Deep-Sky Tour
This month’s star tour starts off with a suitably seasonal theme
Tick the box when you’ve seen each one
1 NGC 2264 PART 1: THE CHRISTMAS TREE CLUSTER
At mag. +6.5, NGC 457 isn’t a tricky object. The official IAU designation NGC 2264 confusingly covers two objects – the Christmas Tree Cluster and the Cone Nebula. For our first target we’ll look at the easier-to-see cluster. The key here is to locate the multiple and variable star S Monocerotis, also known as 15 Monocerotis. The star’s variability range is small, its brightness barely wavering between mag. +4.6 and mag. +4.7. It marks the trunk of a Christmas tree-shaped cluster of stars. Mag. +7.2 HD 47887, located 26 arcminutes to the south, marks the top of the tree. The upside down tree shape is surprisingly easy to pick out with a small telescope using a low power, wide-field eyepiece. We’d recommend making sure you’ve identified it correctly before moving on to target two. SEEN IT
2 NGC 2264 PART 2: THE CONE NEBULA
The Cone Nebula also falls under the heading NGC 2264. To find it, start at the Christmas Tree Cluster. Try to spot the reflection nebulosity surrounding S Monocerotis and the small group of stars 8 arcminutes to the southwest. If you can’t, chances are you’ll not be able to see the Cone. It’s a dark nebula formed by an intervening dust cloud blocking light from a brighter nebula beyond. Its shape mirrors the Christmas tree, the cone and the tree being aligned tip-to-tip. The most obvious part of the Cone Nebula is where the background nebulosity is brightest, just south of HD 47887. A UHC filter will really help if you have one. SEEN IT
3 6+ 7+( )2; )85 1(%8/$
The Fox Fur Nebula is often grouped into the confused label NGC 2264 although it is uniquely designated as Sh2-273. Bearing a remarkable resemblance to a fox stole in long exposure photos, its nose is located roughly between S Monocerotis and variable star V641 Monocerotis to the southwest. The fur stole then sweeps in a broad arc around to the northwest for about 0.5°. This is part of a much larger and fainter region of glowing hydrogen gas. A nebula or UHC filter is recommended for viewing the Fox Fur Nebula visually. SEEN IT
COLLINDER 106
We take a break from confusion for our next target by heading
3.8° south and 1° west of S Monocerotis. Here you’ll find Collinder 106, a mag. +4.6 open cluster. This is a large and sparsely populated object. It has an apparent size measuring 45 arcminutes across and consequently benefits from the use of a low power, wide-field eyepiece. There are around 20-25 stars visible here mostly of 6th or 7th magnitude. Although quite a poor example of an open cluster, it does have the virtue of providing a good comparison with our penultimate target, which can be found 1.7° to the southwest of Collinder 106. SEEN IT
5 NGC 2244
NGC 2244 is the open cluster at the heart of the Rosette Nebula. For many, this cluster is the only part of the Rosette they get to see! Its half dozen brightest stars look like a wobbly version of the dots on the six face of a die. The brightest star, 12 Monocerotis in the southeast corner, is a foreground object and doesn’t belong to the more distant cluster. The star in the six dots’ southwest corner is HD 46223. This is a true cluster member estimated to be 400,000 times brighter than the Sun and 50 times as massive. HD 46150 – the middle-west dot – is even more impressive at 450,000 times brighter than the Sun and 60 times more massive. NCG 2244 is about 18 lightyears across and 5,200 lightyears distant. SEEN IT
6 1*& $1' 7+( ROSETTE NEBULA
While our first target had one designation for multiple objects, our last reverses the trend by having multiple designations for one object. The Rosette Nebula is a large, circular patch of nebulosity surrounding the open cluster NCG 2244. It’s seen as several nebulous patches designated as NGC 2237, 2238, 2239 and 2246. The nebula is being excited by the hot young stars of NGC 2244. Its 1.3° apparent diameter gives it a low surface brightness, so it’s a challenge to see. Observe from a dark sky site with a low-power eyepiece and use a UHC or OIII filter if you have one. A 4-inch scope is enough to see it without filters if the sky’s dark enough. Its size is truly stunning; its physical diameter is 130 lightyears across. SEEN IT