Beta Ursae Majoris

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β 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
U−B color index +0.00[2]
B−V color index -0.02[2]
Variable type Suspected
Radial velocity (Rv) -12.0[3] km/s
Proper motion (μ) RA: +81.43[1] mas/yr
Dec.: +33.49[1] mas/yr
Parallax (π) 40.90 ± 0.16[1] mas
Distance 79.7 ± 0.3 ly
(24.45 ± 0.10 pc)
Mass 2.7[4] M
Radius 3.021 ± 0.038[5] R
Luminosity 63.015 ± 1.307[5] L
Surface gravity (log g) 3.83[6] cgs
Temperature 9377 ± 75[5] K
Rotational velocity (v sin i) 46[7] km/s
Age 500 ± 100[6] Myr
Other designations
Merak, Mirak,[8] 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.[9]
Database references

Beta Ursae Majoris (Beta UMa, β Ursae Majoris, β UMa) is a star in the northern circumpolar constellation of Ursa Major. It has the traditional name Merak.

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" in England, 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.


Based upon parallax measurements of this star, it is located at a distance of 79.7 light-years (24.4 parsecs) from the Earth. The spectrum shows this to have a stellar classification of A1 V,[4] making it a fairly typical main sequence star that is generating energy through the thermonuclear fusion of hydrogen at its core. The effective temperature of the outer envelope is about 9,225 K,[4] giving it a white-hued glow that is typical for A-type stars.[10] 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.[4][11]

Observation of the star in the infrared reveal an excess emission that suggests the presence of a circumstellar debris disk of orbiting dust,[4] much like those discovered around Fomalhaut and Vega The mean temperature of this disk is 120 K,[11] indicating that it is centered at a radius of 47 Astronomical Units from the host star.[4] The dust has an estimated mass of about 0.27% the mass of the Earth.[11]

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.[6] 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 11.0 light-years (3.4 pc); much closer to each other than these stars are to the Earth.[12]

Name and etymology[edit]

In culture[edit]

USS Merak (1918) and USS Merak (AF-21), both of United States navy ship.

In the original Star Trek series episode The Cloud Minders, the planet "Merak II" is said to be suffering from a botanical plague that can only be treated by the use of the mineral zenite.

The video game Devil Survivor 2 features Merak, alongside the other stars of the Big Digger, as the Septentrione, strange beings that appear in Japan on each day of the game. Merak features on the 2nd day in Osaka, Japan.

In the anime Saint Seiya, also known as Knights of the Zodiac, the God Warriors are the main antagonists during the Asgard arc, each star of the Big Dipper serves as a guardian for each God Warrior. Hagen is the name of the God Warrior representing the star Beta Merak.


  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:0708.1752free to read. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. 
  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. ^ Evans, D. S. (June 20–24, 1966). "The Revision of the General Catalogue of Radial Velocities". In Batten, Alan Henry; Heard, John Frederick. Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30. Determination of Radial Velocities and their Applications. 30. University of Toronto: International Astronomical Union. p. 57. Bibcode:1967IAUS...30...57E. 
  4. ^ a b c d e f 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/0703608free to read, Bibcode:2007ApJ...663..365W, doi:10.1086/518404 
  5. ^ 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:1112.3316free to read, Bibcode:2012ApJ...746..101B, doi:10.1088/0004-637X/746/1/101 . See Table 10.
  6. ^ 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 
  7. ^ 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/0610785free to read, Bibcode:2007A&A...463..671R, doi:10.1051/0004-6361:20065224 
  8. ^ a b c Allen, Richard Hinckley (1899), "Star-names and their meanings", New York, G. E. Stechert: 438, 
  9. ^ "MERAK -- Variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-01 
  10. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, retrieved 2012-01-16 
  11. ^ 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/0609555free to read, Bibcode:2007ApJ...660.1556R, doi:10.1086/509912 
  12. ^ 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:1007.0425free to read, Bibcode:2011ApJS..192....2S, doi:10.1088/0067-0049/192/1/2 
  13. ^ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日

Coordinates: Sky map 11h 01m 50.5s, +56° 22′ 57″