Beta Ursae Majoris
Epoch J2000.0 Equinox J2000.0
|Right ascension||11h 01m 50.47654s|
|Declination||+56° 22′ 56.7339″|
|Apparent magnitude (V)||+2.37|
|Spectral type||A1 V|
|U−B color index||+0.00|
|B−V color index||-0.02|
|Radial velocity (Rv)||-12.0 km/s|
|Proper motion (μ)||RA: +81.43 mas/yr
Dec.: +33.49 mas/yr
|Parallax (π)||40.90 ± 0.16 mas|
|Distance||79.7 ± 0.3 ly
(24.45 ± 0.10 pc)
|Radius||3.021 ± 0.038 R☉|
|Luminosity||63.015 ± 1.307 L☉|
|Surface gravity (log g)||3.83|
|Temperature||9377 ± 75 K|
|Rotational velocity (v sin i)||46 km/s|
|Age||500 ± 100 Myr|
The apparent visual magnitude of this star is +2.37, 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, 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, giving it a white-hued glow that is typical for A-type stars. 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.
Observation of the star in the infrared reveal an excess emission that suggests the presence of a circumstellar debris disk of orbiting dust, much like those discovered around Fomalhaut and Vega The mean temperature of this disk is 120 K, indicating that it is centered at a radius of 47 Astronomical Units from the host star. The dust has an estimated mass of about 0.27% the mass of the Earth.
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 by our 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. 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.
Name and etymology 
- The name Merak is derived from the Arabic المراق al-maraqq "the loins" (of the bear).
- This star as Pulaha, one of the Seven Rishis.
- In Chinese, 北斗 (Běi Dǒu), meaning Northern Dipper, refers to an asterism consisting of β Ursae Majoris, α Ursae Majoris, γ Ursae Majoris, δ Ursae Majoris, ε Ursae Majoris, ζ Ursae Majoris and η Ursae Majoris. Consequently, β Ursae Majoris itself is known as 北斗二 (Běi Dǒu èr, English: the Second Star of Northern Dipper) and 天璇 (Tiān Xuán, English: Star of Celestial Rotating Jade).
In culture 
- 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.
- Johnson, H. L. et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory 4 (99), Bibcode:1966CoLPL...4...99J
- 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
- Evans, D. S. (June 20–24, 1966), "The Revision of the General Catalogue of Radial Velocities", in Batten, Alan Henry; John Frederick, Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30, University of Toronto: International Astronomical Union, Bibcode:1967IAUS...30...57E
- 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. See Table 10.
- 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
- 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
- Allen, Richard Hinckley (1899), Star-names and their meanings, G. E. Stechert, p. 438
- "MERAK -- Variable Star", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2012-01-01
- "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, retrieved 2012-01-16
- 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
- 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
- (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日