Globular cluster in Ophiuchus
M10
One of a pair with M12, three degrees apart, near enough that a wide field takes both at once. The brighter and tighter of the two, and therefore the one people find first and remember.
The pairing with M12 is not quite the coincidence it looks: the two are at similar distances and are genuinely neighbours, about two thousand light-years apart. M10 is around eighty light-years across, and its brightest stars sit at about magnitude 13, which is the number that decides whether your telescope will grain it or not.
- Catalogue
- M10
- Type
- Globular cluster
- Constellation
- Ophiuchus
- Distance
- 14,300 light-years
- Apparent magnitude
- 6.6
- Surface brightness
- 21.7 mag/□″
- Apparent size
- 20′
- Right ascension
- 16h 57m
- Declination
- −4.10°
- Found by
- Charles Messier, 1764
Magnitude 6.6 sounds comfortable and is spread over 20′, which brings it down to 21.7 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 | No | No | No | No |
| Bortle 8 18.1 mag/□″ | No | No | No | No | No |
| Bortle 9 17.8 mag/□″ | No | No | No | No | No |
At the eyepiece
Larger and looser than it first appears, and it resolves at around 150mm into a scattering that seems to thin out as you look away from the centre. Sweep three degrees west and M12 is there in the same eyepiece field, which makes them the natural pair to compare.
When, and from where
Highest in the middle of the night in June. Close enough to the celestial equator to rise from almost anywhere on Earth, and to pass overhead for anyone near 4°S.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 26° | |
| 51°N London, Calgary | 35° | |
| 40°N New York, Madrid | 46° | |
| 30°N Cairo, Houston | 56° | |
| 0°N the equator | 86° | |
| 33°S Sydney, Cape Town | 61° |
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 M10 itself, not from the middle of Ophiuchus.
- M12 Globular cluster 3.3° apart Ophiuchus · magnitude 6.7 · binoculars from Bortle 5 or darker
- M14 Globular cluster 10° apart Ophiuchus · magnitude 7.6 · binoculars from Bortle 5 or darker
- M107 Globular cluster 11° apart Ophiuchus · magnitude 8.1 · binoculars from Bortle 5 or darker
- M9 Globular cluster 15° apart Ophiuchus · magnitude 7.7 · binoculars from Bortle 5 or darker
One of 110. Positions J2000; the sky is the final authority.