Physical Properties

M 25 is a loose star cluster, a bright but not particularly rich collection of about 50 stars brighter than magnitude 12 and a few dozen weaker stars. Due to an error, the star cluster was not listed in John Herschel's «General Catalogue» of 1864, was also forgotten by J. L. E. Dreyer in his "New General Catalog" and was finally included in the «Index Catalogue» in 1908 under the number IC 4725. The distance is given as 620 parsecs (2022 light years). ^{[4, 145]}

An unusual member is hidden in M ​​25: U Sagittarii is a bright, classic Cepheid variable, first noticed by J. Schmidt in 1866. According to studies by J. B. Irwin in 1955, the affiliation of the star to the star cluster appears to be fairly certain. U Sagittarii is a normal Cepheid with a magnitude of 6.3 mag to 7.1 mag and a period of 6.77925 days. The spectral range extends from F5 to approximately G1. Cepheids are very rarely found in star clusters. As with all Cepheids, the color of the light becomes more yellowish when the brightness goes towards a minimum. U Sagittarii is also a visual double star. The 9.5 mag bright companion star is located at a distance of 66.5 "at a position angle of 253°. The distance between the two stars is estimated at around 40'000 AU. The weaker star is in turn accompanied by a companion with 10.1 mag at a distance of 0.7 arc seconds (430 AU) circled to a position angle of 122°. The last distance measurement from 2018 is 684 pc ± 21 pc. (2231 ly ± 68 ly) ^{[4, 145]}

Finder Chart

The open star cluster M 25 is located in the constellation Sagittarius, around 3° west of the Small Sagittarius Cloud (M 24). Connect the stars Kaus Borealis (λ Sagittarii) and γ Scuti. Place the Telrad halfway one degree west of it, so that the middle Telrad circle comes to rest on the line. The outer Telrad circle then lies roughly on the line of the three stars π - ξ - μ Sagitarii.