Colliding black holes were once stellar giants

The pair of black holes that smashed into each other 1.3 billion years ago and sent ripples through space-time to Earth – detected for the very first time in September and reported in February – were once giant stars, with masses between 40 and 100 times that of the sun. 

Krzysztof Belczynski from Warsaw University in Poland and colleagues calculated the masses of the gravitational-wave-generating black holes and traced their timeline from meeting in a binary dance to their inevitable cataclysmic death.

The work was published in Nature.

The study is “tremendously exciting”, writes John Eldridge, an astrophysicist at the University of Auckland in New Zealand, in a News and Views piece. It examines every phase of the evolution of binary stars and the universe in the one model.

After word spread that Albert Einstein’s final prediction in his general theory of relativity was found – that a cosmological clash of massive proportions may stretch space-time – thanks to the twin ultra-sensitive Laser Interferometer Gravitational-Wave Observatory (LIGO) instruments in the US in September, came a flurry of interest in the wave-generating black holes. 

How old were they? What kind of stars were they before collapsing into black holes?

Belczynski and colleagues used high-precision simulations of binary black hole formation, which showed they likely started out as binary stars that collapsed to become black holes while spinning around each other – not two separate black holes that “found each other” and were snared by the other’s gravitational pull.

On the basis of these models, they calculated the black holes were once massive main-sequence stars which formed around two billion years after the Big Bang.

Main sequence stars fuse hydrogen to become helium. The sun, for instance, is a main-sequence star, and they comprise around 90% of stars in the universe.

After around 3.8 million years of circling each other, one of the stars – now a helium star – collapsed into a black hole. Around 10% of the star’s material was ejected in the form of neutrinos – slippery subatomic particles with no charge and that travel at near the speed of light.

The black hole and second star, which graduated to become a helium star around 1.2 billion years later and collapsed into a black hole a mere 300 million years after that, kept circling each other and drawing closer and closer.

All the while, they shared material in what’s known as a common envelope, a shroud of gas that surrounds the binary system.

Finally, some five billion years after the second star flipped into a black hole, the two crashed in an explosion so massive it warped space-time. 

Belczynski and colleagues estimate that such mergers are plentiful, and once our ground-based gravitational wave detectors are running at full capacity, we should see 1,000 black hole mergers per year.

And this has Eldridge excited: “With each gravitational wave detection, we’ll learn something new.”