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The Orion Nebula consists of one of the nearest (thus in the Milky Way Galaxy), massive molecular clouds (30 - 40 light years in diameter) about 1,300 light years from the solar system. This makes the nebula potentially the closest HII region to Earth, a mass of hydrogen that has been ionized by nearby, hot, young stars. Regions like this are called stellar nurseries, nurturing the birth of multiple young stars such as the Orion Nebula Star Cluster. These are a hallmark of the asterism.