Open clusters are groups of dozens or even hundreds of recently born stars, loosely clustered together, and often surrounded by traces of the nebulosity from which they were created. They are usually dominated by a few intensely hot, blue-white stars, the most spectacular of which are called "OB associations," from the spectral types of these stars. Such bright blue stars are rarely found elsewhere in the sky. They are so massive that they live and die in a few tens of millions of years and have no time to get caught up in the eventual dispersion of the cluster's more moderate stars.

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