M 21 Deep sky · Open cluster in Sagittarius · M 21 · NGC 6531
Webb's Cross
A young open cluster just north-east of the Trifid Nebula, with a cross of brighter stars at its centre.
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- Type
- Open cluster
- Designations
- M 21 · NGC 6531
- Constellation
- Sagittarius
- Apparent magnitude
- 5.9
- Apparent size
- 16′
- Position (J2000)
- RA 18h 04m · Dec −22° 29′
The full Moon, about 31′ across, is 1.9 times as wide. The Sun passes within a degree of it around late December, and for about 29 days of the year it sits within 15° of the Sun, lost in its glare.
When to see it
On the night of Sun 20 Jun 2027, when it is highest around midnight, on each city’s own clock:
| From | Rises | Highest | Sets |
|---|---|---|---|
| London GMT+1 | 21:16 | 01:10 · 16° | 05:04 |
| New York EDT | 20:29 | 01:05 · 27° | 05:40 |
| Singapore GMT+8 | 19:19 | 01:15 · 66° | 07:12 |
| Sydney GMT+10 | 17:03 | 00:06 · 79° | 07:10 |
Common questions
- Can I see it from 50° north?
- Yes. It rises and sets, and at its highest it stands 18° up in the south.
- Can I see it from 35° south?
- Yes. It rises and sets, and at its highest it stands 77° up in the north.
- When is the best time of year to see it?
- Around mid June, when it stands opposite the Sun and is highest at midnight. It comes round about two hours earlier each month, so by mid September it is highest in the early evening, and in late March not until the hours before dawn.
- Naked eye, binoculars or telescope?
- The naked eye, from a dark country sky, which shows stars to about magnitude 7.0; from a city, binoculars.
Look for
Put the Trifid Nebula in a low-power eyepiece and M 21 sits in the same field — the stars of a cluster beside the glow of a nursery.
Look for
From 50° north it climbs to 18°; from 35° north it climbs to 33°; from the equator it climbs to 68°; from 35° south it climbs to 77°.
Look for
Magnitude 5.9 against the faintest star each can show: the eye under a city sky, 4.5 (1.4 short); the eye under a dark country sky, 7.0 (1.1 to spare); 50 mm binoculars under that dark sky, 11.0 (5.1 to spare); a 100 mm telescope there, 12.5 (6.6 to spare). Through 7×50 binoculars, whose field is about 7.1° across, it spans 4% of the field’s width; through a 100 mm telescope at 50×, a field about 1° across, it spans 27% of the field’s width.
Words used here
Its constellation
Nearby in Sagittarius
- Nebula Lagoon Nebula
- Nebula Trifid Nebula
- Globular cluster Great Sagittarius Cluster
- Nebula Omega Nebula
- Open cluster Black Swan Cluster
- Open cluster M 23
- Open cluster Small Sagittarius Star Cloud
- Open cluster M 25
- Globular cluster M 28
- Globular cluster M 54
- Globular cluster Specter Cluster
- Globular cluster M 69
- Globular cluster M 70
- Globular cluster M 75