Globular cluster in Hercules
M92
A superb globular that would be famous anywhere else in the sky, and is instead nine degrees from M13 and permanently in its shadow. It is older — perhaps fourteen billion years — and among the most metal-poor clusters known.
The stars in it hold about a two-hundredth of the Sun's heavy elements, which puts its formation about as early as anything in the galaxy. It is one of the few globulars, too, whose orbit brings it near enough to the plane to be disturbed, and yet it has held together — a dense enough core to survive what stripped M12.
- Catalogue
- M92
- Type
- Globular cluster
- Constellation
- Hercules
- Distance
- 26,700 light-years
- Apparent magnitude
- 6.4
- Surface brightness
- 20.8 mag/□″
- Apparent size
- 14′
- Right ascension
- 17h 17m
- Declination
- +43.13°
- Found by
- Johann Elert Bode, 1777
Magnitude 6.4 sounds comfortable and is spread over 14′, which brings it down to 20.8 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 6.
| 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 | Easy | Easy | Easy | Easy |
| Bortle 7 18.6 mag/□″ | No | Challenge | Challenge | Challenge | Challenge |
| Bortle 8 18.1 mag/□″ | No | Challenge | Challenge | Challenge | Challenge |
| Bortle 9 17.8 mag/□″ | No | No | No | No | No |
At the eyepiece
Would be the showpiece of any other constellation. Smaller and tighter than M13 but brighter per unit area, so it takes magnification well and its core stays stubbornly unresolved. Put the two side by side in one evening; the comparison flatters M92 more than people expect.
When, and from where
Highest in the middle of the night in June. Well placed for the northern hemisphere and overhead for anyone near 43°N. South of 47°S it stays below the horizon.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 73° | Never sets |
| 51°N London, Calgary | 82° | Never sets |
| 40°N New York, Madrid | 87° | |
| 30°N Cairo, Houston | 77° | |
| 0°N the equator | 47° | |
| 33°S Sydney, Cape Town | 14° | Low — in the murk |
Nearby in the sky
The whole catalogue →Nothing else in the catalogue is especially close; the nearest is 9.5° apart, which is a separate trip rather than a nudge of the tube. Everything below is measured from M92 itself, not from the middle of Hercules.
- M13 Hercules Cluster 9.5° apart Hercules · magnitude 5.8 · binoculars from Bortle 5 or darker
- M57 Ring Nebula 21° apart Lyra · magnitude 8.8 · binoculars from Bortle 6 or darker
- M102 Spindle Galaxy 24° apart Draco · magnitude 9.9 · binoculars from Bortle 4 or darker
- M56 Globular cluster 27° apart Lyra · magnitude 8.3 · binoculars from Bortle 5 or darker
One of 110. Positions J2000; the sky is the final authority.