Globular cluster in Sagitta
M71
Argued about for two centuries — too loose for a globular, too rich for an open cluster. The modern answer is a very sparse globular, and it is one of the nearest, which is why it resolves so easily. The argument was settled by its stars rather than its shape: they are old and poor in heavy elements, which open clusters are not.
It is around thirteen thousand light-years away and perhaps twenty-seven across, which is small — and it sits in Sagitta, the third-smallest constellation, in a rich enough field to be worth the detour.
- Catalogue
- M71
- Type
- Globular cluster
- Constellation
- Sagitta
- Distance
- 13,000 light-years
- Apparent magnitude
- 8.2
- Surface brightness
- 21.1 mag/□″
- Apparent size
- 7′
- Right ascension
- 19h 54m
- Declination
- +18.78°
- Found by
- Philippe Loys de Chéseaux, 1745
Small enough that its 8.2 magnitudes arrive concentrated rather than smeared: 21.1 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 10×50 binoculars from Bortle 5.
| Sky | Eye | 10×50 | 80mm | 130mm | 200mm |
|---|---|---|---|---|---|
| Bortle 1 21.9 mag/□″ | No | 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
Loose enough that it resolves easily at 150mm and looks more like a rich open cluster than a globular, which is exactly why its classification took two hundred years to settle. Small, in a pretty field within Sagitta, and a good demonstration that these categories have edges.
When, and from where
Highest in the middle of the night in July. Passes overhead at 19°N and rises from everywhere except the far south — below 71°S it never clears the horizon.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 49° | |
| 51°N London, Calgary | 58° | |
| 40°N New York, Madrid | 69° | |
| 30°N Cairo, Houston | 79° | |
| 0°N the equator | 71° | |
| 33°S Sydney, Cape Town | 38° |
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 M71 itself, not from the middle of Sagitta.
- M27 Dumbbell Nebula 4.2° apart Vulpecula · magnitude 7.4 · binoculars from Bortle 5 or darker
- M56 Globular cluster 14° apart Lyra · magnitude 8.3 · binoculars from Bortle 5 or darker
- M57 Ring Nebula 20° apart Lyra · magnitude 8.8 · binoculars from Bortle 6 or darker
- M29 Open cluster 21° apart Cygnus · magnitude 6.6 · binoculars from Bortle 9 or darker
One of 110. Positions J2000; the sky is the final authority.