Open
Cluster and Planetary Nebula
aka
NGC 2818, PNG 261.9+08.5, PK 261+08.1, He 2-23, ARO 508, ESO
372-13 (PN)
aka
NGC 2818A, Melotte 96, Collinder 206, Raab 82, OCL 743 (OC)
RA:
09h16m01.5s Dec: -36°36'37" (PN) (Pyxis)
Integrated
Visual Magnitude: 13.0 (PN) 9.9 (OC)
Angular
Diameter: 50" (PN) 9' (OC)
Mean
Surface Brightness: 21.2 Mag/arc-secē (PN)
Distance
6000-10000 ly
Age:
600 to 1000 Myrs (OC)
Minimum
requirements to detect: 4-inch under dark skies |
|
 Where
the heck have I been? That's what I found myself thinking as I
looked at this wonderful planetary nebula embedded in an open
cluster. These two objects both share an NGC number and some
references use the designation 2818A to describe one or the other of
the two. Planetary nebulae in open clusters make for a rare
visual treat and the planetary that appears to lie in M46 is one of
my all-time favorite objects. So how had I overlooked NGC 2818
all these years? And why isn't it better known? The only
answer I can come up with is that NGC 2818 are pretty far south and
lie in a little-observed constellation (at least for us
northerners). From the latitude of Los Angeles (33N) this pair
only transits 20o
of above the horizon. But provided you observe near maximum altitude
this is high enough for a pretty good view. In mid-February
NGC 2818/A transit between 11 PM and midnight.
The planetary nebula
NGC 2818 is relatively large and bright with interesting structure.
This planetary is similar in size and brightness to the much better
known "Bug" (NGC 6302) in Scorpius. It's relatively
high surface brightness takes magnification well. Look for the
appearance of two separate parallel lobes that arc toward one
another, nearly touching at the ends. Also look for a fainter
outer envelope, particularly off each end, using averted vision.
I didn't detect any color but Steve Coe notes that, "A few dark
lanes are seen in the planetary at 165X, it is light green at all
powers." Although NGC 2818 has a lower surface brightness
than NGC 2438 (the PN in M46) it does compare favorably and its
intriguing structure will surely bring me back again and again.
The NGC 2818 cluster is
fairly faint, mostly made up of 11th to 14th magnitude stars.
It is scattered and irregular and does not immediately jump out at
you as a cluster of stars. It doesn't begin to compare to M46.
Regardless, the many stars do make a nice backdrop for the
planetary. Interestingly, NGC 2818 is the only
cluster-planetary combination that is currently believed to be truly
associated. Although the status of NGC 2438 as a member of M46
is not known for certain, most indications are that the planetary is
a foreground object. And the relatively young age of M46 makes
the possibility of one of it's member stars being the progenitor of
the planetary nebula fairly remote. The NGC 2818 cluster, on
the other hand, is older (perhaps as old as Billion years).
The field in a 12-inch at
75x. North is down and east is to the right. |
