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The Cone Nebula

Diffuse Nebula
aka NGC 2264, Sh 2-273, LBN 911, Bernes 97
RA: 06h41m20.0s Dec: +0926'02" (Monoceros)
Integrated Visual Magnitude: ?

Angular Diameter: 20'

Minimum requirements to detect: 10-inch under dark skies

The Cone Nebula lies in the southern part of NGC 2264, the Christmas Tree cluster, with which is shares the NGC designation.  This is a rich region with much nebulosity and many interesting objects.  The Christmas Tree cluster has at its base the variable star S Mon (also known as 15 Mon).  If viewed with this northern portion at the bottom, the stars of this cluster form the outline of a Christmas Tree, topped by the 7th magnitude star HD 47887.

S Mon is an irregular eruptive variable that varies from 4.62 to 4.68 magnitude.  A hot, massive star of spectral type O8, it is at least 8000 times as luminous as the sun.  The variability is produced by variations in the chromosphere, which are also related to the outflow (or stellar wind) emanating from this recently formed and relatively short lived massive star.  S Mon has an 8th magnitude companion 2.9" away at position angle 213o.

HD 47887, at the top of the Christmas Tree, is a spectral type B3 giant.  Look for a companion 12.8" distant at position angle 153o.  The area around HD 47887 is populated with dozens of Orion variables.

You should be able to see nebulosity surrounding S Mon, as well as a group of stars about 10' to it's southwest.  If you cannot see this nebulosity, there is no point in looking for the much more difficult Cone nebula.  The Cone nebula lies near HD 47887, making a very faint haze around and to the south of this star.  Like the Horsehead nebula, the Cone's shape comes from an intervening dark cloud that lies to the south of HD 47887.

As with the Horsehead, you will need clear dark skies and good seeing.  Try to observe with the nebula as high in the sky as possible.  Make sure that you are comfortable, and spend a lot of time looking.  A quick look just won't do--you have to relax and work at it.  You should be sitting down, if possible.  It can help if you can keep both eyes open as you look.  If there are any stray lights in the area, sometimes placing a cloth or towel over your head at the eyepiece can make all the difference, although you run the risk of looking silly!  According to David Knisely, a UHC filter can really bring out the detail in this nebula, often making the difference in seeing it at all.

Once you are confident that you can see the feeble glow of the nebula, look for a notch or hole due south of HD 47887.  Use averted vision. If you can convince yourself that you have seen a notch, then you can claim success!  As always with faint objects, keep returning again and again.  There are rare nights when you will see much more than you thought possible, and experience can really help.

The NGC 2264 region contains clouds of interstellar hydrogen gas, mixed with small grains of dust.  The majority of the hydrogen is neutral, which means that the nucleus of the hydrogen atom, a single proton, has a single electron attached.  This is called an HI cloud (or region).  In regions near hot, bright stars such as S Mon, the ultraviolet radiation from the hot star strips away the electrons from the majority of the hydrogen atoms.  This process is called ionization and these regions are called HII regions.  The free electrons frequently recombine with the nuclei for brief periods of time.  When they recombine they must give up energy in the form of particles of light.  Thus, the ultraviolet radiation from a nearby star is converted into a red hydrogen glow.  The result is a glowing nebula, which appears red in photographs.  The color is not typically detectable to the eye--all we typically see is a faint gray glow.

Intermixed with these glowing HII regions are the cooler, more dense HI clouds.  Stars are forming in the most dense parts of these clouds (called cores).  The stars of the NGC 2264 cluster were formed very recently from the surrounding  cloud complex.  The Hubble Space Telescope has imaged new stars forming in this region today.

Dark clouds form where the gas isn't glowing and the density is high enough that the dust particles intermixed with the gas can absorb significant amounts of starlight.  If you look at a star through one of these so called dark nebulae, it will appear much dimmer (perhaps it will even be invisible).

When you mix regions of glowing gas with dark nebulae, you can get interesting patterns such as the one which defines the Cone nebula.

  R.A. Dec. Mag Diameter
Cone Nebula 06h41m +09o53' ~4 1o


Millennium Star Atlas Vol I Chart 202
Sky Atlas 2000 Chart 12
Uranometria 2000 Vol I Chart 183
Uranometria 2nd Ed. Chart 95
Herald-Bobroff Astroatlas B-11 C-52