Globular cluster in Aquarius
M72
About as faint and as remote as this catalogue goes for globulars, out in the galaxy's outer halo. In anything small it is indistinguishable from a slightly out-of-focus star, which is presumably why nobody noticed it until 1780.
It is around fifty-five thousand light-years out and around a hundred and six across, and it is approaching us at some 255 kilometres a second, which is quick even for a halo object on a long orbit. Méchain found it in 1780 and Messier confirmed it, and neither of them could have told you it was anything but faint.
- Catalogue
- M72
- Type
- Globular cluster
- Constellation
- Aquarius
- Distance
- 55,400 light-years
- Apparent magnitude
- 9.4
- Surface brightness
- 21.9 mag/□″
- Apparent size
- 6′
- Right ascension
- 20h 53m
- Declination
- −12.53°
- Found by
- Pierre Méchain, 1780
Small enough that its 9.4 magnitudes arrive concentrated rather than smeared: 21.9 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 4.
| 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 | Challenge | Challenge | Challenge | Challenge |
| Bortle 6 19.3 mag/□″ | No | No | 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
Faint, small and unrewarding — at 150mm it is a slightly soft star and at 250mm it is a slightly soft star with a hint of grain. It is on the list for completeness, and finishing the list is the only real reason to look at it.
When, and from where
Highest in the middle of the night in August. Overhead at 13°S and southerly enough to sit low from northern Europe, though never out of reach there — only north of 77°N does it stop rising at all.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 17° | Low — in the murk |
| 51°N London, Calgary | 26° | |
| 40°N New York, Madrid | 37° | |
| 30°N Cairo, Houston | 47° | |
| 0°N the equator | 77° | |
| 33°S Sydney, Cape Town | 70° |
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 M72 itself, not from the middle of Aquarius.
- M73 Asterism 1.3° apart Aquarius · magnitude 9.0 · binoculars from Bortle 6 or darker
- M75 Globular cluster 15° apart Sagittarius · magnitude 8.5 · binoculars from Bortle 5 or darker
- M2 Globular cluster 15° apart Aquarius · magnitude 6.5 · binoculars from Bortle 5 or darker
- M30 Globular cluster 15° apart Capricornus · magnitude 7.2 · binoculars from Bortle 5 or darker
One of 110. Positions J2000; the sky is the final authority.