Mensa (“the table”) is a small constellation that is the second southernmost in the night sky, meaning that it is not visible from the northern hemisphere. Being so far south means that it is also circumpolar, but the constellation is notoriously difficult to find as its brightest star, a yellow dwarf called Alpha Mensae, has a visual magnitude of just 5.09. Mensa is therefore considered to be the faintest of all the 88 recognized constellations.
Mensa is a southern hemisphere constellation that can be seen by observers located between +4° and -90° of latitude, although best viewed in summer when it reaches its highest elevation. Mensa is the sky’s 75th largest constellation, with its 4 main stars forming a shape which depicts Table Mountain in Cape Town.
Being located so close to the south celestial pole means that southern hemisphere residents can see Mensa all year-round, with the Large Magellanic cloud acting as a useful reference point for finding this faint constellation. Neighboring constellations of Mensa includes Chamaeleon, Dorado, Hydrus, Volans and Octans, the night sky’s southernmost constellation.
Lacaille Constellation Family
Mensa is a member of the Lacaille family of constellations, together with Antlia, Caelum, Circinus, Fornax, Horologium, Microscopium, Norma, Octans, Pictor, Reticulum, Sculptor, and Telescopium.
Mensa is one of 14 southern constellations created by French astronomer Nicolas Louis de Lacaille during his 1751/1752 stay in South Africa. He named it Mons Mensae (“table mountain”) after Table Mountain which overlooks Cape Town, whose clouds also reminded him of the Magellanic clouds which are found in the constellation. British astronomer Francis Baily later shortened its name to just Mensa.
– Alpha Mensae, the constellation’s brightest star, is a yellow dwarf (G5 V) situated 33 years light years away with a magnitude of only 5.09. It is around 5.4 billion years old, and has a similar radius and mass as the Sun, but just 83% its luminosity.
– Gamma Mensae, the second most luminous star in Mensa, is an orange giant (K4III) located 101 light years distant with an apparent visual magnitude of 5.18.
– Beta Mensae, the constellation’s third brightest star, is a yellow giant (G8III) found 640 light years from our solar system of magnitude 5.302.
– Eta Mensae is an orange giant star (K4III) located 712 light years away that shines with a magnitude of 5.47.
Other stars of interest in the constellation includes the blue stars Theta Mensae and Mu Mensae; the white giant Zeta Mensae; the yellow stars Pi Mensae and W Mensae; and the orange giant Lambda Mensae.
Notable Deep Sky Objects
The constellations of Mensa, together with Dorado, contains the Large Magellanic Cloud (LMC), a nearby galaxy that is 14,000 light years across and visible to the naked eye as a faint cloud. It is found 163,000 light years from Earth and due to gravitational attraction it orbits our own Milky Way as a satellite galaxy. The only other galaxies closer to us than LMC is the Canis Major Dwarf Galaxy found 25,000 light-years away, and the Sagittarius Dwarf Spheroidal galaxy located 70,000 light-years distant.
Mensa also contains the globular star cluster NGC 1987, as well as a quasar situated 6 billion light years away called PKS 0637-752, that is 10 trillion times more luminous than our sun, and is believed to be powered by a supermassive black hole.