Mensa (constellation)

Mensa is a constellation in the Southern Celestial Hemisphere near the south celestial pole, one of fourteen constellations drawn up in the 18th century by French astronomer Nicolas-Louis de Lacaille. Its name is Latin for table, though it originally commemorated Table Mountain and was known as "Mons Mensae". One of the eighty-eight constellations designated by the International Astronomical Union (IAU), it covers a keystone-shaped wedge of sky 153.5 square degrees in area. Other than the south polar constellation of Octans, it is the most southerly of constellations and is observable only south of the 5th parallel of the Northern Hemisphere. The faintest constellation in the night sky recognized by the IAU, Mensa contains no apparently bright stars—the brightest, Alpha Mensae, is barely visible in suburban skies. Part of the Large Magellanic Cloud, several star clusters and a quasar lie in the area covered by the constellation, and at least three of its star systems have been found to have exoplanets.

History Originally named "Montagne de la Table" or "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, near the location of Lacaille's observatory. 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. 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 symbolic of the Age of Enlightenment. Sir John Herschel proposed shrinking the name to one word in 1844, noting that Lacaille himself had abbreviated some of his constellations thus. 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.

Characteristics 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. The three-letter abbreviation for the constellation, as adopted by the IAU in 1922, is "Men". The official constellation boundaries, as set by Belgian astronomer 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°. The whole constellation is visible to observers south of latitude 5°N.

Features

Stars

Bright stars Lacaille gave eleven stars in the constellation Bayer designations, using the Greek alphabet to label 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 their naming. Alpha Mensae is the brightest star with a barely visible apparent magnitude of 5.09, making it the only constellation with no star above magnitude 5.0. Overall, there are 22 stars within the constellation's borders brighter than or equal to apparent magnitude 6.5.

Alpha Mensae is a solar-type star (class G7V) 33.32 ± 0.02 light-years from Earth. It came to within 11 light-years from Earth around 250,000 years ago and would have been considerably brighter back then—nearly of second magnitude. An infrared excess has been detected around this star, indicating the presence of a circumstellar disk at a radius of over 147 astronomical units (AU). The estimated temperature of this dust is below 22 K. However, data from Herschel Space Observatory failed to confirm this excess, leaving the finding in doubt. 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. Gamma Mensae is the second-brightest star in the constellation, at magnitude 5.19. Located 104.9 ± 0.5 light-years from Earth, it is an ageing (10.6 billion year-old) star around 1.04 times as massive as the Sun. It has swollen to around 5 times the solar radius, becoming an orange giant of spectral type K2III. Beta Mensae is slightly fainter at magnitude 5.31. Located 660 ± 10 light-years from Earth, 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, and lies in front of the Large Magellanic Cloud. Zeta and Eta Mensae have infrared excesses suggesting they too have circumstellar disks of dust. Zeta Mensae is an ageing white giant of spectral type A5 III around 394 ± 4 light-years from Earth, and Eta Mensae is an orange giant of spectral type K4 III, lying 650 ± 10 light-years away from Earth. Pi Mensae is a solar-type (G1) star 59.62 ± 0.07 light-years distant. In 2001, a substellar companion was discovered in an eccentric orbit. 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. The discovery of a second substellar companion—a super-Earth—was announced on 16 September 2018. It takes 6.27 days to complete its orbit and is the first exoplanet detected by the Transiting Exoplanet Survey Satellite (TESS) submitted for publication.

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