M25 - IC 4725

Messier 25 (IC 4725) is a bright open cluster in the constellation Sagittarius, located approximately 2,000 light-years from Earth. It is one of the few Messier objects cataloged in the Index Catalogue (IC) rather than the more common New General Catalogue (NGC), a historical curiosity resulting from it being missed during the compilation of the NGC. The cluster contains roughly 86 stars within a diameter of about 19 light-years and shines at a combined apparent magnitude of about 4.6, making it visible to the naked eye under reasonably dark skies. M25 is estimated to be around 90 million years old and contains a notable Cepheid variable star, U Sagittarii, which varies between magnitudes 6.3 and 7.1 over a period of 6.74 days. Cepheid variables are critically important in astronomy as standard candles for measuring cosmic distances, and U Sagittarii was among the early examples studied to refine the period-luminosity relationship. The cluster was discovered by Philippe Loys de Cheseaux around 1745 and independently noted by Messier in 1764. Through binoculars, M25 is a striking sight, displaying about two dozen stars in a loose but attractive grouping against the Milky Way. A small telescope at low magnification reveals the full complement of member stars, including a nice color contrast between the brighter blue-white members and the yellow-orange giant U Sagittarii. The cluster spans about 32 arcminutes, roughly the size of the full Moon, so it is best appreciated at low power in wide-field instruments.

Contains about 86 stars spanning 19 light-years at a distance of approximately 2,000 light-years, with an age of around 90 million years.