From one collapsing star, two black holes form and fuse

(Phys.org) --Black holes--massive objects in space with gravitational forces so strong that not even light can escape them--come in a variety of sizes. On the smaller end of the scale are the stellar-mass black holes that are formed during the deaths of stars. At the larger end are supermassive black holes, which contain up to one billion times the mass of our sun. Over billions of years, small black holes can slowly grow into the supermassive variety by taking on mass from their surroundings an...