The physics driving supermassive black holes are difficult to fathom even for scientists who devote their lives to studying such objects. When you add a second black hole, things get even harder to follow. Scientists have never been able to observe the collision of two black holes, but a new simulation from NASA’s Goddard Space Flight Center could offer some clarity on the physics involved.

It’s well-established at this point that large galaxies have supermassive black holes in the center. We also know that galaxies in the universe regularly merge. Yet, we see very few galaxies that have two giant black holes in the center. Those we do see aren’t close enough that their gravitational fields interact, making it difficult to identify merging black holes from light alone — we don’t know what to look for, but the new Goddard simulation could help.

The gravitational waves from smaller black holes merging have been confirmed with instruments like the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO). A supermassive black hole merger would be much more distant, so we can’t rely on gravitational waves to pinpoint them. Earth is too noisy to pick up the signal. We do know something about the emissions from gas orbiting supermassive black holes, and that’s where Goddard researchers have focused their attention.

Supermassive black holes should pull clouds of superheated gas along with them when they merge, and even more gas would end up around a black hole if two galaxies merge. Researchers modeled two supermassive black holes orbiting each other three times to determine how that gas would behave shortly before a collision. They found that this stage of the process would be dominated by intense emissions of UV and X-rays from gas in three distinct regions. There would be a cooler ring of gas around the pair of black holes, as well as smaller, hotter discs circling each individual singularity. A stream of gas would also feed the smaller discs from the surrounding halo.

As matter flows into the black holes, the simulation predicts the UV light would interact with the black hole’s corona to produce higher X-ray emissions. With a lower rate, the UV light would dim. Scientists expect the X-ray emissions from a pair of merging black holes would be significantly brighter than either one could produce on its own.

It took the Blue Waters supercomputer 46 days to produce this simulation with its 9,600 CPU cores, and it’s not even complete. NASA didn’t attempt to model the center of gravity between the two orbiting masses. It’s just a black circle in the animation. There’s still a lot to learn.

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