The night sky is a field of stars. In every direction, stars bright and dim fill the horizon to brimming. Some seem to form distinct patterns, which we recognize as constellations. Yet as beguiling as those patterns may be, most of them are no more than projections of the human mind. The vast majority of stars, in our own galaxy and in others, have no true physical connection to one another. At least, not anymore. Every star actually begins its life in a group, surrounded by siblings of nearly the same age that only later drift apart. Astronomers know this because some of these stellar nurseries, called star clusters, still exist. The Orion nebula cluster is perhaps the most famous one: in images from the Hubble Space Telescope, its stars wink from within churning clouds of dust and gas. You can see the Pleiades cluster from your backyard: it is the fuzzy patch in the constellation Taurus.
Star clusters vary enormously, ranging from fragile associations with just a few dozen members to dense aggregates of up to a million stars. Some groups are very young—only a few million years old—and others date from the dawn of the universe. Within them, we find stars in every stage of the stellar life cycle. Indeed, observations of star clusters provided the main evidence for today's accepted theory of how individual stars evolve over time. The theory of stellar evolution is one of the triumphs of 20th-century astrophysics.