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| 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.
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| 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:
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| Ha channel - stars removed with Starnet |
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| Red channel - starless |
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| Green channel - starless |
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| 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...
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| 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...
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| 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
