Globular cluster in Pegasus
M15
The core of this one has fallen in on itself, and the stars at the centre are packed so tightly that we still cannot say whether what holds them is a black hole or simply an enormous crowd. Nothing else in the catalogue is this dense.
It also contains a planetary nebula, Pease 1 — a star in the act of dying, inside a cluster of stars that died long ago. Only a handful are known in any globular, and finding this one at the eyepiece is among the harder things an amateur telescope can be asked to do.
- Catalogue
- M15
- Type
- Globular cluster
- Constellation
- Pegasus
- Distance
- 33,600 light-years
- Apparent magnitude
- 6.2
- Surface brightness
- 21.1 mag/□″
- Apparent size
- 18′
- Right ascension
- 21h 30m
- Declination
- +12.17°
- Found by
- Jean-Dominique Maraldi, 1746
Magnitude 6.2 sounds comfortable and is spread over 18′, which brings it down to 21.1 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
Its collapsed core shows: the middle is a hard, almost stellar point rather than the soft glow most globulars present, and the contrast between that and the resolved outer stars is the thing worth looking at. Magnify it. Small apertures do it no favours at all.
When, and from where
Highest in the middle of the night in August. Passes overhead at 12°N and rises from everywhere except the far south — below 78°S it never clears the horizon.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 42° | |
| 51°N London, Calgary | 51° | |
| 40°N New York, Madrid | 62° | |
| 30°N Cairo, Houston | 72° | |
| 0°N the equator | 78° | |
| 33°S Sydney, Cape Town | 45° |
Nearby in the sky
The whole catalogue →Nothing else in the catalogue is especially close; the nearest is 13° apart, which is a separate trip rather than a nudge of the tube. Everything below is measured from M15 itself, not from the middle of Pegasus.
- M2 Globular cluster 13° apart Aquarius · magnitude 6.5 · binoculars from Bortle 5 or darker
- M27 Dumbbell Nebula 24° apart Vulpecula · magnitude 7.4 · binoculars from Bortle 5 or darker
- M71 Globular cluster 24° apart Sagitta · magnitude 8.2 · binoculars from Bortle 5 or darker
- M73 Asterism 26° apart Aquarius · magnitude 9.0 · binoculars from Bortle 6 or darker
One of 110. Positions J2000; the sky is the final authority.