18 Ursae Majoris

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18 Ursae Majoris
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Ursa Major
Right ascension 09h 16m 11.32784s[1]
Declination +54° 01′ 18.6878″[1]
Apparent magnitude (V) 4.832[2]
Spectral type A6 V[3]
U−B color index 0.087[2]
B−V color index 0.186[2]
Variable type δ Sct[4]
Radial velocity (Rv) −16.13±1.30[5] km/s
Proper motion (μ) RA: +49.43[1] mas/yr
Dec.: +59.92[1] mas/yr
Parallax (π) 27.90 ± 0.20[1] mas
Distance 116.9 ± 0.8 ly
(35.8 ± 0.3 pc)
Absolute magnitude (MV) 2.03[6]
Mass 1.72±0.02[4] M
Luminosity 12.83[6] L
Surface gravity (log g) 3.98±0.2[4] cgs
Temperature 7,450±150[4] K
Rotational velocity (v sin i) 158±17[5] km/s
Age 1.05+0.10
Other designations
e UMa, 18 UMa, DD UMa, BD+54° 1285, FK5 2734, GJ 3541, HD 79439, HIP 45493, HR 3662, SAO 27191[7]
Database references

18 Ursae Majoris, or e Ursae Majoris, is a single[8] star in the northern circumpolar constellation of Ursa Major. It is faintly visible to the naked eye with an apparent visual magnitude of 4.832.[2] The annual parallax shift measured from Earth's orbit is 27.90 mas, which provides a distance estimate of 117 light years. It is moving closer to the Sun with a heliocentric radial velocity of −16 km/s,[5] and is an unbound and older member of the Ursa Major Moving Group.[4]

The stellar classification assigned to this star is A6 V,[3] which indicates it is an A-type main-sequence star that is generating energy through hydrogen fusion at its core. The star's variability was first noticed by American astronomer Frank Schlesinger in 1914 and it has been given the variable star designation DD UMa. This is a low amplitude Delta Scuti variable with a magnitude change of around 0.4 and pulsation cycles of 9.4 and 15.0 cycles per day.[4] It is rotating rapidly with a projected rotational velocity of about 158 km/s.[5] This is giving the star an oblate shape with an equatorial bulge that is an estimated 6% larger than the polar radius.[9]

18 UMA is about a billion years old with 1.72[4] times the mass of the Sun. It is radiating around 13[6] times the Sun's luminosity from its photosphere at an effective temperature of about 7,450 K.[4]


  1. ^ a b c d e van Leeuwen, F. (2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752Freely accessible, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357. 
  2. ^ a b c d Boyajian, Tabetha S.; et al. (February 2012), "Stellar Diameters and Temperatures. I. Main-sequence A, F, and G Stars", The Astrophysical Journal, 746 (1): 101, arXiv:1112.3316Freely accessible, Bibcode:2012ApJ...746..101B, doi:10.1088/0004-637X/746/1/101. 
  3. ^ a b Abt, Helmut A.; Morrell, Nidia I. (July 1995), "The Relation between Rotational Velocities and Spectral Peculiarities among A-Type Stars", Astrophysical Journal Supplement, 99: 135, Bibcode:1995ApJS...99..135A, doi:10.1086/192182. 
  4. ^ a b c d e f g h Elmaslı, A.; et al. (February 2012), "A spectroscopic study of DD UMa: Ursa Major group member and candidate for BRITE", New Astronomy, 17 (2): 221−226, arXiv:1107.4856Freely accessible, Bibcode:2012NewA...17..221E, doi:10.1016/j.newast.2011.07.006. 
  5. ^ a b c d Becker, Juliette C.; et al. (April 2015), "Extracting Radial Velocities of A- and B-type Stars from Echelle Spectrograph Calibration Spectra", The Astrophysical Journal Supplement Series, 217 (2): 13, arXiv:1503.03874Freely accessible, Bibcode:2015ApJS..217...29B, doi:10.1088/0067-0049/217/2/29, 29. 
  6. ^ a b c Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971Freely accessible, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015. 
  7. ^ "e UMa". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2018-02-22. 
  8. ^ Rodriguez, David R.; et al. (May 2015), "Stellar multiplicity and debris discs: an unbiased sample", Monthly Notices of the Royal Astronomical Society, 449 (3): 3160–3170, arXiv:1503.01320Freely accessible, Bibcode:2015MNRAS.449.3160R, doi:10.1093/mnras/stv483. 
  9. ^ Belle, G. T. (2012), "Interferometric observations of rapidly rotating stars", The Astronomy and Astrophysics Review, 20: 51, arXiv:1204.2572Freely accessible, Bibcode:2012A&ARv..20...51V, doi:10.1007/s00159-012-0051-2.