Most deep-sky thoughts turn to mighty Orion at the start of a new year, but lying a mere 10 degrees to the east-southeast of Betelgeuse, Orion’s brilliant red supergiant star, lies the wonderful Rosette Nebula (NGC 2237), one of the winter sky’s observing gems. Under a very transparent and moonless sky at an observing sight free from significant sources of light pollution, hints of its ghostly ring of nebulosity, over twice the size of the full Moon at its fullest extent, might be glimpsed through a pair of binoculars or a finderscope.
The Rosette is one of the finest emission nebulae in the entire sky, a huge, wreath-shaped cloud of ionised hydrogen gas that’s a stellar nursery, like the nearby Orion Nebula. It’s located in the northern reaches of the constellation of Monoceros, making it an easily accessible target for telescopes and wide-field imaging set-ups.
Embedded in the intricate nebula is the very pretty and youthful fifth-magnitude open cluster NGC 2244. The Rosette Nebula itself can prove difficult to see in hazy skies and from suburban locations, but individual stars in the cluster can be resolved without too much trouble through binoculars or a small telescope.
In common with the nearby Orion Nebula, the Rosette is a huge stellar nursery where ongoing star formation continues to populate the fifth-magnitude open star cluster NGC 2244, whose 23-arcminute form is embedded at the centre of the Rosette. The cluster’s energetic hot young O-type stars are blowing with fierce winds and unleashing torrents of unimaginably hostile ultra-violet radiation, both of which are evacuating the centre of the Rosette and beautifully and intricately sculpting the surrounding nebulosity.
The Rosette is formally designated NGC 2237, but, unsurprisingly and owing to its size and complexity, it boasts several other NGC identifications: NGC 2238, 2239 and 2246. This is thanks to nineteenth-century visual observers seeing and cataloguing different parts of the nebula before the era of photographic plates confirmed its true form.
How to observe
The Rosette Nebula lies in the more northerly reaches of Monoceros, just 10 degrees to the east-south-east of brilliant Betelgeuse, Orion’s red supergiant star. Closer in is magnitude +4.4 epsilon (ε) Monocerotis, which lies just over two degrees to the west. As January turns to February, the Rosette Nebula crosses the southern meridian from London at about 10pm GMT, when it lies at a healthy 43 degrees in elevation.
The open cluster NGC 2244 is well seen in a pair of 10 x 50 binoculars and finderscopes, while the nebula’s low contrast provides a tougher proposition for visual observers. However, it’s by no means impossible to glimpse the brighter arcs of nebulosity with small telescopes once you’ve allowed your eyes to become dark-adapted and your optics to cool to the ambient temperature. Why not try the neat technique of averted vision if you struggle to see a any trace of nebulosity?
From an observing site with a very dark sky, a humble 80–100mm (~ three- to four-inch) refractor working at a low power and with a UHC filter can present a ghostly ring of nebulosity on a moonless and transparent night (the first 10 days or so of December is a good time to go Rosette hunting). If you’re stuck under a typical suburban UK sky, a low-power view through a ‘scope in the 150-200mm (six- to eight-inch) range can show half a dozen or so of the brightest cluster stars together with a splash of fainter suns in the central void, surrounded by a faint nebulous glow.
Deep-sky imagers will have a veritable field day upon shooting the Rosette through seemingly bewildering combinations of filters, narrowband or otherwise. A wide-field set-up is the favoured option, but longer-focus instruments can flourish too by capturing superb zoomed-in vistas, including that of the Rosettes’ central void. Processing fun is mandatory, as the Rosette looks great through whichever colour palette it’s presented in.