M25
IC 4725
Object Data
- Catalog Designation
- M25
- Type
- OpenCluster
- Constellation
- Sagittarius
- Magnitude
- 4.6
- Right Ascension
- 18h 31m 36.0s
- Declination
- -19° 15' 00.0"
- Distance
- 2,000 light-years
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About M25
Description
M25 is a bright open cluster in Sagittarius, about 2,000 light-years from Earth. It contains roughly 600 stars spread across about 19 light-years, with an age of about 90 million years. The cluster includes the notable Cepheid variable star U Sagittarii, one of the few Cepheids in a Messier object.
Observing Tips
Located about 6 degrees north of M24. Easily visible in binoculars as a bright, scattered group of stars. A telescope at 40-60x provides the best view, revealing about 30 stars in a loose but attractive pattern. Look for the yellowish Cepheid variable U Sagittarii, which varies from magnitude 6.3 to 7.1 over 6.74 days. Best observed from June through September.
History
Discovered by Philippe Loys de Cheseaux in 1745-46. Charles Messier cataloged it in 1764. Curiously, M25 was omitted from the NGC catalog due to an error in recording its position, though it later received the IC designation IC 4725.
Fun Facts
The Cepheid variable U Sagittarii in M25 pulsates with a period of 6.74 days, changing its brightness by nearly a full magnitude. Cepheid variables are crucial 'standard candles' in astronomy — their period-luminosity relationship, discovered by Henrietta Leavitt in 1912, is one of the primary tools for measuring cosmic distances.
Community Photos (1)
Credit: Starhopper. License: CC BY-SA 4.0. (Wikimedia Commons)
Skybred Feb 28, 2026
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