NGC 2264 - The Christmas Tree Cluster & Cone Nebula...

Object: NGC 2264
Type: Open Cluster & Emission Nebulae
Constellation: Monoceros
Distance: 2400 light years

Date: 31 January, 06 & 10 February 2018
Equipment: SXV-H9, Vixen 114mm f5.3 ED refractor, NEQ6 mount, guiding with Lodestar X2/PHD

Subframes: 70 x 300s H-alpha, 20 x 300s RGB, no darks (hot pixel removal in Astroart instead).

Subframes were stacked in Astroart, the outputs converted to TIF files and then colour-composited in PaintShop Pro. I gathered the colour data on the third night, but I quite liked the mono H-alpha image on its own, so I posted that as well (below).

The colour data was a bit of a pig to process with the software I have.  First of all it became apparent that 300 second subs in H-alpha probably weren't long enough.  I had to stretch the resultant stack quite a way to get any decent detail, which introduced more noise than I really wanted. Then the blue colour stack turned out to be a slightly different size to the others, despite all being full frame without any need to resize.  I always refocus between filter changes as the colour correction on my old Vixen 114 ED refractor is not brilliant, so I guess this was the cause, particularly on the blue channel.  This made it difficult to stack the images for colour composition.  I did my best to manually rotate and resize in PaintShop, but the stars still show some odd haloes where the channels didn't line up.

I made two colour images, one created by the addition of green and blue channels to the H-alpha image above, which was used as the red channel for an RGB composite. This was a rather violent red, as anticipated, so I blended it with an HaRGB stack, which was the usual washed-out pink.  The blend gave a nice colour balance. The blue brilliance of the brighter stars in the cluster has led to some flaring around them.  I find the effect quite pleasing, however.

Clicking on either image pulls up the image icons, which can each be clicked on to "blink" each image. 

The Christmas Tree Cluster is a relatively young open cluster, formed between 3 and 30 million years ago. It is part of the NGC 2264 region, along with the Cone Nebula and the Fox Fur Nebula (Sharpless 273), and belongs to the Monoceros OB1 association, a loose association of very young stars located in the Orion Arm of the Milky Way.

The Christmas Tree Cluster (discovered by William Herschel in 1784) was given its nickname by astronomer Maurice Leyland for its triangular shape, like that of a Christmas tree, as seen through binoculars or a small telescope.  In the image above, the tip of the inverted “tree” is marked by the seventh magnitude star HD 47887 (just above the Cone Nebula as seen at the bottom of the image) with the bright variable star S Monocerotis (15 Monocerotis) to the north forming its “trunk”.

S Monocerotis is a bright irregular eruptive variable.  It is a multiple star system whose primary component is a hot, massive O-type star 10 times the size of the Sun and 30 times as massive.

The “Fox Fur Nebula” (Sharpless 273) can be seen below S Monocerotis in the above image.  Its resemblance to road-kill is quite remarkable, even in these modest images.  The bluish area arises from the reflection of starlight from dust in the region.  Some images show this reflection nebulosity to be much more extensive than seen here, probably because I did not compensate for the poorer blue sensitivity of the CCD with longer blue subs.

The Christmas Tree Cluster is visible to the naked eye in good conditions and appears quite striking in binoculars. The stars forming the Christmas tree shape, along with dozens of other tiny bright stars within the cluster, are a magical sight as seen in my VC200L with a 25mm eyepiece.  None of the associated nebulosity is visible however, at least, not to me.

NGC 2264 is listed as “Hidden Treasure 38” in Stephen O’Meara’s excellent book "Hidden Treasures" in the Cambridge University Press “Deep Sky Companion” series, the relevant excerpt from which can be found here.