Astronomy:Epsilon Ursae Majoris

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Short description: Star in the constellation Ursa Major
Epsilon Ursae Majoris
Ursa Major constellation map.svg
Red circle.svg
Location of ε Ursae Majoris (circled)
Observation data
Equinox J2000.0]] (ICRS)
Constellation Ursa Major
Right ascension  12h 54m 01.74959s[1]
Declination +55° 57′ 35.3627″[1]
Apparent magnitude (V) 1.77[2]
Characteristics
Spectral type A1III-IVp kB9
U−B color index +0.02[2]
B−V color index −0.02[2]
Variable type α2 CVn
Astrometry
Radial velocity (Rv)−9.3[3] km/s
Proper motion (μ) RA: +111.91[1] mas/yr
Dec.: −8.24[1] mas/yr
Parallax (π)39.51 ± 0.20[1] mas
Distance82.6 ± 0.4 ly
(25.3 ± 0.1 pc)
Absolute magnitude (MV)−0.2[4]
Details
Mass2.91[5] M☉
Radius4.14[6] R☉
Luminosity102[7] L☉
Surface gravity (log g)3.23[8] cgs
Temperature9,020[8] K
Metallicity [Fe/H]+0.00[4] dex
Rotational velocity (v sin i)33[9] km/s
Age300[10] Myr
Other designations
Database references
SIMBADdata

Epsilon Ursae Majoris is a star in the northern constellation of Ursa Major. The designation is Latinised from ε Ursae Majoris and abbreviated Epsilon UMa or ε UMa. It is officially named Alioth /ˈæliɒθ/.[11][12] Despite being designated "ε" (epsilon), it is the brightest star in the constellation and at magnitude 1.77 is the thirty-third brightest star in the sky.

It is the star in the tail of the bear closest to its body, and thus the star in the handle of the Big Dipper (or Plough) closest to the bowl. It is also a member of the large and diffuse Ursa Major moving group. Historically, the star was frequently used in celestial navigation in the maritime trade, because it is listed as one of the 57 navigational stars.[1]

Physical characteristics

Book plate by Sidney Hall depicting Ursa Major's stars
A light curve for Epsilon Ursae Majoris, plotted from TESS data[13]

According to Hipparcos, Epsilon Ursae Majoris is 81 light-years (25 parsecs) from the Sun. Its spectral type is A1p; the "p" stands for peculiar, as its spectrum is characteristic of an α2 Canum Venaticorum variable. Epsilon Ursae Majoris, as a representative of this type, may harbor two interacting processes. First, the star's strong magnetic field separating different elements in the star's hydrogen 'fuel'. In addition, a rotation axis at an angle to the magnetic axis may be spinning different bands of magnetically sorted elements into the line of sight between Epsilon Ursae Majoris and the Earth. The intervening elements react differently at different frequencies of light as they whip in and out of view, causing Epsilon Ursae Majoris to have very strange spectral lines that fluctuate over a period of 5.1 days. The kB9 suffix to the spectral type indicates that the calcium K line is present and representative of a B9 spectral type even though the rest of the spectrum indicates A1.

Epsilon Ursae Majoris's rotational and magnetic poles are at almost 90 degrees to one another. Darker (denser) regions of chromium form a band at right angles to the equator.

It has long been suspected that Epsilon Ursae Majoris is a spectroscopic binary, possibly with more than one companion.[14] A more recent study suggests Epsilon Ursae Majoris's 5.1-day variation may be due to a substellar object of about 14.7 Jupiter masses in an eccentric orbit (e=0.5) with an average separation of 0.055 astronomical units.[15] It is now thought that the 5.1-day period is the rotation period of the star, and no companions have been detected using the most modern equipment.[6]

Epsilon Ursae Majoris has a relatively weak magnetic field, 15 times weaker than α Canum Venaticorum, but it is still 100 times stronger than that of the Earth.[16]

Name and etymology

ε Ursae Majoris (Latinised to Epsilon Ursae Majoris) is the star's Bayer designation.

The traditional name Alioth comes from the Arabic alyat al-hamal ("the sheep's fat tail"). In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[17] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[18] included a table of the first two batches of names approved by the WGSN; which included Alioth for this star.

This star was known to the Hindus as Añgiras, one of the Seven Rishis.[19]

In Chinese, 北斗 (Běi Dǒu), meaning Northern Dipper, refers to an asterism equivalent to the Big Dipper. Consequently, the Chinese name for Epsilon Ursae Majoris itself is 北斗五 (Běi Dǒu wu, English: the Fifth Star of Northern Dipper) and 玉衡 (Yù Héng, English: Star of Jade Sighting-Tube).[20]

Namesakes

The United States Navy's Crater class cargo ship USS Allioth (AK-109) was named after the star.

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, doi:10.1051/0004-6361:20078357, Bibcode: 2007A&A...474..653V 
  2. ↑ 2.0 2.1 2.2 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), Batten, Alan Henry; Heard, John Frederick, eds., "The Revision of the General Catalogue of Radial Velocities", Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30 (University of Toronto: International Astronomical Union) 30: pp. 57, Bibcode: 1967IAUS...30...57E 
  4. ↑ 4.0 4.1 Tektunali, H. G. (June 1981), "The spectrum of the CR star Epsilon Ursae Majoris", Astrophysics and Space Science 77 (1): 41–58, doi:10.1007/BF00648756, Bibcode: 1981Ap&SS..77...41T 
  5. ↑ 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, doi:10.1088/0067-0049/192/1/2, Bibcode: 2011ApJS..192....2S 
  6. ↑ 6.0 6.1 Shulyak, D.; Paladini, C.; Causi, G. Li; Perraut, K.; Kochukhov, O. (2014). "Interferometry of chemically peculiar stars: Theoretical predictions versus modern observing facilities". Monthly Notices of the Royal Astronomical Society 443 (2): 1629. doi:10.1093/mnras/stu1259. Bibcode: 2014MNRAS.443.1629S. 
  7. ↑ Katarzyński, K.; Gawroński, M.; Goździewski, K. (2016). "Search for exoplanets and brown dwarfs with VLBI". Monthly Notices of the Royal Astronomical Society 461 (1): 929. doi:10.1093/mnras/stw1354. Bibcode: 2016MNRAS.461..929K. 
  8. ↑ 8.0 8.1 Gray, R. O.; Corbally, C. J.; Garrison, R. F.; McFadden, M. T.; Robinson, P. E. (2003). "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I". The Astronomical Journal 126 (4): 2048. doi:10.1086/378365. Bibcode: 2003AJ....126.2048G. 
  9. ↑ Royer, F. et al. (October 2002), "Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i", Astronomy and Astrophysics 393 (3): 897–911, doi:10.1051/0004-6361:20020943, Bibcode: 2002A&A...393..897R 
  10. ↑ Nakajima, Tadashi; Morino, Jun-Ichi (2012). "Potential Members of Stellar Kinematic Groups within 30 pc of the Sun". The Astronomical Journal 143 (1): 2. doi:10.1088/0004-6256/143/1/2. Bibcode: 2012AJ....143....2N. 
  11. ↑ 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. 
  12. ↑ "IAU Catalog of Star Names". http://www.pas.rochester.edu/~emamajek/WGSN/IAU-CSN.txt. 
  13. ↑ "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html. 
  14. ↑ Morgan, B. L.; Beddoes, D. R.; Scaddan, R. J.; Dainty, J. C. (1978). "Observations of binary stars by speckle interferometry - I". Monthly Notices of the Royal Astronomical Society 183 (4): 701–710. doi:10.1093/mnras/183.4.701. Bibcode: 1978MNRAS.183..701M. 
  15. ↑ Sokolov, N. A. (March 2008), "Radial velocity study of the chemically peculiar star ɛ Ursae Majoris", Monthly Notices of the Royal Astronomical Society: Letters 385 (1): L1–L4, doi:10.1111/j.1745-3933.2008.00419.x, Bibcode: 2008MNRAS.385L...1S. 
  16. ↑ Kochukhov, O.; Shultz, M.; Neiner, C. (2019). "Magnetic field topologies of the bright, weak-field Ap stars θ Aurigae and ∊ Ursae Majoris". Astronomy and Astrophysics 621: A47. doi:10.1051/0004-6361/201834279. Bibcode: 2019A&A...621A..47K. 
  17. ↑ "IAU Working Group on Star Names (WGSN)". https://www.iau.org/science/scientific_bodies/working_groups/280/. 
  18. ↑ "Bulletin of the IAU Working Group on Star Names, No. 1". http://www.pas.rochester.edu/~emamajek/WGSN/WGSN_bulletin1.pdf. 
  19. ↑ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 438. ISBN 0-486-21079-0. https://archive.org/details/starnamestheirlo00alle/page/438. Retrieved 2012-09-04. 
  20. ↑ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 15 日
  1. ^  This article incorporates text from a publication now in the public domain: Chambers, Ephraim, ed (1728). "Allioth". Cyclopædia, or an Universal Dictionary of Arts and Sciences (first ed.). James and John Knapton, et al. 




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