa Ursae Majoris, Lambda Ursae Majoris, Mu Ursae Majoris and Nu Ursae Majoris. Consequently, Xi Ursae Majoris itself is known as 下台二 (Xià Tái èr, English: Star of Second Lower Step).
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- Nicolet, B (1978). "Catalogue of homogeneous data in the UBV photoelectric photometric system". Astronomy and Astrophysics. 34: 1. Bibcode:1978A&AS...34....1N.
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- Fuhrmann, Klaus (2008). "Nearby stars of the Galactic disc and halo - IV". Monthly Notices of the Royal Astronomical Society. 384 (1): 173–224. Bibcode:2008MNRAS.384..173F. doi:10.1111/j.1365-2966.2007.12671.x.
- Mason, Brian D.; McAlister, Harold A.; Hartkopf, William I.; Shara, M. M.; Shara, M. M. (January 1995), "Binary star orbits from speckle interferometry. 7: The multiple system XI Ursae Majoris", The Astronomical Journal, 109 (1669): 332–340, Bibcode:1995AJ....109..332M, doi:10.1086/117277
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- Spectroscopic binary orbits from photoelectric radial velocities. Paper 142: Xi Ursae Majoris page 274
- Berman, Louis (1931). "The spectroscopic orbit of the fainter component in the system [xi] Ursae Majoris". Lick Observatory bulletin ; no. 432; Lick Observatory bulletins ; no. 432. 15: 109. Bibcode:1931LicOB..15..109B. doi:10.5479/ADS/bib/1931LicOB.15.109B.
- "Archived copy". Archived from the original on July 16, 2011. Retrieved June 29, 2009.CS1 maint: Archived copy as title (link)[dead link]
- Wright, Edward L.; Skrutskie, M. F.; Kirkpatrick, J. Davy; Gelino, Christopher R.; Griffith, Roger L.; Marsh, Kenneth A.; Jarrett, Tom; Nelson, M. J.; Borish, H. J.; Mace, Gregory; Mainzer, Amanda K.; Eisenhardt, Peter R.; McLean, Ian S.; Tobin, John J.; Cushing, Michael C. (2012). "A T8.5 Brown Dwarf Member of the Xi Ursae Majoris System". arXiv:1203.5764v1 [astro-ph.SR].
- Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
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- Piazzi, G., The Palermo Catalogue, Palermo, 1814.
- Bečvář, A., Atlas Coeli (Atlas of the Heavens) II – Catalogue, Plague, 1964.
- Richard Hinckley Allen :Star Names — Their Lore and Meaning – Ursa Major, the Greater Bear
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- (in Chinese) (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 21 日
- Calculated from the bolometric magnitude 4.54 ± 0.06 and the bolometric correction −0.12 ± 0.05 using the formula: BC = Mbol − MV
- Calculated from the bolometric magnitude 5.00 ± 0.06 and the bolometric correction −0.16 ± 0.05 using the formula: BC = Mbol − MV
- This estimates assumes that ξ UMa Ab is a red dwarf.