疏散星团，也称为银河星团，是由同一个巨分子云中的数百颗至多数千颗恒星形成的集团。在银河系中发现的疏散星团已经超过1,100个，并且被认为还有更多存在的。它们环绕着银河中心运转时，只靠着微弱的引力吸引维系在一起，并且很容易因为与其它集团或气体云的近距离接触而瓦解。疏散星团的寿命通常只有几亿年，但少数质量特别大的可以存活数十亿年。相较之下，质量更大的球状星团，拥有更多的恒星，成员彼此间的引力极为强大，可以存活的时间也更长。只有在星系的螺旋臂和不规则星系能发现疏散星团，它们只存在于恒星形成活跃区。

年轻的疏散星团可能仍然在它们形成的分子云中，照亮它们在分子云内创造出来的H II区。随着时间推移，来自星团的辐射压会将分子云吹散。通常情况下，在辐射压将气体驱散之前，大约有10%质量的气体能凝聚形成恒星。

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.