| Open |
These are also known as Galactic clusters.
They are physically related groups of stars held together by means of
gravitational attraction. Believed to have originated from large cosmic
gas and dust clouds in the Milky Way, they continue to orbit the galaxy
through the disk. New open star clusters are forming in diffused nebula
as you read this. These are composed of young population I stars. Open
clusters are found mainly in the same regions as that of diffuse nebula,
most commonly in spiral arms in disk galaxies and irregular galaxies.
Open clusters are found in the band of the Milky Way.
Most open clusters have only a short life as
stellar swarms. As they drift along their orbits, some of their members
escape the cluster, due to velocity changes in mutual closer encounters,
tidal forces in the galactic gravitational field, and encounters with
field stars and interstellar clouds crossing their way. An average open
cluster has spread most of its member stars along its path after several
100 million years; only few of them have an age counted by billions of
years. The escaped individual stars continue to orbit the Galaxy on
their own as field stars: All field stars in our and the external
galaxies are thought to have their origin in clusters. |
