Asterism in Ursa Major
Winnecke 4 (M40)
A mistake, preserved. Messier went looking for a nebula another astronomer had reported here, found nothing but two faint stars close together, and catalogued them anyway. They are not even a real pair — they lie at different distances and are drifting apart.
Messier wrote down what he had actually seen rather than what he had gone looking for, and left it in the list. That is worth more than the object: a catalogue that records a negative result is a catalogue somebody was being honest in, and the double is preserved as a small monument to the practice.
- Catalogue
- M40
- Type
- Asterism
- Constellation
- Ursa Major
- Distance
- about 500 light-years
- Apparent magnitude
- 8.4
- Surface brightness
- 17.0 mag/□″
- Apparent size
- 1′
- Right ascension
- 12h 22m
- Declination
- +58.08°
- Found by
- Johann Hevelius, 1660
Magnitude 8.4 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 10×50 binoculars from Bortle 7; with a 200mm from Bortle 9.
| Sky | Eye | 10×50 | 80mm | 130mm | 200mm |
|---|---|---|---|---|---|
| Bortle 1 21.9 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 2 21.7 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 3 21.4 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 4 20.9 mag/□″ | No | 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 | No | Easy | Easy | Easy |
| Bortle 9 17.8 mag/□″ | No | No | Easy | Easy | Easy |
At the eyepiece
Two faint stars about fifty arcseconds apart, splittable in almost anything. There is nothing else to see and never was. Worth visiting precisely because it is the catalogue's honest record of a mistake, which is a rarer thing in science writing than it should be.
When, and from where
Highest in the middle of the night in March. Well placed for the northern hemisphere and overhead for anyone near 58°N. South of 32°S it stays below the horizon.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 88° | Never sets |
| 51°N London, Calgary | 83° | Never sets |
| 40°N New York, Madrid | 72° | Never sets |
| 30°N Cairo, Houston | 62° | |
| 0°N the equator | 32° | |
| 33°S Sydney, Cape Town | — | Never rises |
Nearby in the sky
The whole catalogue →Nothing else in the catalogue is especially close; the nearest is 5.8° apart, which is a separate trip rather than a nudge of the tube. Everything below is measured from M40 itself, not from the middle of Ursa Major.
- M109 Galaxy 5.8° apart Ursa Major · magnitude 9.8 · binoculars from Bortle 4 or darker
- M97 Owl Nebula 9.8° apart Ursa Major · magnitude 9.9 · binoculars from Bortle 4 or darker
- M108 Galaxy 10.0° apart Ursa Major · magnitude 10.0 · binoculars from Bortle 4 or darker
- M106 Galaxy 11° apart Canes Venatici · magnitude 8.4 · binoculars from Bortle 4 or darker
One of 110. Positions J2000; the sky is the final authority.