Open cluster in Scorpius
Butterfly Cluster (M6)
The Butterfly, and for once the name earns itself — the wings are there at almost any magnification. One star in it burns orange among all the blue, which is an early notice of what is coming for the rest of them.
The orange one is BM Scorpii, a variable that shifts by about half a magnitude on a cycle of some eight hundred days, so the cluster is not quite the same twice. At around a hundred million years old the rest of it is still on the main sequence, and that first defector is simply the heaviest.
- Catalogue
- M6
- Type
- Open cluster
- Constellation
- Scorpius
- Distance
- 1,600 light-years
- Apparent magnitude
- 4.2
- Surface brightness
- 19.8 mag/□″
- Apparent size
- 25′
- Right ascension
- 17h 40m
- Declination
- −32.22°
- Found by
- Giovanni Batista Hodierna, before 1654
Magnitude 4.2 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 8 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/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 9 17.8 mag/□″ | No | Easy | Easy | Easy | Easy |
At the eyepiece
Binoculars show the wings; a telescope at low power shows them better and anything higher takes them apart. The orange star sits slightly off-centre and is obvious once you know it is there — the only colour in an otherwise blue-white field, and it changes brightness over a couple of years.
When, and from where
Highest in the middle of the night in June. A southern object: overhead at 32°S, and low and murky from Britain and the northern United States even at its best. North of 58°N it never rises.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | — | Never rises |
| 51°N London, Calgary | 7° | Low — in the murk |
| 40°N New York, Madrid | 18° | Low — in the murk |
| 30°N Cairo, Houston | 28° | |
| 0°N the equator | 58° | |
| 33°S Sydney, Cape Town | 89° |
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 M6 itself, not from the middle of Scorpius.
- M7 Ptolemy's Cluster 3.9° apart Scorpius · magnitude 3.3 · binoculars from Bortle 9 or darker
- M62 Globular cluster 8.6° apart Ophiuchus · magnitude 6.5 · binoculars from Bortle 6 or darker
- M8 Lagoon Nebula 9.4° apart Sagittarius · magnitude 4.6 · binoculars from Bortle 3 or darker
- M19 Globular cluster 10° apart Ophiuchus · magnitude 6.8 · binoculars from Bortle 5 or darker
Terms on this page
- The main sequence The long, stable middle age a star spends fusing hydrogen in its core.
One of 110. Positions J2000; the sky is the final authority.