Globular cluster in Sagittarius
M22
Brighter and larger than M13 and almost unknown by comparison, entirely because it sits low in the south from Europe and North America. From the tropics it is the finest globular in the sky.
It was one of the first globulars anybody found — Abraham Ihle logged it in 1665, before Messier was born — and it has since turned out to contain a planetary nebula and, on one contested reading of how its stars move, a pair of black holes of a hundred solar masses or so. At thirty-two arcminutes it is the width of the Moon.
- Catalogue
- M22
- Type
- Globular cluster
- Constellation
- Sagittarius
- Distance
- 10,600 light-years
- Apparent magnitude
- 5.1
- Surface brightness
- 21.3 mag/□″
- Apparent size
- 32′
- Right ascension
- 18h 36m
- Declination
- −23.90°
- Found by
- Abraham Ihle, 1665
Magnitude 5.1 sounds comfortable and is spread over 32′, 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 from a Bortle 5 sky or darker, and aperture makes almost no difference to that — spread this thinly, an object is limited by how bright the sky behind it is rather than by how much light the telescope collects. A darker site beats a bigger telescope.
| 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/□″ | Challenge | 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
From a latitude that shows it properly it beats M13 comfortably: resolving in binoculars, filling the field at 150mm, with a slightly elliptical sprawl that M13 has never had. From Britain it is a low smudge, and no aperture fixes an altitude problem.
When, and from where
Highest in the middle of the night in June. Overhead at 24°S and southerly enough to sit low from northern Europe, though never out of reach there — only north of 66°N does it stop rising at all.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 6° | Low — in the murk |
| 51°N London, Calgary | 15° | Low — in the murk |
| 40°N New York, Madrid | 26° | |
| 30°N Cairo, Houston | 36° | |
| 0°N the equator | 66° | |
| 33°S Sydney, Cape Town | 81° |
Nearby in the sky
The whole catalogue →2 more of the catalogue sit within five degrees, close enough to take in on the same night without moving far. Everything below is measured from M22 itself, not from the middle of Sagittarius.
- M28 Globular cluster 2.9° apart Sagittarius · magnitude 6.8 · binoculars from Bortle 6 or darker
- M25 Open cluster 4.8° apart Sagittarius · magnitude 4.6 · binoculars from Bortle 9 or darker
- M24 Sagittarius Star Cloud 7.1° apart Sagittarius · magnitude 2.5 · binoculars from Bortle 9 or darker
- M21 Open cluster 7.4° apart Sagittarius · magnitude 6.5 · binoculars from Bortle 9 or darker
One of 110. Positions J2000; the sky is the final authority.