The Monoceros Constellation

Posted on Friday, 19 October, 2018 by Zheluo Cai

Monoceros is a faint constellation on the celestial equator. Its name is Greek for a unicorn. Its definition is attributed to the 17th-century Dutch cartographer Petrus Plancius. Monoceros is the 35th constellation in size, occupying an area of 482 square degrees. It is located in the second quadrant of the northern hemisphere and can be seen at latitudes between +75° and -90°, and belongs to the Orion family of constellations, along with Canis Major, Canis Minor, Lepus and Orion. It is bordered by Orion to the west, Gemini to the north, Canis Major to the south and Hydra to the east. Other bordering constellations include Canis Minor, Lepus and Puppis.

Monoceros is not easily seen with the naked eye, containing only a few fourth magnitude stars. Alpha Monocerotis has a visual magnitude of 3.93, slightly brighter than Gamma Monocerotis at 3.98. Beta Monocerotis is a triple star system, the three stars forming a triangle which seems to be fixed. The visual magnitudes of the stars are 4.7, 5.2 and 6.1. William Herschel discovered it in 1781 and commented that it is “one of the most beautiful sights in the heavens.” Epsilon Monocerotis is a fixed binary, with visual magnitudes of 4.5 and 6.5. S Monocerotis, or 15 Monocerotis, is a bluish-white variable star and is located at the centre of NGC 2264. The variation in its magnitude is slight (4.2–4.6). It has a companion star of visual magnitude 8. V838 Monocerotis, a variable red supergiant star, had an outburst starting on January 6, 2002; in February of that year, its brightness increased by a factor of 10,000 in one day. After the outburst was over, the Hubble Space Telescope was able to observe a light echo, which illuminated the dust surrounding the star. Monoceros also contains Plaskett’s Star, which is a massive binary system whose combined mass is estimated to be that of almost 100 Suns put together. The nearest known black hole to our Solar System is in this constellation. The binary star system A0620-00 (abbreviated from 1A 0620-00) in the constellation of Monoceros is at a distance of about roughly 3,300 light-years away. The black hole is estimated to be 6.6 solar mass.

Monoceros contains two super-Earth exoplanets in one planetary system: COROT-7b was detected by the COROT satellite, and COROT-7c was detected by HARPS from ground-based telescopes. Until the announcement of Kepler-10b in January 2011, COROT-7b was the smallest exoplanet to have its diameter measured, at 1.58 times that of the Earth (which would give it a volume 3.95 times Earth’s). Both planets in this system were discovered in 2009.

Monoceros contains many clusters and nebulae, most notable among them:

M50, an open cluster
The Rosette Nebula (NGC 2237, 2238, 2239, and 2246) is a diffuse nebula in Monoceros. It has an overall magnitude of 6.0 and is 4900 light-years from Earth. The Rosette Nebula, over 100 light-years in diameter, has an associated star cluster and possesses many Bok globules in its dark areas. It was independently discovered in the 1880s by Lewis Swift (early 1880s) and Edward Emerson Barnard (1883) as they hunted for comets.
The Christmas Tree Cluster (NGC 2264) is another open cluster in Monoceros. Named for its resemblance to a Christmas tree, it is fairly bright at an overall magnitude of 3.9; it is 2400 light-years from Earth. The variable star S Monocerotis represents the tree’s trunk, while the variable star V429 Monocerotis represents its top.
The Cone Nebula (NGC 2264), associated with the Christmas Tree Cluster, is a very dim nebula that contains a dark conic structure. It appears clearly in photographs but is very elusive in a telescope. The nebula contains several Herbig-Haro objects, which are small irregularly variable nebulae. They are associated with protostars.
NGC 2254 is an open cluster with an overall magnitude of 9.7, 7100 light-years from Earth. It is a Shapley class f and Trumpler class I 2 p cluster, meaning that it appears to be a fairly rich cluster overall, though it has fewer than 50 stars. It appears distinct from the background star field and is very concentrated at its centre; its stars range moderately in brightness.
Hubble’s Variable Nebula (NGC 2261) is a nebula with an approximate magnitude of 10, 2500 light-years from Earth. Though it is named for Edwin Hubble, it was discovered in 1783 by William Herschel. Hubble’s Variable Nebula is illuminated by R Monocerotis, a young variable star embedded in the nebula; the star’s unique interaction with the material in the nebula makes it both an emission nebula and a reflection nebula. One hypothesis regarding their interaction is that the nebula and its illuminating star are a very early stage planetary system.
NGC 2170 is a reflection nebula in Monoceros. It was discovered by William Herschel on October 16, 1784.
The Butterfly Nebula (NGC 2346) is a planetary nebula with an apparent magnitude of 11.6. It is approximately 2,000 light years distant. The central star is a spectroscopic binary with a period of about 16 days. It is a variable star, probably as a result of having dust in orbit around it. It is about 3,650 light years distant from Earth. The nebula is sometimes known as the Seagull Nebula. The nebula is located on the border between Monoceros and Canis Major constellation. It is an H2 region centred on the star HD 53367. IC 2177 was discovered by the Welsh amateur astronomer Isaac Roberts. IC 2177 is an emission nebula in Monoceros.
The Red Rectangle Nebula is a protoplanetary nebula in Monoceros. It was discovered in 1973. The binary star system at the centre of the nebula was first observed by the American astronomer Robert Grant Aitken in 1915. The nebula has an apparent magnitude of 9.02 and is approximately 2,300 light years distant from Earth. It is a compact bipolar nebula, and it envelops the binary star completely, obscuring its light. When the cool star at the centre evolves into a hot white dwarf in the next few thousand years, HD 44179 will become a planetary nebula.