Black holes that come together to become one pose a problem for scientists. The massive stars they begin as are far too big to fit into the small orbits required for a cosmic merger.

This is a key mystery in the study of gravitational waves and, while several theories have been proposed, none fully explain the process involved.

However, astrophysicists from the UK and Netherlands have taken a step closer to understanding how black holes merge. In a study published in the journal Nature Communications, the researchers looked at the formation of gravitational waves using a newly developed technique that analyses the sources of these events.

Gravitational waves are ripples in space time caused by huge astronomical events like supernova (exploding stars). First predicted by Albert Einstein a century ago, scientists using the Advanced LIGO (Laser Interferometer Gravitational-wave Observatory) announced their detection in February 2016. The waves detected came from GW150914 and GW151226 – a pair of merging black holes.

In the latest study, researchers looked at the stars that would have formed these black holes. Stars that form black holes are huge, so must be very far apart. However, for black holes to merge, they must be no further from one another than around 18.6 million miles – or a fifth of the distance between the Sun and Earth.

colliding black holes
Representational image of colliding black holes. P. Marenfeld/NOAO/AURA/NSF

They showed that three events can explain how massive stars can end up as black holes that are so close they merge into one. First, the two stars start out very far apart. As the stars expand (as they get close to death) they go through stages of mass transfer.

Finally they go through a highly unstable stage of mass transfer were the cores of both stars are enveloped in a cloud of hydrogen gas. When this gas is ejected, energy is removed from the orbit, bringing the stars closer together.

At this point, gravitational wave emission takes place. It takes around two million years for the two black holes to form, then billions of years for them to merge into one.

Senior author Ilya Mandel said: "This work makes it possible to pursue a kind of 'palaeontology' for gravitational waves. A palaeontologist, who has never seen a living dinosaur, can figure out how the dinosaur looked and lived from its skeletal remains. In a similar way, we can analyse the mergers of black holes, and use these observations to figure out how those stars interacted during their brief but intense lives."