Open cluster in Scorpius
Ptolemy's Cluster (M7)
Ptolemy logged this as a cloudy patch in AD 130, which is why it still carries his name. It is also too large for most telescopes: the correct instrument for it is a pair of binoculars, or none at all.
It spans eighty arcminutes — nearly three Moon widths — and lies almost directly toward the centre of the galaxy, so the background behind it is as rich as the sky gets. Around two hundred million years old, and near enough that its brightest members are ordinary stars rather than giants.
- Catalogue
- M7
- Type
- Open cluster
- Constellation
- Scorpius
- Distance
- 980 light-years
- Apparent magnitude
- 3.3
- Surface brightness
- 21.4 mag/□″
- Apparent size
- 80′
- Right ascension
- 17h 54m
- Declination
- −34.82°
- Found by
- Ptolemy, AD 130
Magnitude 3.3 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?
Bright enough for any sky there is, including the middle of a city. One of the few things on this list of which that is true.
| 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/□″ | Easy | Easy | Easy | Easy | Easy |
At the eyepiece
Big enough that a telescope is the wrong tool. Binoculars from a dark site give the whole cluster at once against the Milky Way behind it, which is the point of it. From northern Europe it skims the horizon and there is no equipment answer to that — you have to travel south.
When, and from where
Highest in the middle of the night in June. A southern object: overhead at 35°S, and low and murky from Britain and the northern United States even at its best. North of 55°N it never rises.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | — | Never rises |
| 51°N London, Calgary | 4° | Low — in the murk |
| 40°N New York, Madrid | 15° | Low — in the murk |
| 30°N Cairo, Houston | 25° | |
| 0°N the equator | 55° | |
| 33°S Sydney, Cape Town | 88° |
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 M7 itself, not from the middle of Scorpius.
- M6 Butterfly Cluster 3.9° apart Scorpius · magnitude 4.2 · binoculars from Bortle 9 or darker
- M69 Globular cluster 8.2° apart Sagittarius · magnitude 7.6 · binoculars from Bortle 6 or darker
- M70 Globular cluster 11° apart Sagittarius · magnitude 7.9 · binoculars from Bortle 5 or darker
- M8 Lagoon Nebula 11° apart Sagittarius · magnitude 4.6 · binoculars from Bortle 3 or darker
One of 110. Positions J2000; the sky is the final authority.