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 a 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. Mensa (a grey patch above Dorado )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 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°. The whole constellation is visible to observers south of latitude 5°N.
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. 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) 33.26 ± 0.05 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. 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. 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 102 ± 3 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 4.99 times the solar radius, becoming an orange giant of spectral type K2III.
- Beta Mensae is slightly fainter at magnitude 5.31. Located 790 ± 40 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 414 ± 9 light-years from Earth, and Eta Mensae is an orange giant of spectral type K4 III.
- Pi Mensae is a solar-type (G1) star 59.7 ± 0.2 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
The Large Magellanic Cloud lies partially within Mensa’s boundaries, although most of it lies in neighbouring Dorado. It is a satellite galaxy of the Milky Way, located at a distance of 163,000 light-years. Among its stars within Mensa are W Mensae, an unusual yellow-white supergiant that belongs to a rare class of star known as an R Coronae Borealis variable, and HD 268835, a blue hypergiant that is girded by a vast circumstellar disk of dust. 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, and NGC 1848, a 27 million-year-old open cluster. Mensa contains several described open clusters.
PKS 0637-752 is a quasar located around 6 billion light-years from Earth, 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. Credit: Wikipedia.