Open cluster in Sagittarius
M23
A hundred-odd stars spread evenly with no real centre, which makes it better in binoculars than in a telescope — true of more of this catalogue than most lists care to admit.
Around three hundred million years old, which is late middle age for something this loose, and it shows: the cluster is already dissolving into the field around it and the boundary is a matter of opinion rather than measurement. In a decent pair of binoculars it is one of the prettier things in Sagittarius.
- Catalogue
- M23
- Type
- Open cluster
- Constellation
- Sagittarius
- Distance
- 2,050 light-years
- Apparent magnitude
- 5.5
- Surface brightness
- 21.3 mag/□″
- Apparent size
- 27′
- Right ascension
- 17h 57m
- Declination
- −19.02°
- Found by
- Charles Messier, 1764
Magnitude 5.5 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 5 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/□″ | 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
A wide, even scattering that low power flatters and high power destroys. Around a hundred stars of similar brightness with no concentration to speak of, which sounds dull and is oddly restful to look at — the visual equivalent of an evenly seeded field.
When, and from where
Highest in the middle of the night in June. Overhead at 19°S and southerly enough to sit low from northern Europe, though never out of reach there — only north of 71°N does it stop rising at all.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 11° | Low — in the murk |
| 51°N London, Calgary | 20° | Low — in the murk |
| 40°N New York, Madrid | 31° | |
| 30°N Cairo, Houston | 41° | |
| 0°N the equator | 71° | |
| 33°S Sydney, Cape Town | 76° |
Nearby in the sky
The whole catalogue →This is a busy patch. 3 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 M23 itself, not from the middle of Sagittarius.
- M21 Open cluster 3.9° apart Sagittarius · magnitude 6.5 · binoculars from Bortle 9 or darker
- M20 Trifid Nebula 4.2° apart Sagittarius · magnitude 6.3 · binoculars from Bortle 3 or darker
- M24 Sagittarius Star Cloud 4.8° apart Sagittarius · magnitude 2.5 · binoculars from Bortle 9 or darker
- M8 Lagoon Nebula 5.6° apart Sagittarius · magnitude 4.6 · binoculars from Bortle 3 or darker
One of 110. Positions J2000; the sky is the final authority.