Open cluster
M25 (M25)
In Sagittarius (Sgr) • Magnitude 4.6 • 32 arcminutes
Open the free AstroPlanner with M25 pre-selected, scored against your telescope, location, and the live cloud forecast.
A bright, scattered open cluster in Sagittarius notable for containing U Sagittarii — a Cepheid variable star that pulsates in brightness with a precise 6.7-day cycle. Cepheid variables are used as "standard candles" to measure cosmic distances, making M25 a tiny but important rung on the cosmic distance ladder.
M25 at a glance
| Catalog IDs | M25 |
| Type | Open cluster |
| Constellation | Sagittarius (Sgr) |
| Right ascension | 18h 31m 37s |
| Declination | -19° 08' 60" |
| Apparent magnitude | 4.60 |
| Angular size | 32.0 × 32.0 arcmin |
| Max altitude at 45°N | 26° |
| Best imaging months | Mar, Apr, May |
How to image M25
M25 sits in the constellation Sagittarius at right ascension 18h 31m 37s and declination -19° 08' 60". 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.