Nebula in Sagittarius
Omega Nebula (M17)
Called the Omega, the Swan and the Horseshoe by different people looking at the same bar of light with a hook on the end. One of the brightest nebulae in the sky and among the easiest to see without a filter.
What is visible is only the hot surface of something much larger: a cloud of several thousand solar masses of gas, most of it dark, with the lit face turned toward us. It is one of the most active star-forming regions in the galaxy, and among the youngest things in the catalogue at under a million years.
- Catalogue
- M17
- Type
- Nebula
- Constellation
- Sagittarius
- Distance
- 5,500 light-years
- Apparent magnitude
- 6.0
- Surface brightness
- 22.7 mag/□″
- Apparent size
- 46′ × 37′
- Right ascension
- 18h 21m
- Declination
- −16.18°
- Found by
- Philippe Loys de Chéseaux, 1745
Magnitude 6.0 sounds comfortable and is spread over 46′, which brings it down to 22.7 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?
Nothing in the common range of amateur equipment makes this straightforward under any sky — it is a photographic target, or one for patience and a very large aperture.
| Sky | Eye | 10×50 | 80mm | 130mm | 200mm |
|---|---|---|---|---|---|
| Bortle 1 21.9 mag/□″ | Challenge | Challenge | Challenge | Challenge | Challenge |
| Bortle 2 21.7 mag/□″ | Challenge | Challenge | Challenge | Challenge | Challenge |
| Bortle 3 21.4 mag/□″ | Challenge | Challenge | Challenge | Challenge | Challenge |
| Bortle 4 20.9 mag/□″ | Challenge | Challenge | Challenge | Challenge | Challenge |
| Bortle 5 20.3 mag/□″ | No | No | No | No | No |
| Bortle 6 19.3 mag/□″ | No | No | No | No | No |
| Bortle 7 18.6 mag/□″ | No | No | No | No | No |
| Bortle 8 18.1 mag/□″ | No | No | No | No | No |
| Bortle 9 17.8 mag/□″ | No | No | No | No | No |
At the eyepiece
The brightest nebula most people have never looked at. The bar and its hook are visible without a filter from a reasonable sky, and an OIII turns it emphatic. Low power, because the surrounding field is part of the effect — this is a nebula that benefits from context.
When, and from where
Highest in the middle of the night in June. Overhead at 16°S and southerly enough to sit low from northern Europe, though never out of reach there — only north of 74°N does it stop rising at all.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 14° | Low — in the murk |
| 51°N London, Calgary | 23° | |
| 40°N New York, Madrid | 34° | |
| 30°N Cairo, Houston | 44° | |
| 0°N the equator | 74° | |
| 33°S Sydney, Cape Town | 73° |
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 M17 itself, not from the middle of Sagittarius.
- M18 Open cluster 58′ apart Sagittarius · magnitude 7.5 · binoculars from Bortle 9 or darker
- M16 Eagle Nebula 2.4° apart Serpens · magnitude 6.0 · binoculars from Bortle 2 or darker
- M24 Sagittarius Star Cloud 2.5° apart Sagittarius · magnitude 2.5 · binoculars from Bortle 9 or darker
- M25 Open cluster 4.0° apart Sagittarius · magnitude 4.6 · binoculars from Bortle 9 or darker
One of 110. Positions J2000; the sky is the final authority.