Star Constellation Facts: Monoceros

Monoceros (“The Unicorn”) is a faint northern sky constellation that lies on the celestial equator, and can be seen by observers located between latitudes +75 and -90 degrees. It is the 35th largest constellation, but despite being faint and difficult to recognise, Monoceros nevertheless contains many notable stars, among which is Plaskett’s Star, the most massive binary system yet discovered. Deep sky objects in Monoceros include among many others, the Rosette Nebula, the Christmas Tree Nebula, the Cone Nebula, and one open cluster Messier object, M50 (NGC 2323). In addition, Monoceros contains 16 stars with confirmed planets.


Monoceros was “invented” in the 17th century by Petrus Plancius, a Dutch astronomer and cartographer, based on observations made by Dutch navigators to fill a perceived gap between the constellations of Orion and Hydra. Plancius’s motivation for creating a constellation where there were none in ancient Greek times, is said to have been the fact that the unicorn appears several times in the Old Testament of the Bible. Whatever the motivation behind the creation of Monoceros, it is one of only a few “new” constellations included in the 88 constellations recognised by the International Astronomical Union (IAU) when many arbitrary “constellations” were removed in the 1920’s.

Orion Family of Constellations

Monoceros borders on the constellations Canis Major, Canis Minor, Gemini, Hydra, Lepus, Orion and Puppis, and forms part of the small Orion family of constellations along with Canis Major, Canis Minor, Lepus and Orion.

Notable Stars

Monoceros has only a few stars brighter than magnitude 4.0, but what the constellation lacks in bright stars, it makes up for with the many massive stars it contains. Below are details on some notable stars in the constellation.

– Alpha Monocerotis; with an apparent visual magnitude of 3.94, the orange giant Alpha Monocerotis is the most luminous star in the constellation. It is about 144 light years away, has a classification of K0 III, and is about 10.1 times as big as the sun, but with only slightly more than twice its mass.

Star Constellation Facts: Monoceros

– Zeta Monocerotis; classified as G2IB, this massive, yellow supergiant is a spectacular 2,535 times as luminous as the Sun, and a whopping 62 times as big. However, its distance from Earth is about 1,852 light years, which gives it an apparent magnitude of only 4.37. Look for this star about three-quarters of a degree from the border between Hydra and Monoceros.

– 13 Monocerotis; the A0Ib white supergiant 13 Monocerotis is about 1,500 light years away, and is even more luminous; it shines 10,800 times brighter than the Sun, has 9 solar masses, and is 37 times as big as the Sun. Together with Epsilon Monoceros, it forms the “gateway” to the Rosette Nebula, which is located a few degrees to the southward of the pair.

It is thought that 13 Monoceros was ejected from the star cluster NGC 2264, which is located only 3.5 degrees to the north of it. One other notable feature of the star is that it appears to be enveloped by a dim reflection nebula, as a result of its light being scattered over a 10-light year area by a relatively dense cloud of interstellar dust.

– HD 48099; as bright stars go, HD 48099 is in a league of its own. Due to its distance of nearly 41,000 light years from Earth, its -9.11 absolute magnitude is dimmed to only an apparent magnitude of 6.37, which means that this star must be at least 350,000 times as bright as the Sun. HD 48099 is in fact a very close binary system with an orbital period of only 3.078 days. Both components are thought to be O-class stars, the one being an O5.5 star, and the other an O9 class star.

V838 Monocerotis– V838 Monocerotis, which is about 20,000 light years away, is arguably one of the most famous stars in the entire sky, even though the cause of its outburst is still unknown.
Nevertheless, in its previous life, V838, an M6.31 star, was almost invisible until its sudden outburst in 2002 started lighting up its surroundings. The visible matter in the image does not form part of the star, instead it is matter surrounding the star that has been made visible by the spreading light echo from the outburst. One strange feature of the red supergiant V838 is the fact that it did not blow off its outer layers, which is what normally happens when stars explode, or suffer an outburst. Nonetheless, even though V838’s temperature has increased, it’s diameter has decreased, and it is now about 15,000 times brighter, and 370 times bigger than the Sun.

Deep Sky Objects

Monoceros contains many deep sky objects, among which is M50, a pretty open cluster. Below are details of some of the more spectacular deep sky objects in The Unicorn.

Messier 50 (NGC 2323); located about 3,200 light years away, this open cluster that was discovered by Charles Messier in 1772, has an apparent visual magnitude of 5.9, and is famous for the arrangement of its constituent stars, which is said to form the popular view of a human heart. However, the shape of a heart is not readily apparent.

Rosette NebulaRosette Nebula (NGC 2237, NGC 2244, NGC 2238, NGC 2246; Caldwell 49); located about 5,200 light years away, the Rosette Nebula is an H II region close to a large molecular cloud, and has an apparent visual magnitude of 9.0. As nebulae go, The Rosette Nebula is large, with a radius of close to 65 light years, which is big enough to have supplied the material from which NGC 2244, the star cluster in the centre of the nebula, formed.

Hubble’s Variable Nebula (NGC 2261, Caldwell 46); located about 2,500 light years away, Hubble’s Variable Nebula has an apparent visual magnitude of 9.0, and its variability is thought to be the result of dust and gas in the nebula that periodically obscures the light from the star R Monocerotis, which is buried deep within the nebula, and therefore not directly visible.

Red Rectangle NebulaRed Rectangle Nebula (HD 44179); first observed by Robert Grant Aiken in 1915, this spectacular nebula located about 2,300 light years away, with an apparent magnitude of 9.02, is in fact a proto-planetary nebula. Before the advent of the Hubble Space Telescope, the nebula appeared to be rectangular, but from this image taken by Hubble, it seems the nebula is more shaped like an X. It is expected that when the relatively cool star in the centre of the structure evolves into a hot white dwarf sometime in the next several thousand years, the structure will develop fully into a planetary nebula on the lines of M57 in Libra.

Meteor Showers

Two meteor showers emanate from Monoceros- the Alpha Monocerids, and the December Monocerids.

Alpha Monocerids; also known as the November Monocerotids, this shower peaks on November 21, but some activity may be observed from the 13th of November to the 2nd of December. However, peaks of more than 100 meteors per hour is said to occur every ten years on the night of November 20th/21st.

December Monocerids; although the December Monocerids peaks on the 8th of December each year, the shower persists from as early as the 9th of November to as late as the 11th of December. However, visual observations rarely exceed two meteors per hour, and it is suggested that an optical aid be used to observe this shower.

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