Collinder 112

Collinder 112 is a very young open star cluster in the constellation Monoceros, also known as Cr 112, NGC 2264 and the Christmas Tree Cluster. NGC 2264 is the catalogue number for the whole region, which also contains the Cone Nebula, the Fox Fur Nebula and the Snowflake Cluster. It is about 2,300 to 2,350 light-years away. It lies in the Monoceros OB1 cloud complex, and the cluster is elongated along a north-west to south-east axis, with two sub-clusters in the southern part. Estimates of the number of young stars range from “more than 600” to many more, because the region is rich in young stellar objects of every evolutionary stage. It stands out for its Christmas-tree shape, with the bright blue O-type binary, 15 Monocerotis, forming the “trunk”. The stars are 1 to 5 million years old, and there is an age spread of about 3 to 4 million years between the older northern region around S Mon and the younger, still-active southern region. The Cone Nebula lies to the south and gets its shape from a dark cone of cold molecular hydrogen and dust in front of a faint emission nebula ionised by S Mon.