Messier 25
Messier 25 is a large, loose, open cluster in Sagittarius. M25 shines at magnitude 4.6, so it can be viewed with the unaided eye under good sky conditions at a dark-sky observing site. It subtends an apparent angle of 25 arcminutes, the same size as the full moon. At a distance of 2,000 light-years, this would correspond to a true size of 16 light-years in diameter in space. It is estimated that there are 86 stars in M25, but there may be up to several hundred true members. More stars can be detected with larger apertures and longer time-exposure images. M25 appears to contain two M-type giant stars, but they are not true members of the cluster, they just happen to lie along the same line of sight. The cluster also contains several blue straggler stars. These large blue stars are thought to have formed through an exchange of mass in a binary system. Blue stragglers are usually found in young open clusters, but most are not type Be stars. In M25, a large number of the blue stragglers are type Be giant stars, a clue that the cluster is not young. M25 also contains the variable star U Sagittarii, a Delta Cephei type variable that ranges from magnitude 6.3 to magnitude 7.1. This is the type of star that provides a standard yardstick for measuring the distances of objects in space. This star was used to determine the distance of 2,000 light-years to the cluster. Delta Cephei type variables also usually occur in older clusters. This star, together with the presence of blue stragglers, leads to an estimate of M25's age as being 90 million years old. M25 was discovered by Philippe Loys de Chéseaux in 1745. North is to the top in the above image.
|
|||||||
|
