Open cluster in Canis Major
M41
Aristotle may have written this down around 325 BC, describing something below Sirius as a star with a faint tail — which would make it the faintest thing recorded in antiquity. It might equally have been a comet, or a stretch of the Milky Way, and the argument has never been settled. It sits four degrees under Sirius and a dark sky gives it up without help.
Whatever Aristotle saw, the cluster holds about a hundred stars and is roughly two hundred million years old, with several orange giants near the middle that give it more colour than most winter clusters manage. It is drifting away from us at about thirty-four kilometres a second and will be measurably fainter in a few million years.
- Catalogue
- M41
- Type
- Open cluster
- Constellation
- Canis Major
- Distance
- 2,300 light-years
- Apparent magnitude
- 4.5
- Surface brightness
- 21.0 mag/□″
- Apparent size
- 38′
- Right ascension
- 6h 46m
- Declination
- −20.73°
- Found by
- Giovanni Batista Hodierna, before 1654
Magnitude 4.5 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 7 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/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 6 19.3 mag/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 7 18.6 mag/□″ | Easy | 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 naked-eye smudge under a dark sky and a fine binocular object anywhere — around eighty stars spread across a full Moon's width, with several orange ones scattered through them. Four degrees below Sirius, which makes it one of the easiest things here to find.
When, and from where
Highest in the middle of the night in December. Overhead at 21°S and southerly enough to sit low from northern Europe, though never out of reach there — only north of 69°N does it stop rising at all.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 9° | Low — in the murk |
| 51°N London, Calgary | 18° | Low — in the murk |
| 40°N New York, Madrid | 29° | |
| 30°N Cairo, Houston | 39° | |
| 0°N the equator | 69° | |
| 33°S Sydney, Cape Town | 78° |
Nearby in the sky
The whole catalogue →Nothing else in the catalogue is especially close; the nearest is 13° apart, which is a separate trip rather than a nudge of the tube. Everything below is measured from M41 itself, not from the middle of Canis Major.
- M50 Open cluster 13° apart Monoceros · magnitude 5.9 · binoculars from Bortle 9 or darker
- M47 Open cluster 14° apart Puppis · magnitude 4.4 · binoculars from Bortle 9 or darker
- M93 Open cluster 14° apart Puppis · magnitude 6.2 · binoculars from Bortle 9 or darker
- M46 Open cluster 15° apart Puppis · magnitude 6.1 · binoculars from Bortle 9 or darker
One of 110. Positions J2000; the sky is the final authority.