Star Clusters fall in two categories: Open Clusters and Globular Clusters.
Open Clusters
They consist of an irregular collection of a few dozen to several hundred
stars, which are gravitationaly bound. They lie in the disk or spiral arms of
our galaxy, the Milky Way, therefore most visible Open Clusters are at a
relative short distance. Dust from our galaxy hides Open Clusters farther away.
Most open clusters are relatively young, some of the youngest being still within
the nebulae in which they formed. It is sometimes impossible to know if the
stars from a cluster are gravitationaly bound or if it is an optical illusion
(known as "asterism").
Globular Clusters
They consist of a spherical collection of up to 100,000 stars and can be
found both in the disk and halo of the Milky Way. Globular Clusters formed early
in galactic history, having ages of 10 billion years or more. |