Open cluster in Puppis
M46
A rich cluster with a planetary nebula, NGC 2438, apparently sitting inside it — apparently, because the nebula is half the distance away and merely lines up. A fine coincidence, and a good demonstration that the sky is a projection. The coincidence is worth dwelling on.
NGC 2438 is about half the distance of the cluster and has no connection to it whatever, yet it sits so convincingly inside the field that the pairing was assumed for years. Two objects, unrelated, at different distances, arranged by the accident of where the Earth happens to sit.
- Catalogue
- M46
- Type
- Open cluster
- Constellation
- Puppis
- Distance
- 5,400 light-years
- Apparent magnitude
- 6.1
- Surface brightness
- 21.9 mag/□″
- Apparent size
- 27′
- Right ascension
- 7h 42m
- Declination
- −14.82°
- Found by
- Charles Messier, 1771
Magnitude 6.1 for the cluster as a whole, though that is a sum rather than a thing you see: what reaches you is a scatter of separate stars, and whether you catch it depends on how bright the brightest of them are. Why surface brightness is the number that matters.
Could you see it?
Straightforward with the naked eye from Bortle 4 or darker; with 10×50 binoculars from Bortle 9.
| Sky | Eye | 10×50 | 80mm | 130mm | 200mm |
|---|---|---|---|---|---|
| Bortle 1 21.9 mag/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 2 21.7 mag/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 3 21.4 mag/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 4 20.9 mag/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 5 20.3 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 6 19.3 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 7 18.6 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 8 18.1 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 9 17.8 mag/□″ | No | Easy | Easy | Easy | Easy |
At the eyepiece
A rich, even carpet of faint stars — several hundred of them, none dominant — with the small ring of NGC 2438 superimposed on the northern part. The nebula needs 150mm and is easier with an OIII, and finding it inside the cluster is one of the more satisfying moments here.
When, and from where
Highest in the middle of the night in January. Overhead at 15°S and southerly enough to sit low from northern Europe, though never out of reach there — only north of 75°N does it stop rising at all.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 15° | Low — in the murk |
| 51°N London, Calgary | 24° | |
| 40°N New York, Madrid | 35° | |
| 30°N Cairo, Houston | 45° | |
| 0°N the equator | 75° | |
| 33°S Sydney, Cape Town | 72° |
Nearby in the sky
The whole catalogue →One more of the catalogue sits within five degrees, close enough to take in on the same night without moving far. Everything below is measured from M46 itself, not from the middle of Puppis.
- M47 Open cluster 1.3° apart Puppis · magnitude 4.4 · binoculars from Bortle 9 or darker
- M93 Open cluster 9.1° apart Puppis · magnitude 6.2 · binoculars from Bortle 9 or darker
- M50 Open cluster 11° apart Monoceros · magnitude 5.9 · binoculars from Bortle 9 or darker
- M48 Open cluster 12° apart Hydra · magnitude 5.8 · binoculars from Bortle 9 or darker
One of 110. Positions J2000; the sky is the final authority.