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M46 & NGC 2438

M46

Open Cluster
aka NGC 2437, Melotte 75, Collinder 159, Raab 62, OCL 601
RA: 07h41m47.8s, Dec: -1448'06" (2000) in Puppis
Galactic lon: +23151', Galactic lat: +0404'
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: -1444'07" (2000) in Puppis
Galactic lon: +23148', Galactic lat: +0407'
Magnitude: 10.10
Size: 1.1'

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.

Millennium Star Atlas Vol I Chart 295
Sky Atlas 2000 Chart 12
Uranometria 2000 Vol II Chart 274
Uranometria 2nd Ed. Chart 135
Herald-Bobroff Astroatlas B-12 C-51