Constellations: Monoceros 'the Unicorn' Content from the guide to life, the universe and everything

Constellations: Monoceros 'the Unicorn'

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The shield of the Science, Mathematics and Engineering faculty of the h2g2 University.Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
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V838 Monocerotis.
The unicorn... because of its intemperance, not knowing how to control itself before the delight it feels towards maidens, forgets its ferocity and wildness, and casting aside all fear it will go up to the seated maiden and sleep in her lap, and thus the hunter takes it.
–  Leonardo da Vinci

Unicorns, horses with a single horn protruding from their forehead, are legendary magical creatures whose blood can supposedly render immortality upon the drinker. However, to slay a unicorn was said to condemn one's soul to eternal damnation.

In ancient Babylon, the cradle of civilisation, people worshipped the unicorn as far back as 3500 BC. The most widely accepted reason for the unicorn's extinction was that they preferred to play in the rain rather than accept Noah's invitation to board the Ark. Consequently, none were saved to repopulate the Earth after the Great Flood. One which is still around but never seen is, of course, the invisible pink unicorn.

The Constellation Monoceros the Unicorn

Name:Monoceros (Greek: 'one horn')
Short form:Mon
Area:482 sq deg
Co-ordinates1:Right Ascension 07h, Declination −05°

The constellation Monoceros, literally 'one horn', is situated between Orion and Hydra, with Orion's hunting dogs Canis Minor above and Canis Major below. Monoceros also shares its borders with Gemini, Lepus and Puppis. The unicorn straddles the brightest star in the night sky, Sirius (alpha Canis Majoris).

A unicorn2 is mentioned in the Old Testament of the Bible. Because of this, the Dutch cartographer Petrus Plancius (1552 - 1622) is credited with introducing the constellation. Johann Bayer (1572 - 1625) included Monoceros in his Uranometria, a 1603 sky atlas.


The scientific star names are simple to understand (if you know your Greek alphabet). For example: 'alpha Monocerotis' means it is the brightest star in the constellation Monoceros. The next brightest is designated 'beta', and so on. Combined with the genitive name, this is known as the 'Bayer designation'. Some stars have proper names as well, for example, alpha Monocerotis is Ctesias. Others are known by their catalogue number.

Monoceros Stars

There are no stars above fourth magnitude in the unicorn, so Monoceros is not a prominent constellation. Sometimes stars fluctuate in brightness, or small errors were made at the time of measuring, so the scientific designations are a little off. In this case the brightest star of the constellation is the beta star Cerastes - a magnificent three-star system which forms a triangle shape. Its discoverer, Sir William Herschel, described it as 'one of the most beautiful sights in the heavens'.

R Monocerotis is (probably) a rare, isolated, massive star surrounded by NGC 2261, known as Hubble's Variable Nebula. Some astronomers dismiss the existence of R Monocerotis, as it does not display a normal stellar spectrum. It was not even listed in the official 2000 sky atlas. Other prominent astronomers insist it is an irregular variable star, may be a protoplanetary system, and is the fuelling source of Hubble's Variable Nebula.

We know the nebula exists so, to try to solve the puzzle, the area has been studied extensively by powerful telescopes at the Mauna Kea Observatories in Hawaii. Other data was collated by the Hubble Space Telescope. The conclusion is that R Monocerotis is probably an RW Aurigae-type variable with a T Tauri-type dwarf star companion. So we're going to have to live with the mystery a while longer.

S Monocerotis is not that unusual a star, except when hosted by the Astronomy Picture of the Day website: you not only see the bright star with its companions but also the fabulous red gas and blue reflection of two nebulae and two star clusters. Where the two colours mix is a lovely shade of pink. What a diva! S Monocerotis is visible to the naked eye. But you need a powerful optical lens to view the surrounding nebulosity of NGC 2264.

Plaskett's Star is the most massive binary system found at the time of writing. Consisting of two blue giant stars, the system was discovered in 1922 and studied extensively by Canadian astronomer John Stanley Plaskett, who had helped to persuade his government to build the Dominion Astrophysical Observatory near Victoria, British Columbia. It housed a 72-inch (183cm) reflecting telescope, the biggest in the world in the early 20th Century.

The 2002 nova V838 Monocerotis can be seen rapidly expanding in the NASA HubbleSite images. The star is a supergiant, 20,000 light years3 away, at the outer edge of our Milky Way galaxy. In 2004 a new Hubble image of the expanding halo surrounding the star was likened to Starry Night, a painting by Vincent van Gogh.

Star Table

StarDesignationName or
catalogue number
Brightness (m)Distance
(light years)
Spectral classification
and/or comments
β Monbeta MonCerastes+3.7690Triple star system
α Monalpha MonCtesias+3.9144Yellow giant
γ Mongamma MonTempestris+3.9645Orange giant
δ Mondelta MonKartajan+4.1370White dwarf
ζ Monzeta Mon29 Mon+4.31,800Yellow giant
ε Monepsilon Mon8 Mon+4.4130White subgiant
S MonHD 4783915 Mon+4.6 var1,000Blue giant
HR 2422 MonGC 8631Plaskett's Star+6.056,600Massive binary star system
V838 MonGSC 04822-00039Nova Monocerotis 2002Var20,000Major outburst 2002

Red Rectangle Nebula

The webbed, X-shaped protoplanetary Red Rectangle Nebula creates an intriguing mystery: why is it such a strange shape? It has been speculated that the central binary star system creates a pair of jets that spin. More than 2,300 light years away, we see the 'drag' of the fast spinning, suspended like the still of a film stuck on freeze-frame; hence the webbed effect between the jets.

New General Catalogue (NGC)

The NGC was compiled by John Louis Emil Dreyer (director of the Armagh Observatory from 1882 - 1916).

NGC in Monoceros

NGC 2261 aka Hubble's Variable Nebula is illuminated by the star R Monocerotis, which has ten times the mass of our own Sun. It is thought the nebula changes shape every month because the dust and gas pass so close to R Monocerotis that they are affected by its solar wind. The gasses excite and flare up, and the movement creates shadows on the rest of the nebula. Edwin Hubble took great fancy to this unusual nebula and, in his honour, it was subsequently named after him.

NGC 2264 is the designation for four objects: the Christmas Tree Cluster, the Snowflake Cluster, the Cone Nebula and the Fox Fur Nebula.

NGC 2237 aka the Rosette Nebula has several super-hot blue giant stars which emit radiation and intense solar winds, making the surrounding area particularly volatile. If there are any cooler stars in the way of these dangers, their planet-forming dust is likely to be blown away. The safe zone is reckoned to be 1.6 light years (ten trillion miles).

There is one Messier object in Monoceros - an open star cluster which was discovered by the Italian astronomer Giovanni Domenico Cassini (1625 - 1712) prior to Messier's cataloguing of it as M50 in 1772. Its NGC designation is NGC 2323.

NGC Table

CatalogueNameTypeBrightness (m)Distance
(light years)
NGC 2237Rosette NebulaEmission nebula+95,200X-ray emitter
NGC 2244Open cluster+4.85,200Within Rosette Nebula
NGC 2261Hubble's Variable NebulaReflection nebula+92,500Fan-shaped nebula illuminated by
R Monocerotis
NGC 2264Four objects+3.92,600Two nebulae and two clusters
NGC 2323M50Open cluster+63,200Early Cassini discovery

Extrasolar Planets in Monoceros

Many extrasolar planetary systems have been found in the constellation Monoceros up to 2009; the first was discovered in 2000. The figures given in the table below are the length of the planet's orbital period around its parent star, which we know of as a year. The mass of the extrasolar planet is compared with that of Jupiter, our solar system's largest planet. It is known by astronomers as the 'Jovian scale'.

The planet CoRoT-1 b in Monoceros was the first to be discovered by the French-led CoRoT satellite mission launched on 27 December, 2006. It is hoped the instrumentation onboard is sensitive enough to detect the passage of rocky planets transiting their stars.

Extrasolar Planets Table

Star name or
catalogue number
catalogue number
Planet mass
(Jovian scale)
Orbital period
(Earth days)
Year of discoveryComments
HD 52265HD 52265 b1.11202000Eccentric orbit
HD 46375HD 46375 b0.21002000Hot sub-Saturn
CoRoT-1CoRoT-1 b1.31.52007First CoRoT discovery
CoRoT-4CoRoT-4 b0.729.22008Hot gas giant
CoRoT-5CoRoT-5 b0.4674.032008Hot gas giant
CoRoT-7CoRoT-7 b0.0150.852009Hot super Earth
CoRoT-7CoRoT-7 c0.023.72009Hot super Earth
HD 45652HD 45652 b0.47432008Hot gas giant
HD 66428HD 66428 b2.821,9732006Orbits Sun-like star
HD 44219HD 44219 b0.584722009Gas giant
CoRoT-12CoRoT-12 b0.92.832010Hot gas giant
CoRoT-13CoRoT-13 b1.34.042010Hot gas giant
CoRoT-14CoRoT-14 b7.61.52010Hot superjovian
CoRoT-15CoRoT-15 b63.33.062010Brown dwarf
CoRoT-18CoRoT-18 b3.51.92011Hot superjovian
CoRoT-19CoRoT-19 b1.13.92011Hot gas giant
CoRoT-20CoRoT-20 b4.249.242011Hot superjovian
CoRoT-21CoRoT-21 b2.262.722011Hot superjovian
CoRoT-24CoRoT-24 b0.15.12011Hot sub-Saturn
CoRoT-24CoRoT-24 c0.1711.752011Hot sub-Saturn

Unicorns in Modern Culture

  • The unicorn is a heraldic supporter that appears in many coats of arms, including the royal coat-of-arms of Great Britain, in which the unicorn stands for Scotland.

  • Powdered unicorn horn is prized for its aphrodisiac properties.

  • A group of unicorns appear in Disney's Fantasia.

  • The Qilin is a legendary Chinese unicorn.

  • Sir Thomas Browne stated in his Pseudodoxia Epidemica that unicorn horn was an antidote to poison.

  • The Honda Unicorn is a silver and black motorcycle.

  • In the 'Harry Potter' universe, the unicorn lives in the Forbidden Forest. The book Harry Potter and the Sorceror's Stone4 features a unicorn on the cover.

  • The Unicorn Tapestries are among the most beautiful and complex works of art to survive from the Middle Ages. They can be seen at The Cloisters, a branch of the Metropolitan Museum of Art, New York.

1Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.2Although the accuracy of the translation is still being debated, the original words refer to a 'large wild animal'.3A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.4The American edition of Harry Potter and the Philosopher's Stone.

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