GG: Constellations: Mensa 'the Table Mountain'
Created | Updated Feb 15, 2016
He was born under The Small Boring Group of Faint Stars which, as you know, lies between The Flying Moose and The Knotted String. It is said that even the ancients couldn't find anything interesting to say about the sign.
- Terry Pratchett, The Light Fantastic, describing the wizard Rincewind.
| Name: | Mensa ('MEN-sah') |
| Genitive: | Mensae ('MEN-say') |
| Meaning: | The Table Mountain |
| Short form: | Men |
| Area: | 153 square degrees |
| Co-ordinates: | 5h 30', -80° |
| Origin: | 18th Century |
Rincewind's 'Small Boring Group of Faint Stars' is a fictitious one, but if any real constellation were to bear the name, it would be Mensa. Mensa is a small constellation close to the celestial south pole, which means that it is fully visible only from the Earth's southern hemisphere. It is bordered by Octans to the south, Chamaeleon and Volans to the east, Dorado to the north and Hydrus to the west. The constellation features no bright stars at all and is the faintest of all the 88 officially recognised constellations. It has no Messier objects. The only real item of interest in the constellation is a portion of the Large Magellanic Cloud, a dwarf galaxy which is closely connected to our own Milky Way Galaxy. Within this portion of the Cloud are nine star clusters which are featured in the NGC Catalogue. Most of the Cloud, however, is in the neighbouring constellation of Dorado.
History
Mensa is so far south that it is in a region of the sky never seen by the ancient Greeks and Babylonians, who devised the ancient constellations, so it has no mythological associations.
The word Mensa is a Latin one meaning 'table', but Mensa is not named after a piece of furniture. The constellation was invented by 18th Century French astronomer Nicolas de Lacaille (1713 - 1762), who was working in Cape Town, South Africa. He was impressed by the bulk of the Table Mountain, which looms above the town, and named the constellation Mons Mensae after it. Four of the five brightest stars in the constellation, alpha, gamma, eta and beta, do in fact bear a resemblance to the outline of the Table Mountain1. The name of the constellation was shortened to Mensa by the International Astronomical Union in 1922 when they set in stone the present 88 constellations.
Mensa thus bears the distinction of being the only constellation named after an actual geographical feature. That's about the only distinction it bears, unfortunately.
Stars
There are a number of different systems used to name the stars. The brightest and most interesting ones have proper names given to them by various cultures over the centuries, such as Aldebaran, Sirius and Vega. None of the stars in Mensa have such names, as the constellation was unknown to the ancient people. The Bayer system was devised by Johann Bayer: each star is identified by a Greek letter followed by the genitive (possessive) form of the constellation name. Bayer usually assigned the letters in order of brightness, so that alpha is normally the brightest star in the constellation2. Due to difficulty in measuring exact brightnesses, the order of letters doesn't always correspond with the brightness.
The stars of Mensa are so faint that they wouldn't appear in lists of stars in any other constellation. They're given here in order of their brightness, but since they are all 5th magnitude, there is very little difference between them.
| Star | Designation | Brightness (m) | Distance (light years3) | Spectral classification |
| α Men | Alpha Mensae | +5.09 | 33 | G6V |
| γ Men | Gamma Mensae | +5.19 | 160 | K2III |
| β Men | Beta Mensae | +5.31 | 250 | G8III |
| θ Men | Theta Mensae | +5.45 | B9V | |
| η Men | Eta Mensae | +5.47 | 130 | K4III |
| κ Men | Kappa Mensae | +5.47 | B9V |
The Large Magellanic Cloud
Our galaxy is known as the Milky Way, and is one of a group of about 30 galaxies of all shapes and sizes known as the Local4 Group. The Milky Way is the second largest galaxy in the group, the biggest being the Andromeda Galaxy (M31). Third in the list is another spiral galaxy, the Triangulum Galaxy (M33). Fourth largest in the local group is an irregular shaped blob known as the Large Magellanic Cloud. Most of the Cloud lies in the constellation of Dorado, but the southern edge of it strays into the constellation of Mensa, which is why it is mentioned here.
The Large Magellanic Cloud is a dwarf galaxy, about one tenth the size of the Milky Way, and is believed to be in orbit around the Milky Way. This is not certain, though, as recent reports have suggested that it may be just passing. The Cloud was recorded as early as 964 AD by Arabic astronomer Al Sufi, and may have been mentioned by early seafarer Amerigo Vespucci, but only really came to the attention of the Western World when Ferdinand Magellan came back from his travels in 1519, so it is named after him, along with its brother, the Small Magellanic Cloud.
The Cloud, when examined closely, appears to contain all the same sort of things that you get in our own galaxy, including nebulas, clusters and even supernovae. Examination of the supernova SN1987A in the LMC in 1987 as it occurred provided an accurate distance to the Cloud of 168,000 light years.
New General Catalogue (NGC)
The NGC catalogue is a list of interesting deep-space objects (that is, objects outside our solar system). It was compiled by Dreyer at Armagh Observatory based on the observations of Sir William Herschel. Mensa has nine objects in the NGC catalogue and predictably enough, they are all very faint. These are all globular clusters in the Large Magellanic Cloud, and are all at a distance of 168,000 light years away.
| Catalogue | Brightness (m) |
| NGC 1711 | +10.1 |
| NGC 2019 | +10.9 |
| NGC 2134 | +11.1 |
| NGC 2065 | +11.2 |
| NGC 2107 | +11.5 |
| NGC 2058 | +11.9 |
| NGC 1943 | +12.0 (blue) |
| NGC 1987 | +12.1 (blue) |
| NGC 2121 | +12.4 |
A Quasar
Quasars are objects which appear to emit a huge amount of radiation from a very small volume of space. The name quasar comes from 'quasi-stellar' because originally they were thought to be like stars. Only when it was found that they are extremely far away did it become evident that quasars are really energetic. There is a quasar in Mensa called PKS 0637-752 which produces as much energy as 10 trillion suns, all from a volume smaller than our own solar system. The quasar is 6 billion light years away, so we're not in any danger of being fried by this intense blast. The leading theory is that quasars are the cores of very distant galaxies and that they are powered by supermassive black holes similar to the one believed to lurk at the heart of our own Milky Way.
Meteor Showers
The space debris which creates a meteor shower usually comes from the tail of a comet, as the Earth crosses the point where the comet has passed previously on its own orbit. Imagine a trail of breadcrumbs; now imagine breadcrumbs travelling at kilometre per second speeds and burning up in the Earth's atmosphere.
The meteor shower connected with this constellation is called the Delta Mensids, because they appear to radiate from a point in the sky close to the star delta Mensae. The shower extends over the week of 14 - 21 March. Analysis shows there are in fact two separate streams of debris, one giving rise to a maximum occurrence of meteors on 18 March, and the other on 19 March. Like all things to do with Mensa, this shower is a fairly insignificant affair, with only 1 or 2 meteors per hour at the peak. The debris is thought to come from Comet Pons, which was visible in 1804.
Mensa Trivia
The word 'Mensa' is traditionally one of the first words of Latin to be taught, as it has a very simple treatment in the language and is completely regular.
Mensa International is an organisation of supposedly super-intelligent people. The aim of the society is to provide a forum for intellectual exchange among its members. Members must have an IQ in the top 2% of the world's population. The exact IQ figure depends on which test is used, but typically lies in the range of 130 - 150. The society is named after a table because it encourages 'round table discussion'5 with no racial or other prejudices. It is likely the choice of name is also influenced by the phrase 'Mens sana' meaning a healthy mind.
The Table Mountain after which the constellation is named is a flat-topped mountain outside Cape Town in South Africa. It is about 3km wide and 1km high, with sloped ends. The mountain is a haven for exotic plant species, with over 2,200 different species being recorded in the area. The most important is the Cape fynbos, which is unique to the region. Parts of the Table Mountain are preserved as a UNESCO World Heritage site.
In ecclesiastical terminology, the mensa was the portion of church funds which an individual received to provide food for himself and his dependants, as opposed to looking after the building he lived in. So a bishop's mensa would provide for the daily expenses of the bishop and all his staff, but separate funds were allocated for the upkeep of the bishop's cathedral.
The association with dining tables has led to the canteen in a German university being referred to as 'mensa'.
