After the hot stars near the center of an emission nebula push away the remaining gas and dust, a group of a few dozen to a few hundred young stars remain clustered together. These groups are called open star clusters, and they are often found along the Milky Way. They are beautiful to observe, especially in dark sky where they look like dazzling jewels set against the black velvet of deep space.
Open star clusters are found mostly near the arms of spiral and irregular galaxies where there is abundant gas and dust for new star formation. For that reason, they’re sometimes called “galactic star clusters”. The greatest concentration of open clusters in our sky lies along the band of the Milky Way in the constellations Cygnus, Scutum, Scorpius, Sagittarius, Crux, Centaurus, Cassiopeia, and Perseus.
There are nearly 1,000 known open clusters in our skies, and likely 10,000 more hidden behind the dusty disk of our galaxy. Some famous open clusters include the Pleiades (M45) and Hyades in Taurus, the Beehive (M44) in Cancer, and the Double Cluster in the constellation Perseus.
Since all the stars in an open cluster are about the same distance from us, their relative brightness is proportional to their true brightness, which in turn depends on their mass and chemical composition. The stars also form at about the same time. So open clusters are like laboratories that help astronomers learn more about the evolution of stars.
Over many tens of millions of years, as an open cluster revolves around the galaxy, it encounters other stars and dust clouds that disrupt the cluster and eject its members into the spiral arms of the galaxy. There, they continue to revolve about the galactic center by themselves or in loose associations of stars. Some of the stars of Ursa Major are part of an association and were once members of an open cluster. The Sun was likely once a member of an open cluster whose members have long scattered into the plane of the Milky Way galaxy.