Open cluster in Sagittarius
M18
A sparse scattering of about twenty stars, catalogued because it sits between M17 and M24 in a part of the sky Messier was sweeping carefully. On its own it would not have been noticed. About thirty million years old and perhaps six light-years across, which is small by any measure.
The honest appeal of it is positional: it sits in the middle of the richest sweep of sky in the northern summer, and stopping on it costs nothing on the way between two far better things.
- Catalogue
- M18
- Type
- Open cluster
- Constellation
- Sagittarius
- Distance
- 4,900 light-years
- Apparent magnitude
- 7.5
- Surface brightness
- 20.9 mag/□″
- Apparent size
- 9′
- Right ascension
- 18h 20m
- Declination
- −17.13°
- Found by
- Charles Messier, 1764
Magnitude 7.5 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 1 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/□″ | 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 | Easy | Easy | Easy | Easy |
| Bortle 9 17.8 mag/□″ | No | Easy | Easy | Easy | Easy |
At the eyepiece
Twenty-odd stars and not much else. Worth a minute on the way between M17 and M24, which is exactly how Messier came across it, and not worth a special trip. Binoculars serve it as well as any telescope.
When, and from where
Highest in the middle of the night in June. Overhead at 17°S and southerly enough to sit low from northern Europe, though never out of reach there — only north of 73°N does it stop rising at all.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 13° | Low — in the murk |
| 51°N London, Calgary | 22° | |
| 40°N New York, Madrid | 33° | |
| 30°N Cairo, Houston | 43° | |
| 0°N the equator | 73° | |
| 33°S Sydney, Cape Town | 74° |
Nearby in the sky
The whole catalogue →This is a busy patch. 4 more of the catalogue sit within five degrees, which is a binocular field or two — worth planning a session around rather than visiting once. Everything below is measured from M18 itself, not from the middle of Sagittarius.
- M17 Omega Nebula 58′ apart Sagittarius · magnitude 6.0 · needs more than binoculars
- M24 Sagittarius Star Cloud 1.5° apart Sagittarius · magnitude 2.5 · binoculars from Bortle 9 or darker
- M16 Eagle Nebula 3.4° apart Serpens · magnitude 6.0 · binoculars from Bortle 2 or darker
- M25 Open cluster 3.5° apart Sagittarius · magnitude 4.6 · binoculars from Bortle 9 or darker
One of 110. Positions J2000; the sky is the final authority.