Stellar black hole binaries were the cause of the three gravitational wave detections so far and researchers have two ideas for how these binaries formed: they either met randomly or they formed together from a massive star pair.

Dutch astronomers from the University of Amsterdam and Leiden University have been crunching the numbers and it appears that massive stellar binaries form black hole pairs more often than previously thought. They worked this out using a sophisticated computer simulation and their results have been accepted in the Monthly Notices of the Royal Astronomical Society.

“If our calculations are correct, double black holes, with a combined mass of fifteen to thirty times that of the sun, arise more often than expected," lead author Professor Ed van den Heuvel said in a statement. "In our own Milky Way, for example, according to the new calculations, such a merging black hole develops once in 100,000 years. Of course, that is still rare for humans, but it's ten times more often than was thought."

Theoretical arguments suggest that massive stars merge over time into a single object that then turns into a black hole. The conditions to create two black holes from two closely orbiting stars were considered “extreme” or at least rare enough that they likely don't explain the full population of binary black holes.

But a simulation, conducted on the supercomputer Little Green Machine in Leiden, showed that the conditions actually don’t have to be so extreme.  

“When the heaviest of the two stars collapses into a black hole, there is a stable situation in which the second star can survive for a long time before it forms the second black hole," Simon Portegies Zwart, who performed the new simulations, added. "In the meantime the first black hole sucks in a lot of matter from the second star, and it ejects much of it again. This mass emission causes considerable shrinking of the double star's orbit. So, when the second star collapses into a black hole, a close double star is formed of two black holes that will fuse together later.”

More gravitational wave observations will be necessary to solve the issue of binary black holes and where they come from, but this research definitely raises interesting questions and has implications for where to look in our own galaxy.