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δ Ursae Minoris
Ursa Minor constellation map.svg
Red circle.svg
Location of δ Ursae Minoris (circled)
Observation data
Epoch J2000       Equinox J2000
Constellation Ursa Minor [1]
Right ascension 17h 32m 12.99671s [2]
Declination 86° 35′ 11.2584″ [2]
Apparent magnitude (V) 4.36 [3]
Spectral type A1 Van [4]
U−B color index +0.03 [3]
B−V color index +0.02 [3]
Radial velocity (Rv)−7.6±2.7 [5] km/s
Proper motion (μ) RA: +10.17 [2]  mas/ yr
Dec.: +53.97 [2]  mas/ yr
Parallax (π)18.95 ± 0.14  mas [2]
Distance172 ± 1  ly
(52.8 ± 0.4  pc)
Absolute magnitude (MV)0.62 [6]
Mass2.35 [7]  M
Radius2.8 [1]  R
Luminosity47.77 [8]  L
Surface gravity (log g)4.04 [7]  cgs
Temperature9,911±337 [7]  K
Rotation19 hours [1]
Rotational velocity (v sin i)154 [7] km/s
Age327 [7]  Myr
Other designations
Yildun, Vildiur, Gildun [9], δ UMi, 23 Ursae Minoris, BD+86°269, FK5 913, GC 24236, HD 166205, HIP 85822, HR 6789, SAO 2937 [10]
Database references

Delta Ursae Minoris, Latinized from δ Ursae Minoris, formally named Yildun /jɪlˈdʌn/, [11] is a white-hued star in the northern circumpolar constellation of Ursa Minor, forming the second star in the bear's tail. [12] It is visible to the naked eye with an apparent visual magnitude of 4.36. [3] Based upon an annual parallax shift of 18.95  mas as seen from Earth, [2] it is located 172  light years from the Sun. The star is moving closer to the Sun with a radial velocity of about −8 km/s. [5]


This is an A-type main-sequence star with a stellar classification of A1 Van, [4] where the 'n' indicates "nebulous" absorption lines in the spectrum due to rapid rotation. It is spinning with a projected rotational velocity of 180 km/s, which is giving the star an oblate shape with an equatorial bulge that is an estimated 7% larger than the polar radius. [13] The star is about 327 [7] million years old with 2.35 [7] times the mass of the Sun. It is radiating about 48 [8] times the Sun's luminosity from its photosphere at an effective temperature of 9,911 K. [7]

δ Ursae Minoris has a 12th magnitude common proper-motion companion at an angular separation of 4.5  arc seconds along a position angle of 67.3°, which is a projected separation of 237.4  au at the distance of δ Ursae Minoris. The pair were resolved using adaptive optics. The companion is assumed to be gravitationally bound and to have a mass of approximately 0.16 M. [14]


δ Ursae Minoris ( Latinised to Delta Ursae Minoris) is the star's Bayer designation.

It bore the traditional name Yildun (also spelled Vildiur or Gildun) from the Turkish yıldız "star". [9] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) [15] to catalogue and standardize proper names for stars. The WGSN approved the name Yildun for this star on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names. [11]


  1. ^ a b c Jim Kaler. "YILDUN (Delta Ursae Minoris)". Retrieved 2012-03-05.
  2. ^ a b c d e f van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv: 0708.1752, Bibcode: 2007A&A...474..653V, doi: 10.1051/0004-6361:20078357, S2CID  18759600
  3. ^ a b c d Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99): 99, Bibcode: 1966CoLPL...4...99J.
  4. ^ a b Gray, R. O.; Garrison, R. F. (December 1987), "The Early A-Type Stars: Refined MK Classification, Confrontation with Stroemgren Photometry, and the Effects of Rotation", Astrophysical Journal Supplement, 65: 581, Bibcode: 1987ApJS...65..581G, doi: 10.1086/191237.
  5. ^ a b de Bruijne, J. H. J.; Eilers, A.-C. (October 2012), "Radial velocities for the HIPPARCOS-Gaia Hundred-Thousand-Proper-Motion project", Astronomy & Astrophysics, 546: 14, arXiv: 1208.3048, Bibcode: 2012A&A...546A..61D, doi: 10.1051/0004-6361/201219219, S2CID  59451347, A61.
  6. ^ Jim Kaler. "The Polar Project". University of Illinois. Retrieved 2012-03-04.
  7. ^ a b c d e f g h David, Trevor J.; Hillenbrand, Lynne A. (2015), "The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets", The Astrophysical Journal, 804 (2): 146, arXiv: 1501.03154, Bibcode: 2015ApJ...804..146D, doi: 10.1088/0004-637X/804/2/146, S2CID  33401607.
  8. ^ a b Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv: 1108.4971, Bibcode: 2012AstL...38..331A, doi: 10.1134/S1063773712050015, S2CID  119257644.
  9. ^ a b Richard Hinckley Allen (1899). Star-names and Their Meanings. G.E. Stechert. pp.  447–460.
  10. ^ "del UMa". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2012-03-04.
  11. ^ a b "IAU Catalog of Star Names". Retrieved 28 July 2016.
  12. ^ Ridpath, Ian (1988), Star Tales, James Clarke & Co., p. 131, ISBN  0718826957.
  13. ^ Belle, G. T. (2012), "Interferometric observations of rapidly rotating stars", The Astronomy and Astrophysics Review, 20 (1): 51, arXiv: 1204.2572, Bibcode: 2012A&ARv..20...51V, doi: 10.1007/s00159-012-0051-2, S2CID  119273474.
  14. ^ De Rosa, R. J.; Patience, J.; Wilson, P. A.; Schneider, A.; Wiktorowicz, S. J.; Vigan, A.; Marois, C.; Song, I.; MacIntosh, B.; Graham, J. R.; Doyon, R.; Bessell, M. S.; Thomas, S.; Lai, O. (2014). "The VAST Survey - III. The multiplicity of A-type stars within 75 pc". Monthly Notices of the Royal Astronomical Society. 437 (2): 1216. arXiv: 1311.7141. Bibcode: 2014MNRAS.437.1216D. doi: 10.1093/mnras/stt1932. S2CID  88503488.
  15. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.

External links