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How many ultracompact H II regions are estimated to exist in the Milky Way Galaxy?
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H II region
Centre of the Orion Nebula (M42). Astronomers have identified some 700 young stars in this 2.5-light-year-wide area. They have also detected over 150 protoplanetary disks, or proplyds, which are believed to be embryonic solar systems that will eventually form planets. These stars and proplyds generate most of the nebula's light. This picture is a mosaic combining 45 images taken by the Hubble Space Telescope.
It is estimated that there are approximately 2,500 ultracompact H II regions within the Milky Way Galaxy. These regions represent between 10 and 20 percent of the total population of O-type stars in the galaxy.1