When stars collapse under their own gravity, they can leave behind a neutron star or black hole — provided that the star is massive enough. In a binary star system, this can lead to some pretty wicked interactions.

When stars collapse under their own gravity, they can leave behind a neutron star or maybe a black hole — provided that the star is massive enough. Neutron stars and black holes are called compact objects by astrophysicists. They are extremely dense, but not necessarily extremely massive. For instance, some black holes can be less massive than stars, at 5 – 10 solar masses — while supergiants like Betelgeuse can have 19 solar masses.

Just because our Sun is a lone wolf star doesn’t mean all the other stars are alone out there. In fact, many stars, (especially massive ones with more than 8 solar masses) are in binary star systems.

A binary star is not just one star orbiting the other neatly like in the solar system models we see in school — what celestial bodies are actually orbiting is each other’s center of mass. If a star is much more massive than the other, it would be the leader of the orbital dance and pull the center of mass closer to it — just like in our case, the Earth is considerably more massive than the Moon, so the center of mass is closer to our planet.

Binaries can form in stellar nurseries, dense molecular clouds that can collapse and form stars. If one of the two is substantially more massive than the other, it can make things extra interesting. What sometimes happens is that the more massive star holding the stronger gravitational field can start accreting (stealing) gas from its companion. In the case of neutron stars, they can do that even when their companion is bigger. We also know that black holes can also munch on an orbiting star, as was reported by the Laser Interferometer Gravitational-Wave Observatory (LIGO) detection GW200105 in 2020. The other scenario (where a star would absorb its more massive partner) is hard to imagine.

Researchers detected a radio source called VT J121001+495647 from the Very Large Array Sky Survey and later looked for the same event using different telescopes, and found X-ray signals. The X-ray emission lasted 15 seconds, with 4 trillion trillion trillion joules per second, the only objects powerful enough to emit this much energy in the X-ray band are supernovae. 

The newly-discovered binary system, 480 million light-years away from Earth has two objects that were probably formed together but had very different life cycles: one of them was probably an ordinary star with regular nuclear fusion activity, like most main sequence stars, while the other is more mysterious. It could be a neutron star or a black hole, but probably, the result of a very hot fast-burning fuel life-cycle that ended in a compact object.

The reason we see a radio emission is that the big star (not the compact object munching on it) went from the normal phase to the supergiant phase. Simply put, it grew in size, (just like our sun will, at some point, grow enough to envelop Earth), and eventually, the companion was wrapped by it. A cataclysmic dance started, the denser companion messed with the core collapse of the bigger friend. Ultimately, this will probably end in one big boom, astronomers explain.

“The companion star was going to explode eventually, but this merger accelerated the process,” said Dillon Dong – leading author of the discovery.

The compact object remained inspiraling towards the star’s core, ejecting mass which formed a disk with a jet coming out of its axis – in a doughnut shape. When finally reaching the core of a titanic boom, the collapse of the star’s core forms a supernova.

This was the first time scientists found evidence of a star eating a neutron star/black hole, something only discussed theoretically until now. Hopefully, future observations will shed more light on this unusual process. The study was published in Science.