Cepheids are luminous and can be used to establish both the distances to remote galaxies and the expansion rate of the Universe ( Hubble constant). Open clusters are often dominated by hot young blue stars, because although such stars are short-lived in stellar terms, only lasting a few tens of millions of years, open clusters tend to have dispersed before these stars die.Įstablishing precise distances to open clusters enables the calibration of the period-luminosity relationship shown by Cepheids variable stars, which are then used as standard candles. The Double Cluster of h+ Chi Persei can also be prominent under dark skies. The most prominent open clusters are the Pleiades and Hyades in Taurus. Close encounters between cluster members can also result in the ejection of stars, a process known as 'evaporation'. Being much less densely populated than globular clusters, they are much less tightly gravitationally bound, and over time, are disrupted by the gravity of giant molecular clouds and other clusters.

Open clusters typically have a few hundred members and are located in an area up to 30 light-years across. They form H II regions such as the Orion Nebula. They are generally young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years, such as Messier 67 (the closest and most observed old open cluster) for example.

Unlike the spherically distributed globulars, they are confined to the galactic plane, and are almost always found within spiral arms. Open clusters are very different from globular clusters. The Pleiades, an open cluster dominated by hot blue stars surrounded by reflection nebulosity

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