“Black holes can bang against space-time as mallets on a drum and have a very characteristic song.” -Janna Levin
If you had told an astrophysicist five years ago that binary black holes were common, that would’ve been news, but not surprising. If you had told them that ~30 solar masses was a good estimate for each one of their masses, though, you might have had to pick their jaws up off the floor. Yet LIGO’s very first detection showed us exactly that, much to the surprise of many.
So how did these black holes come to be? After much numerical study, it appears that young, metal-poor stars about 40-100 solar masses each conspired to create these binary black hole pairs, with only one of the members resulting in a supernova. The rest is cosmic history.