Monoceros (Greek: Μονόκερως, "unicorn") is a faint constellation on the celestial equator. Its definition is attributed to the 17th-century cartographer Petrus Plancius. 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 contains only a few fourth magnitude stars, making it difficult to see with the naked eye. One of the nearest known black holes to the Solar System is in this constellation. The binary star system A0620-00 in the constellation of Monoceros is at a distance of roughly 3,300 light-years (1,000 parsecs) away. The black hole is estimated to be 6.6 solar masses. Monoceros contains two super-Earth exoplanets in one planetary system: CoRoT-7b was detected by the CoRoT satellite and CoRoT-7c was detected by the High Accuracy Radial Velocity Planet Searcher 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. Part of the galactic plane goes through Monoceros, so background galaxies are concealed by interstellar dust. Monoceros contains many clusters and nebulae; most notable among them are: Messier 50, 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 center; 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. It is named for Edwin Hubble, and was discovered in 1783 by 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. IC 447, a reflection nebula.