Sharpless 2-273

Sharpless 2-273

Object: Sh2-273 (centred on NGC 2264, the "Christmas Tree Cluster" and Cone Nebula)

Type: Emission and reflection nebulae, open cluster 
Constellation: Monoceros
Distance: 2400 light years

Date: February 26th., March 5th., 6th., 7th., 2021
Equipment: ATIK 460EX with EFW2, Samyang 135mm lens @f2.8, Vixen GPDX mount, guiding with Lodestar X2/PHD

Subframes: 10 x 600s each for Ha, red, green and blue, flats, no darks (hot pixel removal in Astroart).

Map showing image field of view

The dim winter constellation of Monoceros is overshadowed by its more brilliant neighbours, with the twins of Gemini to the north and Orion to the west.  Nevertheless, within it can be found several interesting deep sky objects, one of which is the huge nebula complex Sharpless 2-273, an area approximately 5 degrees across. The Stellarium map opposite shows the location of the field of view of the main image above.

At the centre of the nebulosity is the 4th-magnitude open cluster NGC 2264, which was discovered on Jan 18, 1784 by William Herschel.

The surrounding nebulosity is a complex consisting of dark absorption nebulae, emission nebulae, reflection nebulae, and the stars that illuminate or outline their structures. At one end of the brightest central area, the “Cone Nebula” is a dense cloud of gas and dust sculpted by stellar winds from an extremely hot, bright star which is completely hidden in visible light by the gas and dust in front of it. Scattered across the central area are a number of bright stars which look like lights strung on a Christmas tree (hence the cluster’s popular name of the “Christmas Tree cluster”) with the Cone Nebula at the apex of the tree, and the bright star S Monocerotis and the “Fox Fur Nebula” near the base. The Fox Fur nebula is not generally considered a part of NGC 2264, but is certainly an extension of the gas and dust filling the region, as all the stars and clouds of gas and dust lie at about the same distance from us. 

I took a narrower field image of the area back in 2019, which shows this colourful area in more detail.

A Hubble Space Telescope view (below left) of the Cone Nebula shows the dense clouds of gas and dust in a region only a couple of light years across. The overall size of the Cone is about 7 light years.

Cone nebula in visible light (left), NGC 2264 in IR (right)

 The Spitzer Space Telescope image (above right) shows open cluster NGC 2264 in infra-red. The brilliant star near the Cone nebula is NGC 2264 IRS, the source of the stellar winds sculpting the Cone. Despite its brilliance, this star is completely hidden by the gas and dust in front of it. Only infrared images can penetrate the dust and reveal the star; but when they do, its brilliance dwarfs that of the other stars in the region.

Just to the southwest of the Cone Nebula lies a tiny fan of nebulosity designated as NGC 2261.  It appears as a tiny comet-shaped blob on the image above (enlarged below).

Although discovered on Dec 26 1783, by William Herschel, the nebula is named after the American astronomer Edwin P. Hubble, who carried out some of the early studies of this object.

It is a fan-shaped cloud of gas and dust which is illuminated by R Monocerotis (R Mon), the bright star at the bottom end of the nebula. Dense condensations of dust near the star cast shadows out into the nebula, and as they move the illumination changes, giving rise to the variations first noted by Hubble. The star itself, lying about 2,500 light-years from Earth, cannot be seen directly, but only through light scattered off of dust particles in the surrounding nebula. R Mon is believed to have a mass of about 10 times that of the Sun, and to have an age of only 300,000 years. There is probably a symmetrical counterpart of the fan-shaped nebula on the southern side of the star, but it is heavily obscured from view by dust lying between this lobe and our line of sight.

This image was my first in over 3 months, thanks to almost continuous cloud cover in my corner of the world.  As always seems the way, the evening of February 26th. was plagued by a high haze and a 100% full moon, but was adequate for H-alpha imaging. Of the 12 x 600 sub-frames I collected before the haze became too dense, I had to discard two due to bright aircraft trails (what lockdown travel restrictions are they subject to, then?). Fortunately, a later bonus (and completely un-forecast) string of clear, moonless evenings in early March allowed me to get some RGB colour data as well. 

The subs for each channel were sigma stacked in Astroart, with the result treated with AA’s gradient removal tool and then given a log stretch. Each stacked channel was then run through Starnet and the resultant starless image given an edge preserving smooth and “clarified” in Paint Shop Pro.  This proved particularly effective for the R, G and B stacks, where the stars were otherwise overpowering: Starnet allowed the dog to see the rabbit when attempting to pull out nebulosity, especially in the blue channel.

The starless stacks are shown below:

Ha channel - stars removed with Starnet

Red channel - starless

Green channel - starless

Blue channel - starless

The R(R = 70:30 R/Ha), G and B channels were colour combined in PaintShop Pro to give this rather striking RGB starless image...

RGB starless combination

The starless Ha layer was pasted back over the above as a luminosity layer at around 30%: this helped to improve detail in the central Cone and Fox Fur area. 

A less aggressively stretched version of each of the R, G and B stacks to exclude nebulosity was given a Gaussian blur to tidy up the rather under-sampled stars, then RGB combined, star-reduced, then given a hefty saturation tweak to produce a colour star layer...

RGB star layer

This was pasted back over the starless HaRGB image in screen mode, with some final selective sharpening and contrast/colour adjustment in PSP to give the final image.   

References:

1)           https://cseligman.com/text/atlas/ngc22a.htm#2264

2)           https://hubblesite.org/contents/media/images/1999/35/904-Image.html