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.
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
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.
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.