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There are
only a few clusters among tens of thousands in the sky that have
such a prominent planetary nebula superimposed on them. This
is the best example in the entire sky. M46 is a moderately rich
cluster, spanning some 30 arc minutes of sky - equal in size
to the full moon in angular extent. At 6.1 magnitude it is barely
seen with the naked eye on a dark night, and contains over 100
known cluster members. There are two additional objects in this
image superimposed on M46. Foremost is the small blue green planetary
nebula (a dying star) NGC2438 just north of the clusters center
seen here as a round object just below the center of the cluster.
It is a bright 10th magnitude, and a whopping 64 arc seconds
in size (large for a planetary). Its central star is 18th magnitude
and not seen here in this image.
The color of the
planetary nebula is teal with a red rim. This is from triple
ionised Oxygen for the green and hydrogen for the reds. I could
never get the color right with my CCD on this one. It always
shot as blood red, probably because of the edges of the filters
band pass was too close to the green emission lines. I am thrilled
to get the actual color here!
The second superimposed
object is an 18th magnitude galaxy, located a few ring diameters
to the lower right of the nebula. Its recorded as a dull orange
color in this image.
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