A black hole desert
The merger signal, called GW190521, lasted only a tenth of a second — but scientists immediately realized it was extraordinary in comparison to LIGO’s first detection in 2015.
“This doesn’t look much like a ‘chirp,' which is what we typically detect,” said Virgo member Nelson Christensen in LIGO’s press release. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”
Unsurprisingly, this strange signal was produced by the merger of two equally weird black holes with masses of about 66 and 85 solar masses, which raises a few questions regarding their formation.
During a typical stellar lifetime, stars are able to support their weight because internal fusion generate an outward force that balances the inward crush of gravity. But if a star is massive enough, once it runs out of fuel, it can no longer fight gravitational collapse. Ultimately, the core of such a star collapses under its own weight before rebounding back out as a dramatic supernova.
But any star that could theoretically form a black hole between 65 to 120 solar masses, like either progenitor of this unique merger, doesn’t explode as supernova. That means there shouldn’t be any black holes born from collapsing stars in that mass range.
Instead, when a star that large begins its death throes, a phenomenon known as “pair instability” kicks in, and the star becomes unstable to the point it avoids gravitational collapse — at least, for a while. And when it does finally explode, it leaves nothing behind. (On the other end of the spectrum, stars above 120 solar masses never go supernova because they collapse directly into black holes.)
“Several scenarios predict the formation of black holes in the so-called pair instability mass gap: they might result from the merger of smaller black holes,” said Virgo collaboration member Michela Mapelli in Virgo’s press release. “However, it is also possible that we have to revise our present understanding of the final stages of the star's life.”