Nebula in Sagittarius
Trifid Nebula (M20)
The Trifid's three dark lanes are dust in front of the glow, not gaps in it. Photographs make the division look obvious and the eyepiece does not, which is why picking it out is one of the honest tests of a sky.
It is also one of very few objects that shows both kinds of nebula at once: red emission where gas is being made to glow from within, and blue reflection above it where dust is merely catching light. At a few hundred thousand years old it is about as young as anything gets, and the star doing the lighting is a triple.
- Catalogue
- M20
- Type
- Nebula
- Constellation
- Sagittarius
- Distance
- 4,100 light-years
- Apparent magnitude
- 6.3
- Surface brightness
- 22.2 mag/□″
- Apparent size
- 28′
- Right ascension
- 18h 03m
- Declination
- −23.03°
- Found by
- Charles Messier, 1764
Magnitude 6.3 sounds comfortable and is spread over 28′, which brings it down to 22.2 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 3 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/□″ | No | Challenge | Challenge | Challenge | Challenge |
| Bortle 5 20.3 mag/□″ | No | Challenge | Challenge | Challenge | Challenge |
| 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
Under a poor sky this is one faint glow. Under a good one, with 150mm and patience, the dark lanes appear and split it into three, and the blue reflection portion to the north becomes a separate thing rather than part of the same cloud. The difference between those two nights is entirely the sky.
When, and from where
Highest in the middle of the night in June. Overhead at 23°S and southerly enough to sit low from northern Europe, though never out of reach there — only north of 67°N does it stop rising at all.
| Latitude | Highest it gets | |
|---|---|---|
| 60°N Oslo, Anchorage | 7° | Low — in the murk |
| 51°N London, Calgary | 16° | Low — in the murk |
| 40°N New York, Madrid | 27° | |
| 30°N Cairo, Houston | 37° | |
| 0°N the equator | 67° | |
| 33°S Sydney, Cape Town | 80° |
In the archive
Nearby in the sky
The whole catalogue →This is a busy patch. 3 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 M20 itself, not from the middle of Sagittarius.
- M21 Open cluster 42′ apart Sagittarius · magnitude 6.5 · binoculars from Bortle 9 or darker
- M8 Lagoon Nebula 1.4° apart Sagittarius · magnitude 4.6 · binoculars from Bortle 3 or darker
- M23 Open cluster 4.2° apart Sagittarius · magnitude 5.5 · binoculars from Bortle 9 or darker
- M28 Globular cluster 5.3° apart Sagittarius · magnitude 6.8 · binoculars from Bortle 6 or darker
One of 110. Positions J2000; the sky is the final authority.