M25 is a bright, mag. +4.6, naked eye open cluster in Sagittarius that's a wonderful sight in binoculars and small telescopes. It was discovered by Philippe Loys de Chéseaux in 1745 and subsequently catalogued by Charles Messier on June 20, 1764. There is however an unusual twist to the history of M25. For such a bright cluster it's reasonable to assume that it would have been included by John Herschel in his comprehensive 19th century General Catalogue. For unknown reasons it wasn't. This is despite the cluster been catalogued by Johann Elert Bode in 1777, observed by William Herschel in 1783 and described by Admiral Smyth in 1836. M25 was finally included in 1908, by J.L.E. Dreyer, in the supplementary Index Catalogue (as IC 4725).
Finding M25 is relatively easy. It's positioned 6.5 degrees north and a little east of the top star of the bright teapot asterism of Sagittarius, Kaus Borealis (λ Sgr - mag. +2.8). Only 3.5 degrees west of M25 is M24, the large Sagittarius Star Cloud.
The cluster is best seen from southern and equatorial regions during the months of June, July and August. For mid-latitude Northern Hemisphere observers, it appears low down during the summer months.
To the naked eye, M25 appears as a faint fuzzy patch of light. It's better defined in 10x50 binoculars with the brightest stars resolvable. An 80mm (3.1-inch) telescope displays a large loosely defined irregular shaped grouping of about 30 mainly white stars. In medium size telescopes, it's easily resolvable and more striking as individual colours are pronounced. A 200mm (8-inch) scope reveals up to 60 stars of many colours spread across 32 arc minutes of apparent space.
The cluster contains four brighter stars, one of which is a Cepheid variable (U Sagittarii) that varies between mag. +6.3 and +7.1 over a period of 6.745 days. It appears yellowish in colour and is located towards the centre of the cluster. Since M25 contains many bright comparison stars, it's possible to accurately estimate the brightness of U Sagittarii over the full variation period.
In total, M25 contains at least 80 stars and is estimated to be 90 million years old. It's located 2,000 light-years from Earth and has a spatial diameter of 20 light-years.
M25 Data Table
|Object Type||Open cluster|
|RA (J2000)||18h 31m 47s|
|DEC (J2000)||-19d 06m 54s|
|Apparent Size (arc mins)||32 x 32|
|Age (years)||90 Million|
|Number of Stars||>80|
|Other Names||Collinder 382, Melotte 204|
|Notable Feature||A Delta Cephei type variable star (U Sgr) is a cluster member|