The binary system known as M33 X-7 is an odd duck indeed. Discovered in 2007, the two objects, a 16-solar-mass black hole and a 70-solar-mass blue star, fit no known mold. The black hole is too massive, the star too dim, and both circle each other too erratically. So a team of astronomers threw away the mold. They performed several hundred thousand simulations until they found out what process had created the black hole and its binary partner, both located in a galaxy some 3 million light-years away in the constellation Triangulum. Four million years previously, the team reports online today in Nature, the black hole's progenitor was actually the more massive of the two, tipping the scales at 97 solar masses. But the big star fused its hydrogen and helium into heavier elements with lightning speed, expelling gigantic solar winds. The companion star lapped up gobs of that expelled material, growing quickly to its current size. When the progenitor star had no more helium to fuse, it collapsed into a black hole—which we now see is pulling matter back in from the companion star at a furious clip.
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