Stellar black holes form when stars with masses around 20 times that of the sun collapse under the weight of their own gravity at the ends of their lives. Most stellar black holes weigh in at around 10 solar masses when the smoke blows away, and computer models of star evolution have difficulty producing black holes more massive than this.
The newly weighed black hole is 16 solar masses. It orbits a companion star in the spiral galaxy Messier 33, located 2.7 million light-years from Earth. Together they make up the system known as M33 X-7."
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