Stars in our Milky Way galaxy tend not to exist alone.   The groupings are either small, with just a few stars, or large, with thousands of stars.    
Star Systems
When stars interact with each other as in multiple star systems or clusters, We can gain great insight into their physical nature.  Consider this:  in these closed gravitational systems, we can make the assumption that all of the stars:
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Carpe Caelum
Small groups of two or more stars are called binary stars.  These systems operate with the basic rules of gravitational attraction.  In fact, it is in binary systems where we discover the masses of stars.
Star clusters come in one of two sizes;  open clusters, the younger variety, have a mere thousand stars or so.  Globular clusters, the ancient type, can contain up to one million stars.
Carpe Caelum Stellar Astronomy