Globular cluster
M55 (M55)
In Sagittarius (Sgr) • Magnitude 7.4 • 19 arcminutes
Open the free AstroPlanner with M55 pre-selected, scored against your telescope, location, and the live cloud forecast.
A large, loosely concentrated globular cluster whose stars are spread out enough that it looks strikingly diffuse compared to typical globulars. Its 100,000 stars are simply far less compressed, giving it an almost "dissolved" look at low magnification that led early observers to debate whether it was a true globular at all.
M55 at a glance
| Catalog IDs | M55, N 6809 |
| Type | Globular cluster |
| Constellation | Sagittarius (Sgr) |
| Right ascension | 19h 40m 01s |
| Declination | -30° 58' 12" |
| Apparent magnitude | 7.42 |
| Surface brightness | 13.7 mag/arcsec² |
| Angular size | 19.0 × 19.0 arcmin |
| Max altitude at 45°N | 14° |
| Best imaging months | Apr, May, Jun |
How to image M55
M55 sits in the constellation Sagittarius at right ascension 19h 40m 01s and declination -30° 58' 12". 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.