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Mensa (constellation)

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Abbreviation Men
Genitive Mensae
Pronunciation /ˈmɛnsə/
genitive: /ˈmɛns/
Symbolism the Table Mountain
Right ascension 03h 12m 55.9008s - 07h 36m 51.5289s
Declination −71° - −85.5°
Quadrant SQ1
Area 153 sq. deg. (75th)
Main stars 4
Stars with planets 2
Stars brighter than 3.00m none
Stars within 10.00 pc (32.62 ly) none
Brightest star α Men (5.09m)
Messier objects none
Meteor showers none
Visible at latitudes between +4° and −90°.
Best visible at 21:00 (9 p.m.) during the month of January.
The constellation Mensa as it can be seen by the naked eye.

Mensa is a constellation in the Southern Celestial Hemisphere near the south celestial pole, one of twelve constellations drawn up in the 18th century by French astronomer Nicolas Louis de Lacaille. Its name is Latin for table, though it originally depicted Table Mountain and was known as Mons Mensae. One of the 88 modern constellations, it covers a keystone-shaped wedge of sky approximately 153.5 square degrees by area. Other than the south polar constellation of Octans, it is the most southerly of constellations and is only observable south of the 5th parallel of the Northern Hemisphere.

It is one of the faintest constellations in the night sky and does not contain any apparently bright stars. Its brightest star, Alpha Mensae is barely visible in suburban skies. Two star systems in Mensa have been found to have planets, and part of the Large Magellanic Cloud lies within the constellation's borders. Several star clusters and a quasar lie within the area covered by the constellation.


Initially known as Mons Mensae, Mensa was created by Nicolas Louis de Lacaille out of dim Southern Hemisphere stars in honor of Table Mountain, a South African mountain overlooking Cape Town. He recalled that the Magellanic Clouds were sometimes known as Cape clouds, and that Table Mountain was often covered in clouds when a southeasterly stormy wind blew. Hence he made a "table" in the sky under the clouds.[1] Lacaille had observed and catalogued 10,000 southern stars during a two-year stay at the Cape of Good Hope. He devised 14 new constellations in uncharted regions of the Southern Celestial Hemisphere not visible from Europe. Mensa was the only constellation that did not honor an instrument that symbolised the Age of Enlightenment.[2] John Herschel proposed shrinking the name to one word in 1844, noting that Lacaille himself had abbreviated some of his constellations thus.[3]

Although the stars of Mensa do not feature in any ancient mythology, the mountain it is named after has a rich mythology. Called "Tafelberg" in Dutch and German, it has two neighboring mountains called "Devil's Peak" and "Lion's Head". Table Mountain features in the mythology of the Cape of Good Hope, notorious for its storms. Explorer Bartolomeu Dias saw the mountain as a mythical anvil for storms. Another myth relating to its dangers comes from Sinbad the Sailor, an Arabic folk hero who saw it as a magnet pulling his ships to the bottom of the sea.[4]


Mensa is bordered by Dorado to the north, Hydrus to the northwest and west, Octans to the south, Chamaeleon to the east and Volans to the northeast. Covering 153.5 square degrees and 0.372% of the night sky, it ranks 75th of the 88 constellations in size.[5] The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is 'Men'.[6] The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of eight segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 03h 12m 55.9008s and 07h 36m 51.5289s, while the declination coordinates are between −69.75° and −85.26°.[7] The whole constellation is visible to observers south of latitude 5°N.[5][a]



Lacaille gave eleven stars in the constellation Bayer designations, labelling them Alpha through to Lambda Mensae (excluding Kappa). Gould later added Kappa, Mu, Nu, Xi and Pi Mensae. Stars as dim as these were not generally given designations; however, Gould felt their closeness to the South Celestial Pole warranted naming.[1] Alpha Mensae is the brightest star with a barely visible apparent magnitude of 5.09,[8] making it the only constellation with no star above magnitude 5.0.[9] Overall, there are 22 stars within the constellation's borders brighter than or equal to apparent magnitude 6.5.[b][5]

  • Alpha Mensae is a solar-type star (class G7V) 33.26 ± 0.05 light-years from Earth.[11] It came to within 11 light-years from Earth around 250,000 years ago and would have been considerably brighter back then.[12] An infrared excess has been detected around this star, indicating the presence of a circumstellar disk with a radius of over 147 AU. The temperature of this dust is below 22 K.[13] No planetary companions have yet been discovered around it. It has a red dwarf companion star at an angular separation of 3.05 arcseconds; equivalent to a projected separation of roughly 30 AU.[8][14][15]
  • Gamma Mensae is the second brightest star in the constellation at magnitude 5.19.[12] Located 102 ± 3 light-years from Earth,[11] it is an ageing (10.6 billion year-old) star around 1.04 times as massive as the Sun. It has swollen to around 4.99 times the solar radius,[16] becoming an orange giant of spectral type K2III.[17]
  • Beta Mensae is slightly fainter at magnitude 5.31.[12] Located 790 ± 40 light-years from Earth,[11] it is a yellow giant of spectral type G8III, around 3.6 times as massive and 513 times as luminous as the Sun. It is 270 million years old,[18] and lies in front of the Large Magellanic Cloud.[12]
  • Zeta and Eta Mensae have infrared excesses suggesting they too have circumstellar disks of dust.[19][20] Zeta Mensae is an ageing white giant of spectral type A5 III around 414 ± 9 light-years from Earth,[21] and Eta Mensae is an orange giant of spectral type K4 III.[20]
  • Pi Mensae is a solar-type (G1) star 59.7 ± 0.2 light-years distant.[11] In 2001, a substellar companion was discovered in an eccentric orbit.[22] Incorporating more accurate Hipparcos data yields a mass range for the companion to be anywhere from 10.27 to 29.9 times that of Jupiter. This confirms its substellar nature with the upper limit of mass putting it in the brown dwarf range.[23]
  • HD 38283 is a sun-like star of spectral type F9.5V of magnitude 6.7, located 125 ± 1 light-years distant. In 2011, a gas giant around a third as massive as Jupiter with an Earth-like orbital period of 363 days was discovered by radial velocity method.[24]
  • TZ Mensae is an eclipsing binary that varies between magnitude 6.2 and 6.9 every 8.57 days.[9] It is composed of two white main sequence stars in close orbit. One of these is of spectral type A0V, has a radius twice as that of the Sun and is 2.5 times as massive. The other, an A8V spectral type, has a radius 1.4 times larger than that of the Sun and is 1.5 time as massive.[25][26]
  • AO Mensae is a faint star of magnitude 9.8. An orange dwarf that has 80% the size and mass of the Sun,[27] it is also a BY Draconis variable.[28] These are a class of stars that have sunspots that are prominent enough so that the star changes brightness as it rotates.[29] It is a member of the Beta Pictoris moving group, a loose association of young stars moving across the galaxy.[27]

Deep-sky objects[edit]

The Large Magellanic Cloud lies partially within Mensa's boundaries,[31] although most of it lies in neighbouring Dorado.[9] It is a satellite galaxy of the Milky Way, located at a distance of 163,000 light-years.[32] Among its stars within Mensa are W Mensae, an unusual yellow-white supergiant that belongs to a rare class of star known as a R Coronae Borealis variable,[33] and HD 268835, a blue hypergiant that is girded by a vast circumstellar disk of dust.[34] Also within the galaxy is NGC 1987, a globular cluster estimated to be around 600 million years old that has a significant number of red ageing stars,[35] and NGC 1848, a 27 million year old open cluster.[36] Mensa contains several described open clusters, most of which can be only be clearly observed from large telescopes.[37]

PKS 0637-752 is a quasar located around 6 billion light-years from Earth,[38] with a redshift of z = 0.651. It was chosen as the first target of the then newly-operational Chandra X-Ray Observatory in 1999. The resulting images revealed a gas jet approximately 326,000 light-years long. It is visible at radio, optical and x-ray wavelengths.[39]


  1. ^ While parts of the constellation technically rise above the horizon to observers between 5°N and 20°N, stars within a few degrees of the horizon are to all intents and purposes unobservable.[5]
  2. ^ Objects of magnitude 6.5 are among the faintest visible to the unaided eye in suburban-rural transition night skies.[10]



  1. ^ a b Wagman 2003, pp. 207–08.
  2. ^ Wagman 2003, pp. 6–7.
  3. ^ Herschel, John (1844). "Farther Remarks on the Division of Southern Constellations". Monthly Notices of the Royal Astronomical Society. 6 (5): 60–62. doi:10.1093/mnras/6.5.60a. 
  4. ^ Staal 1988, p. 259.
  5. ^ a b c d Ridpath, Ian. "Constellations: Lacerta–Vulpecula". Star Tales. Self-published. Retrieved 4 March 2016. 
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  19. ^ Ishihara, Daisuke; Takeuchi, Nami; Kobayashi, Hiroshi; et al. (2017). "Faint warm debris disks around nearby bright stars explored by AKARI and IRSF". Astronomy & Astrophysics. 601: 18. arXiv:1608.04480Freely accessible. Bibcode:2017A&A...601A..72I. doi:10.1051/0004-6361/201526215. A72. 
  20. ^ a b Kim, Sungsoo S.; Zuckerman, B.; Silverstone, Murray (2001). "Extent of Excess Far-Infrared Emission around Luminosity Class III Stars". The Astrophysical Journal. 550 (2): 1000–06. arXiv:astro-ph/0012001Freely accessible. Bibcode:2001ApJ...550.1000K. doi:10.1086/319803. 
  21. ^ Belle, Gerard 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. 
  22. ^ Jones, H. R. A.; Paul Butler, R.; Tinney, C. G.; et al. (2002). "A probable planetary companion to HD 39091 from Anglo-Australian Planet Search". Monthly Notices of the Royal Astronomical Society. 333 (4): 871–875. arXiv:astro-ph/0112084Freely accessible. Bibcode:2002MNRAS.333..871J. doi:10.1046/j.1365-8711.2002.05459.x.  (web Preprint)
  23. ^ Reffert, S.; Quirrenbach, A. (2011). "Mass constraints on substellar companion candidates from the re-reduced Hipparcos intermediate astrometric data: nine confirmed planets and two confirmed brown dwarfs". Astronomy & Astrophysics. 527. id.A140. arXiv:1101.2227Freely accessible. Bibcode:2011A&A...527A.140R. doi:10.1051/0004-6361/201015861. 
  24. ^ Tinney (2011). "The Anglo-Australian Planet Search. XXI. A Gas-giant Planet in a One Year Orbit and the Habitability of Gas-giant Satellites". The Astrophysical Journal. Bibcode:2011ApJ...732...31T. doi:10.1088/0004-637x/732/1/31. 
  25. ^ Graczyk, Dariusz; Konorski, Piotr; Pietrzyński, Grzegorz; et al. (2017). "The Surface Brightness-color Relations Based on Eclipsing Binary Stars: Toward Precision Better than 1% in Angular Diameter Predictions". The Astrophysical Journal. 837 (1): 19. arXiv:1611.09976Freely accessible. Bibcode:2017ApJ...837....7G. doi:10.3847/1538-4357/aa5d56. 7. 
  26. ^ Andersen, J.; Clausen, J. V.; Nordstrom, B. (1987). "Absolute dimensions of eclipsing binaries. XII - TZ Mensae". Astronomy and Astrophysics. 175 (1-2): 60–70. Bibcode:1987A&A...175...60A. 
  27. ^ a b Messina, S; Desidera, S; Turatto, M; Lanzafame, A. C; Guinan, E. F (2010). "RACE-OC project: Rotation and variability of young stellar associations within 100 pc". Astronomy and Astrophysics. 520: A15. arXiv:1004.1959Freely accessible. Bibcode:2010A&A...520A..15M. doi:10.1051/0004-6361/200913644. 
  28. ^ Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S. 
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  30. ^ Leggett, S. K.; et al. (2017). "The Y-type Brown Dwarfs: Estimates of Mass and Age from New Astrometry, Homogenized Photometry, and Near-infrared Spectroscopy". The Astrophysical Journal. 842 (2). 118. arXiv:1704.03573Freely accessible. Bibcode:2017ApJ...842..118L. doi:10.3847/1538-4357/aa6fb5. 
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  • Ridpath, Ian; Tirion, Wil (2017), Stars and Planets Guide, Princeton University Press, ISBN 978-0-00-823927-5 
  • Staal, Julius D.W. (1988), The New Patterns in the Sky: Myths and Legends of the Stars, The McDonald and Woodward Publishing Company, ISBN 0-939923-04-1 
  • Wagman, Morton (2003). Lost Stars: Lost, Missing and Troublesome Stars from the Catalogues of Johannes Bayer, Nicholas Louis de Lacaille, John Flamsteed, and Sundry Others. Blacksburg, Virginia: The McDonald & Woodward Publishing Company. ISBN 978-0-939923-78-6. 

External links[edit]

Coordinates: Sky map 05h 00m 00s, −80° 00′ 00″