‘Stellar’ black holes form when massive stars collapse under their own gravity at the end of their lives. We know that a star must have an initial mass of between 15–20 solar masses in order to collapse and form a stellar black hole. We also have a pretty good idea of the distribution of stellar masses in the Milk y Way. These two facts tell us that about one in 1,000 stars has the potential to become a black hole. An estimate of the number of stars in the Milk y Way, 100 billion stars, then implies there could be up to 100 million stellar black holes in the Milk y Way.
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