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They are extremely hot and bright, with surface temperatures of between 20,000 - 50,000°C. They typically have 10 to 50 solar masses on the Hertzsprung-Russell diagram, and can have radii in excess of 1000 solar radii. These rare and enigmatic stars are amongst the hottest, largest and brightest in the known Universe.
Because of their extreme masses they have relatively short lifespans of only 10 to 50 million years and are only observed in young cosmic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies. They are not observed in spiral galaxy cores, elliptical galaxies, or globular clusters, all of which are believed to be old.
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