How Big Can a Black Hole Get?

Black holes are everywhere in the Universe. A typical galaxy might have millions of stellar-mass black holes, ones created when a massive star explodes (this is usually the kind you think of when you think "black hole"). But some black holes are true monsters, with millions or even billions of times the Sun's mass. We think that every large galaxy has one of these supermassive black holes in its core. Study after study has shown that these black holes have a symbiotic relationship with their hos...