Globular cluster in Sagittarius
M54
For two centuries an ordinary if distant globular. In 1994 the Sagittarius Dwarf Galaxy was discovered behind it, and M54 turned out to be that galaxy's nucleus — the first globular found to belong to something other than the Milky Way.
The Sagittarius Dwarf is being pulled apart by our own galaxy and its stars are being spread through the halo, which means M54 is the surviving core of something in the process of being eaten. Everything else in the catalogue is a piece of the Milky Way; this is a piece of something else, caught mid-absorption.
- Catalogue
- M54
- Type
- Globular cluster
- Constellation
- Sagittarius
- Distance
- 87,000 light-years
- Apparent magnitude
- 7.6
- Surface brightness
- 21.0 mag/□″
- Apparent size
- 9′
- Right ascension
- 18h 55m
- Declination
- −30.48°
- Found by
- Charles Messier, 1778
Small enough that its 7.6 magnitudes arrive concentrated rather than smeared: 21.0 per square arcsecond, which is why it stands out from the sky better than its brightness alone suggests. 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 5.
| 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 | 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
Small, bright and utterly ordinary-looking, which is the joke: this is a fuzzy dot that turned out to belong to another galaxy. There is nothing to resolve at any amateur aperture. What you are actually looking at is the centre of something the Milky Way is in the middle of eating.
When, and from where
Highest in the middle of the night in July. A southern object: overhead at 30°S, and low and murky from Britain and the northern United States even at its best. North of 60°N it never rises.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | — | Never rises |
| 51°N London, Calgary | 9° | Low — in the murk |
| 40°N New York, Madrid | 20° | Low — in the murk |
| 30°N Cairo, Houston | 30° | |
| 0°N the equator | 60° | |
| 33°S Sydney, Cape Town | 87° |
Nearby in the sky
The whole catalogue →One more of the catalogue sits within five degrees, close enough to take in on the same night without moving far. Everything below is measured from M54 itself, not from the middle of Sagittarius.
- M70 Globular cluster 3.1° apart Sagittarius · magnitude 7.9 · binoculars from Bortle 5 or darker
- M69 Globular cluster 5.4° apart Sagittarius · magnitude 7.6 · binoculars from Bortle 6 or darker
- M22 Globular cluster 7.8° apart Sagittarius · magnitude 5.1 · binoculars from Bortle 5 or darker
- M28 Globular cluster 8.8° apart Sagittarius · magnitude 6.8 · binoculars from Bortle 6 or darker
One of 110. Positions J2000; the sky is the final authority.