Globular cluster
Great Hercules Cluster (M13)
In Hercules (Her) • Magnitude 5.8 • 20 arcminutes
Open the free AstroPlanner with Great Hercules Cluster pre-selected, scored against your telescope, location, and the live cloud forecast.
The most famous globular cluster in the northern sky, visible to the naked eye as a faint smudge on dark nights and containing roughly 300,000 stars in a ball 145 light-years wide. In 1974 scientists aimed the Arecibo radio telescope directly at M13 and broadcast humanity's first deliberate message to the stars — it will arrive in about 25,000 years.
Great Hercules Cluster at a glance
| Catalog IDs | M13, N 6205 |
| Type | Globular cluster |
| Constellation | Hercules (Her) |
| Right ascension | 16h 41m 42s |
| Declination | +36° 27' 36" |
| Apparent magnitude | 5.78 |
| Surface brightness | 12.0 mag/arcsec² |
| Angular size | 20.0 × 20.0 arcmin |
| Max altitude at 45°N | 81° |
| Best imaging months | Feb, Mar, Apr |
How to image Great Hercules Cluster
Great Hercules Cluster sits in the constellation Hercules at right ascension 16h 41m 42s and declination +36° 27' 36". 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.