Globular cluster in Scorpius
M4
The only globular on Messier's list that Messier could actually resolve — he wrote it down as a cluster of very small stars while everything else of its kind stayed a smudge to him. It is the nearest one to us, which is the whole explanation.
A dark lane of stars runs across the middle, unusual enough in a globular to be worth looking for. Orbiting a pulsar inside it is a planet of about Jupiter's mass and roughly thirteen billion years' age — near enough as old as the galaxy, and formed when there was barely anything heavy to build a planet from.
- Catalogue
- M4
- Type
- Globular cluster
- Constellation
- Scorpius
- Distance
- 7,200 light-years
- Apparent magnitude
- 5.6
- Surface brightness
- 21.3 mag/□″
- Apparent size
- 26′
- Right ascension
- 16h 24m
- Declination
- −26.53°
- Found by
- Philippe Loys de Chéseaux, 1746
Magnitude 5.6 sounds comfortable and is spread over 26′, which brings it down to 21.3 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 from a Bortle 5 sky or darker, and aperture makes almost no difference to that — spread this thinly, an object is limited by how bright the sky behind it is rather than by how much light the telescope collects. A darker site beats a bigger telescope.
| 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/□″ | Easy | 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
The easiest globular in the sky to resolve, and the bar of stars running north to south through the middle is genuinely visible rather than a photographic artefact. From Britain it never gets high, so the trick is catching it on the meridian in June and accepting whatever the atmosphere gives you.
When, and from where
Highest in the middle of the night in May. A southern object: overhead at 27°S, and low and murky from Britain and the northern United States even at its best. North of 63°N it never rises.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 3° | Low — in the murk |
| 51°N London, Calgary | 12° | Low — in the murk |
| 40°N New York, Madrid | 23° | |
| 30°N Cairo, Houston | 33° | |
| 0°N the equator | 63° | |
| 33°S Sydney, Cape Town | 84° |
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 M4 itself, not from the middle of Scorpius.
- M80 Globular cluster 3.9° apart Scorpius · magnitude 7.3 · binoculars from Bortle 6 or darker
- M19 Globular cluster 8.7° apart Ophiuchus · magnitude 6.8 · binoculars from Bortle 5 or darker
- M62 Globular cluster 9.0° apart Ophiuchus · magnitude 6.5 · binoculars from Bortle 6 or darker
- M107 Globular cluster 14° apart Ophiuchus · magnitude 8.1 · binoculars from Bortle 5 or darker
One of 110. Positions J2000; the sky is the final authority.