An open star cluster is a collection of 10 to several 1000 stars with a very faint center. Within the Milky Way, up to 15,000 such open star clusters are suspected. Until today 1,200 are known.
Since it is assumed that the stars in such an accumulation were formed at about the same time, their ages can be easily studied on the basis of this. In principle, open star clusters are rather young, since the clusters dissolve under the influence of external gravitational forces and therefore do not persist over a particularly long period of time.
In contrast to the more densely populated globular clusters, open star clusters have a different size, localization, age and formation, but especially a lower star density. The stars within an open cluster have similar properties. Thus the individual stars that are part of such a society have the same proper motion and the same radial velocity.
In terms of size, a star cluster is divided into its core and corona. The core of an open star cluster is 3 to 4 light years across. The corona extending around it covers a range of up to 20 light years.
Globular clusters contain mostly old stars. Open star clusters, on the other hand, disperse before the majority of the stars die, which is why the light is usually produced by the young, blue, hot stars. These have a higher weight and thus a shorter life expectancy.