As a giant cloud of gas and dust coalesces in the arms of a spiral galaxy, tiny clumps in the cloud collapse and ignite into groups of hundreds to thousands of young stars. The hottest of these stars light up the remaining gas and dust, resulting in glowing gas clouds like the Carina and Orion nebulae.
After several million years, the hottest stars of the new cluster blow off the remaining gas and dust. What remains is a group of dazzling new stars loosely held together by gravity.
Over many tens of millions of years, as the 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 “stellar associations”. Some of the stars of Ursa Major are part of an association and were once members of an open cluster.
Open star clusters are only found nears the arms of spiral and irregular galaxies, where there is abundant gas and dust. Elliptical galaxies no longer have enough gas and dust to sustain the creation of new stars.
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The greatest concentration of open clusters in our skies lies along the Milky Way in Cygnus, Scutum, Scorpius, and Sagittarius. For that reason, open clusters are sometimes called “galactic clusters”. There are nearly 1000 known open clusters in our skies, and likely 10,000 more hidden behind the disk of our galaxy.
Most open clusters stay together for a few tens of millions of years. The famous Pleiades, easily visible in the northern winter sky, is about 100 million years old. The double cluster is about 5 million years old. Some are massive enough to hold together much longer; the cluster NGC 188 is an amazing 5 billion years of age.
Since all the stars in a cluster are about the same distance from us, their relative brightness is proportional to their true brightness, which in turn is proportional to their mass and chemical composition. So open clusters makes it possible for astronomers to learn more about the evolution of stars.