Globular cluster in Sagittarius
M55
A large, loose globular that resolves into stars more readily than almost any other, because it is nearby and sparse. Badly placed for northern observers, which is the only reason it is not better known.
It is around a hundred light-years across and unusually loose for its class, with a core that never collapsed. That combination — near, sparse, uncondensed — is exactly what makes it resolve so readily, and a 150mm from a southern latitude takes the whole face of it apart, edge to core.
- Catalogue
- M55
- Type
- Globular cluster
- Constellation
- Sagittarius
- Distance
- 17,600 light-years
- Apparent magnitude
- 6.3
- Surface brightness
- 21.3 mag/□″
- Apparent size
- 19′
- Right ascension
- 19h 40m
- Declination
- −30.97°
- Found by
- Nicolas-Louis de Lacaille, 1752
Magnitude 6.3 sounds comfortable and is spread over 19′, which brings it down to 21.3 per square arcsecond — the number that actually decides whether there is anything to see. 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 5.
| 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 | Challenge | Challenge | Challenge | Challenge |
| Bortle 7 18.6 mag/□″ | No | Challenge | Challenge | Challenge | Challenge |
| Bortle 8 18.1 mag/□″ | No | No | No | No | No |
| Bortle 9 17.8 mag/□″ | No | No | No | No | No |
At the eyepiece
Large, loose and resolves more easily than almost any other globular — at 150mm it is a field of individual stars rather than a glow with grain in it. From southern latitudes it is a highlight; from Britain it crawls along the horizon and is barely worth the attempt.
When, and from where
Highest in the middle of the night in July. A southern object: overhead at 31°S, and low and murky from Britain and the northern United States even at its best. North of 59°N it never rises.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | — | Never rises |
| 51°N London, Calgary | 8° | Low — in the murk |
| 40°N New York, Madrid | 19° | Low — in the murk |
| 30°N Cairo, Houston | 29° | |
| 0°N the equator | 59° | |
| 33°S Sydney, Cape Town | 88° |
Nearby in the sky
The whole catalogue →Nothing else in the catalogue is especially close; the nearest is 9.7° apart, which is a separate trip rather than a nudge of the tube. Everything below is measured from M55 itself, not from the middle of Sagittarius.
- M54 Globular cluster 9.7° apart Sagittarius · magnitude 7.6 · binoculars from Bortle 5 or darker
- M75 Globular cluster 11° apart Sagittarius · magnitude 8.5 · binoculars from Bortle 5 or darker
- M70 Globular cluster 12° apart Sagittarius · magnitude 7.9 · binoculars from Bortle 5 or darker
- M69 Globular cluster 15° apart Sagittarius · magnitude 7.6 · binoculars from Bortle 6 or darker
One of 110. Positions J2000; the sky is the final authority.