Open cluster
M21 (M21)
In Sagittarius (Sgr) • Magnitude 6.5 • 13 arcminutes
Open the free AstroPlanner with M21 pre-selected, scored against your telescope, location, and the live cloud forecast.
A young open cluster in Sagittarius whose stars formed roughly 4.6 million years ago — around the same time as some of the earliest human ancestor fossils. By comparison, the stars of our own Sun are roughly a thousand times older.
M21 at a glance
| Catalog IDs | M21, N 6531 |
| Type | Open cluster |
| Constellation | Sagittarius (Sgr) |
| Right ascension | 18h 04m 37s |
| Declination | -22° 30' 00" |
| Apparent magnitude | 6.50 |
| Angular size | 13.0 × 13.0 arcmin |
| Max altitude at 45°N | 23° |
| Best imaging months | Mar, Apr, May |
How to image M21
M21 sits in the constellation Sagittarius at right ascension 18h 04m 37s and declination -22° 30' 00". To frame and integrate it well, AstroPlanner will compute the optimal moonless window for tonight from your location, the field-of-view fit against your sensor and focal length, the suggested total integration time given your aperture and sky Bortle class, and a cloud-aware schedule that drops it from the plan if your nearest cloud forecast spike overlaps the best altitude window.