Messier 25 (IC 4725) is one of the more remarkable open clusters in the constellation Sagittarius.
M 25 was discovered by Philippe Loys de Cheseaux in 1746 and re-observed by Messier in 1764. However, for unknown reasons William Herschel failed to include it in his General Catalog. Thereforem, M 25 was never added to the NGC, and thus only has an Index Catalog (IC) number.
Visible to the naked eye under dark skies as a blurry spot of magnitude 4.6, Messier 25 is a fine open cluster for binoculars and small telescopes. It contains three dozen bright stars irregularly scattered over a 32' area, and may actually contain up to 600 members. Its more prominent stars form two east-to-west streams, divided by a starless lane. The outlying stars are brighter to the north and south of the cluster's core, than to the east and west.
Sources agree that this cluster's distance is about 2,000 light-years, making its physical diameter about 20 light years. M 25 has an absolute magnitude of -6.5, corresponding to a luminosity of 33,000 suns. It is also notable for containing a Cepheid variable, U Sagittarii, which varies between magnitudes 6.3 and 7.0 over a period of 6.75 days. U Sgr's cluster membership is confirmed by its common radial velocity, shared with the other cluster members (they are all receding at 4 km/sec), and helps provide an accurate distance measurement.
Two giants of spectral type M and G can be seen in this cluster; the G-type giants appear to be actual members (the M's are not). These well-evolved giant stars provide evidence that M 25 is not a very young cluster; it probably has an age of about 90 million years.