Globular cluster in Hercules
Hercules Cluster (M13)
The showpiece globular of northern skies, and the target of the Arecibo message in 1974 — a radio transmission aimed at it as a demonstration, not an attempt at contact. It will arrive in about 25,000 years, by which time the cluster will have moved.
Around three hundred thousand stars, and the famous detail is the propeller — three dark lanes meeting at an angle across the south-east face, first noticed by a nineteenth-century observer and still argued over as to whether it is real structure or an accident of how the stars fall. Either way, once found it is unmissable.
- Catalogue
- M13
- Type
- Globular cluster
- Constellation
- Hercules
- Distance
- 22,200 light-years
- Apparent magnitude
- 5.8
- Surface brightness
- 20.9 mag/□″
- Apparent size
- 20′
- Right ascension
- 16h 42m
- Declination
- +36.47°
- Found by
- Edmond Halley, 1714
Magnitude 5.8 sounds comfortable and is spread over 20′, which brings it down to 20.9 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 4 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/□″ | Easy | 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
Deserves its reputation and rewards patience more than aperture. At 150mm the outer half breaks into stars while the core stays solid; at 250mm the dark lanes across the face — the propeller — become findable, and once you have seen them you cannot unsee them.
When, and from where
Highest in the middle of the night in May. Well placed for the northern hemisphere and overhead for anyone near 36°N. South of 54°S it stays below the horizon.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 66° | Never sets |
| 51°N London, Calgary | 75° | |
| 40°N New York, Madrid | 86° | |
| 30°N Cairo, Houston | 84° | |
| 0°N the equator | 54° | |
| 33°S Sydney, Cape Town | 21° |
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 M13 itself, not from the middle of Hercules.
- M92 Globular cluster 9.5° apart Hercules · magnitude 6.4 · binoculars from Bortle 6 or darker
- M102 Spindle Galaxy 25° apart Draco · magnitude 9.9 · binoculars from Bortle 4 or darker
- M57 Ring Nebula 27° apart Lyra · magnitude 8.8 · binoculars from Bortle 6 or darker
- M101 Pinwheel Galaxy 32° apart Ursa Major · magnitude 7.9 · needs more than binoculars
One of 110. Positions J2000; the sky is the final authority.