The 15 Solar Mass Threshold: How a Black Hole’s Mass Governs Its Spin ✦
Scientists studied black hole mergers and found that their spin depends on mass. Black holes lighter than 15 solar masses spin differently than heavier ones: lighter ones more often have reverse spin. Imagine spinning tops of different weights set in motion in different ways — they spin differently. This might mean that black holes are born from different “stellar families”.
Objects predicted by Schwarzschild and now caught through gravitational waves (thanks to Weiss and LIGO) spin like tops when paired up. In some cases, their rotation is in sync: both spin in the same direction as the pair. In others, it's as if they were kicked out of alignment. An analysis of 259 mergers laid bare a sharp transition: if the heavier black hole is less than 15 Suns, its spin almost always 'points' along the orbit. Above that, the spins become chaotic, like tossed coins.
The 15-solar-mass boundary marks two evolutionary paths: the lightweights apparently get their main 'kick' during a supernova explosion (when the star's core, having exhausted its hydrogen and helium, fills up with carbon), while the heavyweights result from other processes. Interestingly, as it nears this limit, a black hole can spin up its rotation to near-light speeds, dragging the fabric of spacetime with it. Further observations will help piece together these dramatic histories in distant galaxies.
🎯 A 15-solar-mass black hole has an event horizon roughly the size of a small town—about 45 kilometers across.
🎬 In the film Interstellar, the rapid spin of the black hole Gargantua bends time. Real black holes, however, reveal their spin through gravitational waves.