|Right Ascension||18 : 31.6 (h:m)
|Declination||-19 : 15 (deg:m)
|Visual Brightness||6.5 (mag)
|Apparent Dimension||40.0 (arc min)
Discovered 1745-46 by Philippe Loys de Cheseaux.
Although a conspicuous cluster, even in the smallest telescopes or opera glasses, M25 has only obtained an IC number. This is because due to unknown reason, John Herschel did not include it in his General Catalog, although it had been observed by de Cheseaux in 1745-46, Messier in 1764, as well as by Bode (1774-77) and Admiral Smyth (1836). It was finally rediscovered by Schmidt in 1866 and added to the second Index Catalog in 1908, using a position obtained by Bailey.
Two giants of spectral type M and two of type G can be found in this cluster, where the G type giants appear to be actual members (the M's not). It moreover contains the Cepheid variable U Sagittarii, which has a period of 6.74 days, a typical period for these variables "in our neighborhood," as Cecilia Payne-Gaposhkin put it. It was discovered by J.B. Irwin in 1956, its membership was confirmed by radial velocity measurements conducted by M.W. Feast of Radcliffs Observatory (the common RV of the cluster members is +4 km/sec).
The occurance of the Cepheid is consistent with the fact that it is not a very young cluster, its age may be about 90 million years (the Sky Catalog 2000 gives 89 million).
The sources agree unusually good in this cluster's distance and give about 2,000 light years. This makes its 40 arc minute diameter corresponding to about 23 light years.
This cluster contains at least 86 probable members, according to Ake Wallenquist. While the Sky Catalog 2000 gives its Trumpler type as I,2,p, Götz classifies it as I,3,m, and Kenneth Glyn Jones quotes Trumpler with IV,3,r. Quite a difference !
Last Modification: 9 Dec 1999, 22:58 MET