S Monocerotis, or 15 Monocerotis, is a bluish whitevariable star and is located at the center ofNGC 2264. The variation in its magnitude is slight (4.2–4.6). It has a companion star of visual magnitude 8.
V838 Monocerotis, a variablered 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, theHubble Space Telescope was able to observe alight echo, which illuminated the dust surrounding the star.[1]
Monoceros also containsPlaskett's Star, a massive binary system whose combined mass is estimated, per 2008 calculations, to be almost 100 solar masses.[2]
One of the nearest knownblack holes to the Solar System is in this constellation. The binary star systemA0620-00 in the constellation of Monoceros is at a distance of roughly 3,300light-years (1,000parsecs) away. The black hole is estimated to be 6.6 solar masses.
Monoceros contains twoSuper-Earthexoplanets in oneplanetary system:CoRoT-7b was detected by theCoRoT satellite andCoRoT-7c was detected by theHigh Accuracy Radial Velocity Planet Searcher from ground-based telescopes. Until the announcement ofKepler-10b in January 2011, CoRoT-7b was the smallest exoplanet to have its diameter measured, at 1.58 times that of theEarth (which would give it a volume 3.95 times Earth's). Both planets in this system were discovered in 2009.
Part of thegalactic plane goes through Monoceros, so background galaxies are concealed byinterstellar dust. Monoceros contains many clusters and nebulae; most notable among them are:
TheRosette Nebula (NGC 2237, 2238, 2239, and 2246) is adiffuse 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 manyBok globules in its dark areas. It was independently discovered in the 1880s byLewis Swift (early 1880s) andEdward Emerson Barnard (1883) as they hunted forcomets.[4]
TheChristmas Tree Cluster (NGC 2264) is another open cluster in Monoceros. Named for its resemblance to aChristmas 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 starV429 Monocerotis represents its top.[5][6]
TheCone 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 severalHerbig–Haro objects, which are small irregularly variable nebulae. They are associated withprotostars.[7]
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 center; its stars range moderately in brightness.[8]
Hubble's Variable Nebula (NGC 2261) is a nebula with an approximate magnitude of 10, 2500 light-years from Earth. It is named forEdwin Hubble, and was discovered in 1783 by Herschel. Hubble's Variable Nebula is illuminated byR Monocerotis, a young variable star embedded in the nebula; the star's unique interaction with the material in the nebula makes it both anemission nebula and areflection nebula. One hypothesis regarding their interaction is that the nebula and its illuminating star are a very early stage planetary system.[9]
In Western astronomy, Monoceros is a relatively modern constellation, not one ofPtolemy's 48 in theAlmagest.[10] Its first certain appearance was on a globe created by the cartographerPetrus Plancius in 1612 or 1613[11] and it was later charted by German astronomerJakob Bartsch as Unicornu on his star chart of 1624.[12]
German astronomersHeinrich Wilhelm Olbers andLudwig Ideler[13]indicate (according toRichard Hinckley Allen's allegations) that the constellation may be older, quoting an astrological work[14]from 1564 that mentioned "the second horse between theTwins and theCrab has many stars, but not very bright"; these references may ultimately be due to the 13th century ScotsmanMichael Scot, but refer to a horse and not a unicorn, and its position does not quite match.Joseph Scaliger (died 1609) is reported[15] to have found Monoceros on an ancientPersian sphere. AstronomerCamille Flammarion (died 1925) believed that a former constellation, Neper (the "Auger"), occupied the part of that sky now deemed Monoceros andMicroscopium, but this is disputed.[15]
Chinese asterisms Sze Fūh, the Four Great Canals; Kwan Kew; and Wae Choo, the Outer Kitchen, all lay within the boundaries of Monoceros.[15]
