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M46
Open
Cluster
aka NGC 2437, Melotte 75, Collinder 159, Raab 62, OCL 601
RA: 07h41m47.8s, Dec: -14°48'06" (2000) in Puppis
Galactic lon: +231°51', Galactic lat: +04°04'
Magnitude: 6.60
Size: 27.0'
Minimum
requirements to detect: 4-inch scope under dark skies |
NGC
2438
Planetary Nebula
aka PN G231.8+04.1, PK 231+04.2, ARO 46
RA: 07h41m50.6s, Dec: -14°44'07" (2000) in Puppis
Galactic lon: +231°48', Galactic lat: +04°07'
Magnitude: 10.10
Size: 1.1' |
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This is one of my all-time favorite
pairs of objects to observe. The view of a small, oval planetary
nebula embedded in what Burnham's describes as a "fine circular
cloud of small stars" always leaves me with a grin on my face.
M46 is an excellent open cluster,
particularly for telescopes under 12". It contains some 150
stars from 9th to 13th magnitude within a circle of 30'. At a
distance of 4500 light years, the stars are contained within a
region 40 light years across.
The 10th magnitude planetary nebula
NGC 2438 was first noticed by William Herschel in 1827. Small (1.1')
and round, with some structure visible in larger instruments, NGC
2438 lies about 7' to the north of the cluster center. Look for a
tiny hazy spot, as if one of the stars is out of focus.
The combination of M46 and NGC 2438
could have been of great value to our understanding of planetary
nebulae. Our understanding of celestial objects is often hindered
greatly by our inability to measure accurate distances. The
distances to planetary nebulae are particularly difficult to
measure, leaving many of their basic properties uncertain. The
distances to open clusters, on the other hand, can be determined
with reasonable accuracy. Had the NGC 2438 planetary nebula proven
to be within the M46 cluster, we could have inferred the distance to
the nebula from that of the cluster. Unfortunately, the motion of
the nebula differs substantially from those of the stars within the
cluster, all but ruling this out. Alas, NGC 2438 appears to be a
foreground object.
Eyepiece view in six inch at 50x. North is down and
east is to the right.
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