Asterism in Sagittarius
Sagittarius Star Cloud (M24)
Not an object at all but a hole in the dust — a window through the arm of the galaxy in front of us, looking out onto the star fields beyond. Messier catalogued the view rather than a thing, which is the only entry of its kind on the list.
The window is something like six hundred light-years wide and the stars visible through it are ten to sixteen thousand light-years away, in the next arm of the galaxy inward. Two small dark nebulae, Barnard 92 and 93, sit inside the field as black notches — dust in front of the hole in the dust.
- Catalogue
- M24
- Type
- Asterism
- Constellation
- Sagittarius
- Distance
- about 10,000 light-years to the near edge
- Apparent magnitude
- 2.5
- Surface brightness
- 20.9 mag/□″
- Apparent size
- 90′
- Right ascension
- 18h 17m
- Declination
- −18.48°
- Found by
- Charles Messier, 1764
Magnitude 2.5 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?
Bright enough for any sky there is, including the middle of a city. One of the few things on this list of which that is true.
| 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/□″ | Easy | Easy | Easy | Easy | Easy |
| Bortle 9 17.8 mag/□″ | Easy | Easy | Easy | Easy | Easy |
At the eyepiece
Point binoculars at it and the field simply fills with stars, more than you can separate, which is the whole experience. There is no object to focus on and no centre to find. It is the one entry on this list where the correct instrument is the smallest one you own.
When, and from where
Highest in the middle of the night in June. Overhead at 18°S and southerly enough to sit low from northern Europe, though never out of reach there — only north of 72°N does it stop rising at all.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 12° | Low — in the murk |
| 51°N London, Calgary | 21° | |
| 40°N New York, Madrid | 32° | |
| 30°N Cairo, Houston | 42° | |
| 0°N the equator | 72° | |
| 33°S Sydney, Cape Town | 75° |
Nearby in the sky
The whole catalogue →This is a busy patch. 4 more of the catalogue sit within five degrees, which is a binocular field or two — worth planning a session around rather than visiting once. Everything below is measured from M24 itself, not from the middle of Sagittarius.
- M18 Open cluster 1.5° apart Sagittarius · magnitude 7.5 · binoculars from Bortle 9 or darker
- M17 Omega Nebula 2.5° apart Sagittarius · magnitude 6.0 · needs more than binoculars
- M25 Open cluster 3.6° apart Sagittarius · magnitude 4.6 · binoculars from Bortle 9 or darker
- M16 Eagle Nebula 4.7° apart Serpens · magnitude 6.0 · binoculars from Bortle 2 or darker
One of 110. Positions J2000; the sky is the final authority.