Nine hundred million years ago, an event occurred 8,550 million trillion kilometers away from Earth. And we just found out about it in August 2019!

That event? A black hole consumed a very dense neutron star. What happens when a black hole overtakes a very dense star? That star is snuffed out instantly, says Susan Scott, leader of the General Relativity Theory and Data Analysis Group at Australian National University.

So what happened?

Black holes and neutron stars are the leftovers after stars die.

Neutron stars that are left behind after supernovae die are the smallest stars out there. But, by Earth standards, they’re not that small. They are about as wide as the city of Chicago or Atlanta but are said to be even denser than our own sun. The diameter of the sun is thought to be approximately 1,391,000 kilometers. Packing all the mass of a sun 1,391,000 kilometers in diameter into a neutron star the width of Chicago or Atlanta makes for a very dense neutron star indeed.

Black holes are areas of gravity that speed up so quickly after a massive star dies that nothing—not even light, and not even neutron stars—can escape it. But they aren’t holes at all. Rather, they consist of enormous amounts of matter packed into a very small area.  NASA has described it like this: “…think of a star ten times more massive than the Sun squeezed into a sphere approximately the diameter of New York City.”

And that is what was discovered in August. Far, far away and long, long ago, a neutron star was swallowed up by a black hole.

How do we know it happened?

Gravitational-wave detectors in Italy and the United States detected ripples in spacetime. They traced those ripples back to the swelling event that is believed to have occurred 8,550 million trillion kilometers away from Earth.

Those ripples are disturbances in the curvature of spacetime. Those disturbances are generated by accelerated masses. They spread outward from the source of the event like waves at the speed of light. Those waves traveling toward Earth at the speed of light are what LIGO and VIRGO detected in August. They were detected by the National Science Foundation’s LIGO (Laser Interferometer Gravitational-Wave Observatory) in Washington and Louisiana and the VIRGO gravitational wave-detector in Italy.

No visual confirmation of the phenomenon has been made yet, despite the SkyMapper Telescope scanning the area. Researchers are still crunching the data to determine the size and mass of the objects that caused the ripples detected by LIGO and VIRGO.

Determining the mass of these objects is important. Objects greater than five times the mass of the sun are considered black holes. Objects smaller than three times the mass of the sun are considered neutron stars.

So what?

Astronomers and physicists have theorized that there are binary systems in space, systems where a black hole and a neutron star circle each other. Detection and confirmation of this event would provide evidence in support of that theory.

What else?

The detectors at VIRGO and LIGO have made at least two other significant discoveries and detections this decade.

Earlier, scientists at VIRGO and LIGO detected gravitational waves and neutron star collisions. In fact, the April 2019 detection of a neutron star collision occurred just weeks after the VIRGO and LIGO detectors were re-activated after upgrades to make them more sensitive to the very waves they later detected. That sensitivity allowed the detectors to find the remnants of a collision between two stars that is believed to have occurred 1,200,000,000 (1.2 billion) light-years away.

With those upgrades, LIGO and VIRGO’s detectors survey larger volumes of the universe than before, with the capacity to detect even more extreme stellar events.