M25

From South Dublin Astronomical Society

M25
Type	Open cluster
NGC	IC 4725
Constellation	Sagattarius
Right Ascension	18h 31.6m
Declination	-19° 15’
Magnitude	6.5
Size	40 arc min.
Image:M25 messier image.jpg

M25 is a beautiful loose open cluster for binoculars and can be found 3¼º almost due east of the Small Sagittarius Star Cloud, M24. Two bright stars will be seen at the top edge of the association with a fainter sun in between. The Cepheid variable star U Sagittarii lies at the heart of the group and fluctuates between magnitudes 6·3 and 7·1 over a period of 6·74 days. A prominent feature of the cluster is the way the stars appear to be arranged in two curved chains backing each other — one observer commented to the author that they looked like two brackets back-to-back and indeed this is a very apt description. The stars in the upper chain are noticeably brighter than those of the lower. The cluster was discovered in 1745-46 by Philippe Loys de Chéseaux and is about 2,000 light years distant. Although not a particularly rich group with only about 90 stars claiming membership, it’s distinctive shape is appealing.