Beta Ursae Majoris

Coordinates: Sky map 11h 01m 50.5s, +56° 22′ 57″
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Beta Ursae Majoris
Beta Ursae Majoris is located in 100x100
Beta Ursae Majoris

Merak in Ursa Major
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Ursa Major
Right ascension 11h 01m 50.47654s[1]
Declination +56° 22′ 56.7339″[1]
Apparent magnitude (V) +2.37[2]
Spectral type A1IVps[3]
U−B color index +0.00[2]
B−V color index -0.02[2]
Variable type Suspected
Radial velocity (Rv)-12.0[4] km/s
Proper motion (μ) RA: +81.43[1] mas/yr
Dec.: +33.49[1] mas/yr
Parallax (π)40.90 ± 0.16 mas[1]
Distance79.7 ± 0.3 ly
(24.45 ± 0.10 pc)
Absolute magnitude (MV)+0.61[5]
Mass2.7[6] M
Radius3.021 ± 0.038[7] R
Luminosity63.015 ± 1.307[7] L
Surface gravity (log g)3.83[8] cgs
Temperature9377 ± 75[7] K
Rotational velocity (v sin i)46[9] km/s
Age500 ± 100[8] Myr
Other designations
Merak, Mirak,[10] β Ursae Majoris, β UMa, Beta UMa, 48 Ursae Majoris, BD+57°1302, FK5 416, GC 15145, HD 95418, HIP 53910, HR 4295, PPM 32912, SAO 27876[11]
Database references

Beta Ursae Majoris (β Ursae Majoris, abbreviated Beta UMa, β UMa), formally named Merak /ˈmɪəræk/,[12][13] is a star in the northern constellation of Ursa Major.

The apparent visual magnitude of this star is +2.37,[2] which means it is readily visible to the naked eye. It is more familiar to northern hemisphere observers as one of the "pointer stars" in the Big Dipper, or the Plough (UK), which is a prominent asterism of seven stars that forms part of the larger constellation. Extending an imaginary straight line from this star through the nearby Alpha Ursae Majoris (Dubhe) extends to Polaris, the north star.

Spectral classification[edit]

In 1943, β Ursae Majoris was listed as a spectral standard for the class of A1 V.[14] When improved instruments made it possible to identify subgiant luminosity classes for early A-class stars, β Ursae Majoris was assigned that class A0 IV.[15] This was later revised to A1 IV.[3] It is considered to be a mild Am star, a type of chemically peculiar star with unusually strong lines of certain metallic elements.[16]


Based upon parallax measurements, β Ursae Majoris is located at a distance of 79.7 light-years (24.4 parsecs) from the Sun. It is a subgiant, a star that has exhausted the hydrogen in its core and is now cooling as it generates energy through the thermonuclear fusion of hydrogen in a shell outside the core. The effective temperature of the outer envelope is about 9,225 K,[6] giving it a white-hued glow that is typical for A-type stars.[17] It is larger than the Sun, with about 2.7 times the mass and 2.84 times the solar radius. If they were viewed from the same distance, Beta Ursae Majoris would appear much brighter than the Sun, as it is radiating 68 times the Sun's luminosity.[6][18]

Observation of the star in the infrared reveal an excess emission that suggests the presence of a circumstellar debris disk of orbiting dust,[6] much like those discovered around Fomalhaut and Vega. The mean temperature of this disk is 120 K,[18] indicating that it is centered at a radius of 47 AU from the host star.[6] The dust has an estimated mass of about 0.27% the mass of the Earth.[18]

Beta Ursae Majoris is one of five stars in the Big Dipper that form a part of a loose open cluster called the Ursa Major moving group, sharing the same region of space and not just the same patch of sky from Earth's perspective. This group has an estimated age of about 500 (± 100) million years. As the members of this group share a common origin and motion through space, this yields an estimate for the age of Beta Ursae Majoris.[8] Two stars are known to be located in relatively close proximity: 37 Ursae Majoris at 5.2 light-years (1.6 pc) and Gamma Ursae Majoris at 11 light-years (3.4 pc); much closer to each other than these stars are to the Earth.[19]


β Ursae Majoris (Latinised to Beta Ursae Majoris) is the star's Bayer designation.

It bore the traditional name Merak derived from the Arabic المراق al-maraqq 'the loins' (of the bear).[10] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[20] 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 Merak for this star.[21]

The Hindus called the star Pulaha, one of the Seven Rishis.[10]

In Chinese, 北斗 (Běi Dǒu), meaning Northern Dipper, refers to an asterism equivalent to the Big Dipper. Consequently, the Chinese name for Beta Ursae Majoris itself is 北斗二 (Běi Dǒu èr, English: the Second Star of Northern Dipper) and 天璇 (Tiān Xuán, English: Star of Celestial Rotating Jade).[22]

In culture[edit]

USS Merak (1918) and USS Merak (AF-21) are both United States navy ships.

See also[edit]


  1. ^ a b c d e 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.
  2. ^ 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
  3. ^ a b Phillips, N. M.; Greaves, J. S.; Dent, W. R. F.; Matthews, B. C.; Holland, W. S.; Wyatt, M. C.; Sibthorpe, B. (2010). "Target selection for the SUNS and DEBRIS surveys for debris discs in the solar neighbourhood". Monthly Notices of the Royal Astronomical Society. 403 (3): 1089. arXiv:0911.3426. Bibcode:2010MNRAS.403.1089P. doi:10.1111/j.1365-2966.2009.15641.x. S2CID 119262858.
  4. ^ Evans, D. S. (June 20–24, 1966). "The Revision of the General Catalogue of Radial Velocities". In Batten, Alan Henry; Heard, John Frederick (eds.). Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30. Determination of Radial Velocities and Their Applications. Vol. 30. University of Toronto: International Astronomical Union. p. 57. Bibcode:1967IAUS...30...57E.
  5. ^ Eggen, Olin J. (August 1998), "The Sirius Supercluster and Missing Mass near the Sun", The Astronomical Journal, 116 (2): 782–788, Bibcode:1998AJ....116..782E, doi:10.1086/300465.
  6. ^ a b c d e Wyatt, M. C.; et al. (July 2007), "Steady State Evolution of Debris Disks around A Stars", The Astrophysical Journal, 663 (1): 365–382, arXiv:astro-ph/0703608, Bibcode:2007ApJ...663..365W, doi:10.1086/518404, S2CID 18883195
  7. ^ a b c 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:1112.3316, Bibcode:2012ApJ...746..101B, doi:10.1088/0004-637X/746/1/101, S2CID 18993744. See Table 10.
  8. ^ a b c Monier, R. (November 2005), "Abundances of a sample of A and F-type dwarf members of the Ursa Major Group", Astronomy and Astrophysics, 442 (2): 563–566, Bibcode:2005A&A...442..563M, doi:10.1051/0004-6361:20053222
  9. ^ Royer, F.; Zorec, J.; Gómez, A. E. (February 2007), "Rotational velocities of A-type stars. III. Velocity distributions", Astronomy and Astrophysics, 463 (2): 671–682, arXiv:astro-ph/0610785, Bibcode:2007A&A...463..671R, doi:10.1051/0004-6361:20065224, S2CID 18475298
  10. ^ a b c Allen, Richard Hinckley (1899), "Star-names and their meanings", New York, G. E. Stechert: 438,
  11. ^ "MERAK -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-01
  12. ^ Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  13. ^ "IAU Catalog of Star Names". Retrieved 28 Apr 2019.
  14. ^ Morgan, William Wilson; Keenan, Philip Childs; Kellman, Edith (1943). "An atlas of stellar spectra, with an outline of spectral classification". Chicago.
  15. ^ Barry, Don C. (1970). "Spectral Classification of a and F Stars". The Astrophysical Journal Supplement Series. 19: 281. Bibcode:1970ApJS...19..281B. doi:10.1086/190209.
  16. ^ Renson, P.; Manfroid, J. (2009). "Catalogue of Ap, HGMN and Am stars". Astronomy and Astrophysics. 498 (3): 961. Bibcode:2009A&A...498..961R. doi:10.1051/0004-6361/200810788.
  17. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on 2012-03-18, retrieved 2012-01-16
  18. ^ a b c Rhee, Joseph H.; et al. (May 2007), "Characterization of Dusty Debris Disks: The IRAS and Hipparcos Catalogs", The Astrophysical Journal, 660 (2): 1556–1571, arXiv:astro-ph/0609555, Bibcode:2007ApJ...660.1556R, doi:10.1086/509912, S2CID 11879505
  19. ^ Shaya, Ed J.; Olling, Rob P. (January 2011), "Very Wide Binaries and Other Comoving Stellar Companions: A Bayesian Analysis of the Hipparcos Catalogue", The Astrophysical Journal Supplement, 192 (1): 2, arXiv:1007.0425, Bibcode:2011ApJS..192....2S, doi:10.1088/0067-0049/192/1/2, S2CID 119226823
  20. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  21. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  22. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日