Constellations: Mensa 'the Table Mountain' Content from the guide to life, the universe and everything

Constellations: Mensa 'the Table Mountain'

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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-ordinates1:Right Ascension 05h 30', Declination −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 that is close to the celestial south pole, which means it is fully visible only from the Earth's Southern Hemisphere. It is bordered by Octans to the south, Chamæleon and Volans to the east, Dorado to the north and Hydrus to the west. The constellation features no bright stars 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. Within this portion of the Cloud are nine star clusters which are featured in the NGC (New General Catalogue). Most of the Cloud, however, is in the neighbouring constellation of Dorado.


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 the constellation is not named after a piece of furniture. The star grouping was invented by the 18th Century French astronomer Nicolas Louis de Lacaille (1713 - 1762), who was working in Cape Town, South Africa. He was impressed by the bulk of Table Mountain, which looms above the town, and named the constellation Mons Mensae after it.

Four of the five brightest stars, alpha, gamma, eta and beta, do in fact bear a resemblance to the outline of Table Mountain2. The name of the constellation was shortened to Mensa by the International Astronomical Union in 1922 when it set in stone the present 88 constellations.

Mensa thus bears the distinction of being the only constellation named after an actual geographical feature. That is about the only eminence it bears, unfortunately.


A number of different systems are used to name the stars. The brightest and most interesting ones have proper names (such as Aldebaran, Sirius and Vega) given to them by various cultures over the centuries. None of the stars in Mensa have such names, as the constellation was unknown to 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 constellation3. Due to the difficulty in measuring brightness, the order of letters does not always correspond to the brightness.

The stars of Mensa are so faint they would not appear in lists of stars in any other constellation. They are given here in order of brightness, but since they are all fifth magnitude there is little difference between them. One star of interest is the yellow dwarf pi Mensae, because in 2001 a planet was discovered in orbit around it (see extrasolar planets table below).

StarDesignationBrightness (m)Distance
(light years4)
Spectral classification
α Menalpha Mensae+5.0933G6V
γ Mengamma Mensae+5.19160K2III
β Menbeta Mensae+5.31250G8III
θ Mentheta Mensae+5.45 B9V
η Meneta Mensae+5.47130K4III
κ Menkappa Mensae+5.47 B9V
π Menpi Mensae+5.6767G1IV

The Large Magellanic Cloud

Our galaxy, the Milky Way, is one of a group of about 30 galaxies of all shapes and sizes known as the Local5 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 over the border of Mensa, which is why it is mentioned here.

The Cloud is a dwarf galaxy, about a 10th of the size of the Milky Way, and is believed to be in orbit around it. This is not certain, though, as recent reports have suggested it may be just passing. The Cloud was recorded as early as 964 by Arabic astronomer Al Sufi, and may have been mentioned by early seafarer Amerigo Vespucci. It 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 the same sort of things as our own galaxy, including nebulae, 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 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, all globular clusters in the Large Magellanic Cloud, and, predictably enough, they are very faint. All are 168,000 light years away.

CatalogueBrightness (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

Extrasolar Planets

Extrasolar planets are ones outside our Solar System, orbiting other stars. They are a hot topic for astronomers, with new ones being discovered all the time. Because the distances involved are so huge, the planets have to be detected by indirect means, such as by analysing the effect their gravity has on their parent star. Up to 2011, two planets have been discovered in the Mensa constellation:

The star pi Mensae has a planet in orbit around it. This planet bears the title pi Mensae b as well as the more technical-looking HD 39091 b. Astronomers would like to find a planet in the 'habitable zone', at the right distance from the parent star for Earth-like life to be possible6. Although this planet passes through the habitable zone, its orbit is so elliptical it is too hot for life at its closest approach to the star, and too cold when furthest removed from the star. This also means no other planets could exist in the habitable zone around this particular star.

In 2011 a gas giant HD 38283 b was detected in orbit around white dwarf star HD 38283. The star is at the end of its life having used up all its fuel.

Extrasolar Planets Table

Star name or
catalogue number
catalogue number
Planet mass
(Jovian scale)
Orbital period
(Earth days)
Year of discoveryComments
HD 39091HD 39091 b10.352,0642001Highly eccentric (elliptical) orbit
HD 38283HD 38283 b0.343632011Habitable zone

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 six billion light years away, so we are 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 powered by supermassive black holes that are similar to the one believed to lurk at the heart of the 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 one or two 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 per cent of the world's population. The exact IQ figure depends on which test is used, but typically lies within the range of 130 - 150. The society is named after a table because it encourages 'round table discussion'7 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.

  • 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 kinds being recorded in the area. The most important is the Cape fynbos  (fine bush), which is unique to the region. Parts of 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'.

1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2Particularly when you remember that in the Southern Hemisphere south is at the top.3In some constellations (such as Ursa Major) he assigned the letters based on position.4A light year is the distance light travels in one year, roughly 10,000,000,000,000 kilometres.5This is a slightly different meaning of the word 'local' to the normal one.6Of course, other forms of life may be possible at completely different temperatures, but we do not really know what to look for.7Although the symbol of the organisation in fact features a square table.

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