A star cluster is an area of greatly increased density of stars compared to the surrounding area of a galaxy . How clearly its concentration exceeds the star background can, however, vary greatly. The stars in a cluster usually belong together in the sense that they were formed together. A distinction is made between open star clusters , which are relatively young and located in the spiral arms , from globular star clusters , which are ancient structures surrounding the galaxy in a halo .
Open star clusters
Open star clusters represent rather loose collections of stars that have formed together from large molecular clouds (gas and dust). Nevertheless, they mostly dissolve over time because their own movements are somewhat different. Therefore, many open star clusters are “astronomically young” objects from an age up to a few hundred million years. They still have many young, massive stars that shine mainly in white-blue light due to their high temperature. One of the most famous open star clusters are the Pleiades .
In the case of globular clusters with significantly more stars, one often assumes a common star formation; due to gravity , the stars remain tied to each other. Globular clusters surround the galaxy at a greater distance and are much older than the open star clusters, they are part of the halo . The age of many globular clusters is on the order of 10 billion years (the age of the universe is estimated to be 13.8 billion years). The brightest globular cluster that can be observed with the naked eye is Omega Centauri and is located in the southern hemisphere of the starry sky.
A third group are the star associations, also called motion clusters or star currents . They are related to the open star clusters and have predominantly young hot stars that seem to move together towards a vanishing point (vertex). These include the Hyades , whose star distribution made a decisive contribution to the development of the Hertzsprung-Russell diagram .
Super star clusters
The class of super star clusters, which has only recently been discovered, stands between the open and globular clusters. They are areas with extremely high star formation rates, which leads to very compact, massive and relatively long-lived star clusters. They show a large number of bright main sequence stars and are surrounded by ionized H-II regions . Their formation is typical in active starburst galaxies .