One of the faintest constellations in the night sky, 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.
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.
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.[a]
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.[b]
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.
HD 39194 is an orange dwarf of spectral type K0V and magnitude 8.08, located 86.21 ± 0.09 light-years distant. Three planets in close orbit were discovered by the High Accuracy Radial Velocity Planet Searcher (HARPS) in 2011. The three take 5.6, 14 and 34 days to complete an orbit around their star, and have minimum masses 3.72, 5.94 and 5.14 times that of the Earth respectively.
TZ Mensae is an eclipsing binary that varies between magnitude 6.2 and 6.9 every 8.57 days. 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 that of the Sun and is 1.5 times as massive.
UX Mensae is another eclipsing binary system composed of two young stars around 1.2 times as massive as the Sun and 2.2 ± 0.5 billion years of age, orbiting each other every 4.19 days. The system is 338.2 ± 0.9 light-years distant.
TY Mensae is another eclipsing binary system classified as a W Ursae Majoris variable; the two components are so close that they share a common envelope of stellar material. The larger star has been calculated to be 1.86 times as massive, have 1.85 times the diameter and is 13.6 times as luminous, while the smaller is 0.4 times as massive, 0.84 times the diameter, and 1.7 times as luminous as the Sun. Their surface temperatures have been calculated at 8164 and 7183 K respectively.
YY Mensae is an orange giant of spectral type K1III around 2.2 times as massive as the Sun, with 12.7 times its diameter and 70 times its luminosity. A rapidly rotating star with a period of 9.5 days, it is a strong emitter of X-rays and belongs to a class of star known as FK Comae Berenices variables. These stars are thought to have formed with the merger of two stars in a contact binary system. With an apparent magnitude of 8.05, it is 707 ± 6 light-years distant.
AH Mensae is a cataclysmic variable star system composed of a white dwarf and a red dwarf that orbit each other every 2 hours 57 minutes. The stars are close enough that the white dwarf strips material off the red dwarf, creating an accretion disc that periodically ignites with a resulting brightening of the system.
TU Mensae is another cataclysmic variable composed of a red dwarf and white dwarf. The orbital period of 2 hours 49 minutes is one of the longest for cataclysmic variable systems exhibiting brighter outbursts, known as superhumps. The normal outbursts result in an increase in brightness lasting around a day every 37 days, while the superhumps last 5–20 days and take place every 194 days.
PKS 0637-752 is a distant quasar with a calculated 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 330,000 light-years long. It is visible at radio, optical and x-raywavelengths.
^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 practically unobservable.
^Objects of magnitude 6.5 are among the faintest visible to the unaided eye in suburban–rural transition night skies.
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The Deep Photographic Guide to the Constellations: Mensa