Open cluster in Cygnus
M39
Thirty-odd stars spread over most of a Moon-width, and so loose that magnification destroys it. This is a binocular object, and a telescope simply takes it apart.
At eight hundred light-years it is among the nearest clusters in the catalogue, which is the whole reason it is spread so wide — thirty stars looking like a cluster only because they are close to us. At roughly three hundred million years it has already begun coming apart at the edges.
- Catalogue
- M39
- Type
- Open cluster
- Constellation
- Cygnus
- Distance
- 800 light-years
- Apparent magnitude
- 4.6
- Surface brightness
- 20.8 mag/□″
- Apparent size
- 32′
- Right ascension
- 21h 32m
- Declination
- +48.43°
- Found by
- Charles Messier, 1764
Magnitude 4.6 for the cluster as a whole, though that is a sum rather than a thing you see: what reaches you is a scatter of separate stars, and whether you catch it depends on how bright the brightest of them are. Why surface brightness is the number that matters.
Could you see it?
Straightforward with the naked eye from Bortle 7 or darker; with 10×50 binoculars from Bortle 9.
| 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/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 7 18.6 mag/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 8 18.1 mag/□″ | No | Easy | Easy | Easy | Easy |
| Bortle 9 17.8 mag/□″ | No | Easy | Easy | Easy | Easy |
At the eyepiece
So large and so loose that a telescope is actively the wrong instrument. In binoculars it is a neat triangular scattering of about thirty bright stars against the Cygnus Milky Way. At any real magnification it stops being a cluster and becomes a random field.
When, and from where
Highest in the middle of the night in August. Well placed for the northern hemisphere and overhead for anyone near 48°N. South of 42°S it stays below the horizon.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 78° | Never sets |
| 51°N London, Calgary | 87° | Never sets |
| 40°N New York, Madrid | 82° | |
| 30°N Cairo, Houston | 72° | |
| 0°N the equator | 42° | |
| 33°S Sydney, Cape Town | 9° | Low — in the murk |
Nearby in the sky
The whole catalogue →Nothing else in the catalogue is especially close; the nearest is 16° apart, which is a separate trip rather than a nudge of the tube. Everything below is measured from M39 itself, not from the middle of Cygnus.
- M29 Open cluster 16° apart Cygnus · magnitude 6.6 · binoculars from Bortle 9 or darker
- M52 Open cluster 20° apart Cassiopeia · magnitude 6.9 · binoculars from Bortle 9 or darker
- M56 Globular cluster 32° apart Lyra · magnitude 8.3 · binoculars from Bortle 5 or darker
- M27 Dumbbell Nebula 32° apart Vulpecula · magnitude 7.4 · binoculars from Bortle 5 or darker
One of 110. Positions J2000; the sky is the final authority.