Open cluster in Sagittarius
M21
Five million years old, which for a cluster is infancy — nothing in it has had time to age out of blue yet. It condensed from the same gas as the Trifid a degree away, and the two are still neighbours.
Its brightest members are B-class stars, hot and blue and short-lived, and there is nothing red in it at all. In a few tens of millions of years the heaviest of them will have gone as supernovae, which will end the cluster and probably the Trifid with it.
- Catalogue
- M21
- Type
- Open cluster
- Constellation
- Sagittarius
- Distance
- 4,250 light-years
- Apparent magnitude
- 6.5
- Surface brightness
- 20.7 mag/□″
- Apparent size
- 13′
- Right ascension
- 18h 05m
- Declination
- −22.50°
- Found by
- Charles Messier, 1764
Magnitude 6.5 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 3 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/□″ | 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
Small, tight and easily passed over because it sits a degree from the Trifid and loses the comparison. Twenty or thirty stars at moderate power, several of them noticeably bright. Best treated as the second half of a visit rather than a destination.
When, and from where
Highest in the middle of the night in June. Overhead at 22°S and southerly enough to sit low from northern Europe, though never out of reach there — only north of 68°N does it stop rising at all.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 8° | Low — in the murk |
| 51°N London, Calgary | 17° | Low — in the murk |
| 40°N New York, Madrid | 28° | |
| 30°N Cairo, Houston | 38° | |
| 0°N the equator | 68° | |
| 33°S Sydney, Cape Town | 80° |
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 M21 itself, not from the middle of Sagittarius.
- M20 Trifid Nebula 42′ apart Sagittarius · magnitude 6.3 · binoculars from Bortle 3 or darker
- M8 Lagoon Nebula 1.9° apart Sagittarius · magnitude 4.6 · binoculars from Bortle 3 or darker
- M23 Open cluster 3.9° apart Sagittarius · magnitude 5.5 · binoculars from Bortle 9 or darker
- M24 Sagittarius Star Cloud 4.9° apart Sagittarius · magnitude 2.5 · binoculars from Bortle 9 or darker
One of 110. Positions J2000; the sky is the final authority.