Stars are formed from gas and dust compressed together by various forces. Star clusters are formed in several different ways. In what are called open clusters, it is usually the sweeping wave action of the spiral arms of the galaxy thrusting vast stretches of this interstellar material away from the galaxy. In the case of what are known as globulars, it is believed that they formed during the original gravitational collapse that formed the galaxy. This is why the globulars are known to be much older than the open clusters.