While black holes aren’t exactly invading the Milky Way, new research has shed light on a whole lot more of these star corpses than previously thought to exist in our galaxy. Millions. Like, 100 million.
Last year, the Laser Interferometer Gravitational-Wave Observatory (LIGO) picked up something of an anomaly. Spacetime ripples (surprisingly not the weird part here) signaled two immense black holes crashing into each other—immense, meaning each was 30 times the mass of our sun. It wouldn’t be out of place to call the phenomenon Starcrash. This sparked a team of scientists at UCI, who recently published a study in Monthly Notices of the Royal Astronomical Society, to rethink the black hole count in our galaxy after wondering how many of this enormity are hiding in the Milky Way and how often such epic collisions occur.
"Fundamentally, the detection of gravitational waves was a huge deal, as it was a confirmation of a key prediction of Einstein's general theory of relativity," said James Bullock, UCI chair and professor of physics and astronomy. "But then we looked closer at the astrophysics of the actual result, a merger of two 30-solar-mass black holes. That was simply astounding and had us asking, 'How common are black holes of this size, and how often do they merge?'"
Stellar-remnant black holes, which are what is left of massive stars after their last gasp, are usually assumed to be the same mass as our sun. What was so strange about the LIGO evidence was the indication of a head-on smash of two black holes huge enough to make the sun look like a punctuation mark.
UCI theoretically autopsied this unusual phenomenon to make sense out of the crash in terms of what we already know about the formation and evolution of galaxies. Star formation in various galaxies tells astronomers about how many black holes formed in that galaxy, and around when. Typically, the vaster the galaxy, the older the stars and black holes throughout. Metal-rich stars abound in larger galaxies, and these stars use up most of the mass from those heavy elements up until their deaths, until there is only enough left for a lower-mass black hole. Stars that are both enormous and have low metal content keep most of their mass, which makes for a monster black hole.
Few black holes actually merge like the star-crossed ones that LIGO picked up on, but the observations could also help with understanding similar black hole collisions that might send ripples through spacetime in the future. More gravitational wave detections will be needed, but the team feel their calculations could mean that mergers of black holes 50 solar masses and up could be sending gravitational waves our way soon.
Because 30 solar masses was just not enough.