Monoceros

Monoceros lies next to Orion on the band of the Milky Way, and so the area of space it describes is on the plane of our Galaxy. Much of the Milky Way visible in this constellation belongs to the Orion Arm of the Galaxy.

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.

Though Monoceros itself is faint and difficult to detect with the unaided eye, its location in the sky is easy to find, as it is surrounded by bright stars in neighbouring constellations. The unmistakable shape of Orion marks its western boundary, while brilliant Sirius in Canis Major lies directly to its south. Northward and eastward is Procyon in Canis Minor, and directly to the north is Alhena in Gemini. An imaginary triangle formed by Betelgeuse, Sirius and Procyon (the three brightest stars in this area) would have its centre in the heart of Monoceros.

Stars

The naked-eye stars of Monoceros are scattered across the constellation. These are in fact generally luminous giant or supergiant stars, but for the most part they lie hundreds of light years distant from the Sun, and so appear faint in the skies of Earth.

The brightest individual star in the constellation is Alpha Monocerotis (given the proper name Samoht in some sources) in the southeast, near the northern edge of Puppis. This is a yellow giant star with a visual magnitude of +3.9, lying at a distance of some 150 light years. It is slightly outshone by Beta Monocerotis, a blue star to the west; this is a triple system nearly 700 light years distant, in which none of the component stars are brighter than Alpha Monocerotis, but in combination they reach a magnitude of +3.7.

Even more distant than the Beta system is Zeta Monocerotis on the Unicorn's eastern border with neighbouring Hydra. This is an intensely luminous yellow supergiant with an absolute magnitude of -3.2, but its great distance of some 1,100 light years means that it shines in the sky at just magnitude +4.4. The closest of the clearly visible stars in the constellation is the white subgiant Epsilon Monocerotis in the north, a binary system that falls some 134 light years from the Solar System.

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 massive binary 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.

Near the Rosette Nebula in the sky lie various other notable nebulae. Hubble's Variable Nebula (C46) is a reflection nebula illuminated by the variable star R Monocerotis, so that, as the star's brightness changes, so does the reflected light across the tapering shape of the nebula. A little to the north of this nebula is another, the Cone Nebula (NGC 2264), in which a dark mass obscures the material behind it to form a conical shape against the illuminated background of the extensive Christmas Tree Cluster.

Along the southern borders of Monoceros lie a string a star clusters and nebulae, including the open clusters M50 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.