Eastward of Orion lies a stretch of sky with few distinguishing features to the naked eye. The Milky Way passes through this region from north to south, but there are no stars here brighter than fourth magnitude. This part of the sky did not therefore acquire a common traditional identity as a distinct group of stars. Historically, there are only a handful of disputed earlier references to a constellation here, and it was not until the seventeenth century that these sparse and faint stars were definitively collected into a constellation, which acquired the name Monoceros the Unicon.
Even more distant is the binary system known as Plaskett's Star or V640 Monocerotis. This system lies more than 5,000 light years distant (and some estimates place it even farther away). This system is noted as being one of the most massivebinary pairs known, with a combined mass approximately one hundred times greater than that of the Sun.
Much closer than any of these stars, but far, far too faint to be detected with the naked eye, lies an object designated UGPS J0722-0540 in the eastern parts of Monoceros. This is a substellar object many times the mass of Jupiter, probably a brown dwarf or possibly a massive rogue planet. It is notable as lying within the immediate neighbourhood of the Sun, at a distance of just 13.3 light years.
Deep Sky Objects
Towards of the north of Monoceros lies one of the most distinctive nebulae in the sky, the Rosette Nebula. This is a star-forming region composed of ionised hydrogen, and within the centre of the nebula a compact cluster of stars, the Rosette Cluster, has come into existence. Pressure from these young stars has created a central bubble within the nebula, so that the general appearance is that of a rosette, with colourful plumes extending from a circular region in its centre.
Along the southern borders of Monoceros lie a string a star clusters and nebulae, including the open clustersM50 and C54, as well as a wide region of nebulae and clusters collectively known as the Seagull Nebula. Towards the southwest of the constellation is the remarkable object known as the Red Rectangle (formally V777 Monocerotis or HD 44179). This is a binary system in the process of developing a planetary nebula, creating an 'X'-shaped pattern with a strangely geometric and regular form.