Stars do not occur in space at completely arbitrary places. Some, such as the Sun, are single stars, called Field Stars, but others are members of pairs or form multiple-star systems. Still others form clusters of various types, and size. All of them are condensed from clouds of gas and dust. Nearly all stars are formed in groups or clusters of some kind.
Two main types of star clusters occur: the small and sparse open clusters containing tens to thousands of young stars, and the large and dense globular clusters containing up to several million stars. Large-scale groupings of some stellar types are called associations.
An "OB Association" is a group of stars of types O and B that are close together, because they were all formed from a single cloud of gas. Since O and B stars are very bright their lives are short (up to about 20 million years), they do not have time to wander far away from their place of origin. Stars lighter than those of spectral types O and B are also formed from the same cloud of gas, and reside in the same region of space. When the O and B stars have all reached the ends of their lives, only the lesser stars are left and form an open cluster. T associations are groups of new-born stars before the ignition of nuclear burning.
The image on the left shows a dense swarm of stars called Omega Centauri. It is located some 17,000 light-years from Earth. Omega Centauri is a massive globular star cluster of 150 light years in diameter, containing 10 million stars swirling in locked orbits around a common center of gravity. The stars in Omega Centauri are all very old, about 12 billion years. They are packed so densely in the cluster's core that it is difficult for ground-based telescopes to make out individual stars. Those in the core of Omega Centauri are so densely packed that occasionally one of them will actually collide with another one. Even in the dense center of Omega Centauri, stellar collisions will be infrequent. But the cluster is so old that many thousands of collisions must have occurred. When stars collide head-on, they probably just merge together and make one bigger star. But if the collision is a near miss, they may go into orbit around each other, forming a close binary star system. Omega Centauri is the most luminous and massive globular star cluster in the Milky Way. It is one of the few globular clusters that can be seen with the unaided eye.
SOME WELL-KNOWN CLUSTERS
The Pleides are a conspicuous object in the night sky with a prominent place in ancient mythology. The cluster actually contains thousands of stars, but only seven are commonly visible to the unaided eye, and thus they have been called "The Seven Sisters". The stars in the Pleiades are thought to have formed together around 100 million years ago, making them 1/50th the age of our sun, and they lie some 425 light years away. The blue haze around the hot stars is the reflection nebula produced by dust cloud behind the stars. For northern hemisphere viewers, the cluster is above and to the right of Orion the Hunter as one faces south, and it transits -- reaches its highest point in the sky -- around 4am in September, midnight in November, and 8pm in January.
The Scorpius OB association is a loose group of stars. This group contains many hot, extremely luminous OB-type stars. It is the site of recent star formation. The stars in such groups are mostly not gravitationally bound but are expanding away from some common center, which presumably marks their birthplace. A study indicates that the Scorpius association has had 20 supernova explosions over the past 11 million years. The bright, dying star Antares (left), is the next star likely to explode. Compare its size to that of our sun.
The Big Dipper
Finally, the stars within a cluster have barely enough gravitational pull to keep them together. This can create a rather large spread of open cluster members. The average cluster exists for approximately 100 million years, because as stars are created they drift further and further from each other. These stars continue drifting until they are no longer within the grasp of the gravitational forces of the clusters. The star constellation Ursa Major (which we know as the "Big Dipper" or "The Plough") is actually part of a vastly spread out, open star cluster!