When massive stars die, as we understand the universe, it does not happen quietly. When their fuel runs out, they become unstable and are wracked by explosions before ultimately ending their lives in a spectacular supernova.
But some massive stars, scientists have discovered, have simply disappeared, leaving no trace in the night sky. Stars that are clearly visible in older surveys are inexplicably missing in newer surveys. A star isn’t exactly a set of keys; you can’t just lose it at the back of the couch. So where on earth are these stars going?
A new study has given us the most convincing explanation yet. According to an international team led by astrophysicist Alejandro Vigna-Gómez of the Niels Bohr Institute in Denmark and the Max Planck Institute for Astrophysics in Germany, some massive stars may die, not with a bang, but with a whimper.
Their proof? A binary system called VFTS 243 in the Large Magellanic Cloud, consisting of a black hole and a companion star. This system shows no signs of a supernova explosion that, according to our models, should have accompanied the formation of the black hole.
“If someone were to stare up at a visible star in total collapse, it could be, at just the right moment, like seeing a star suddenly dim and disappear from the sky,” Vigna-Gómez explains.
‘The collapse is so complete that no explosion takes place, nothing escapes and you cannot see a single bright supernova in the night sky. Astronomers have actually observed the sudden disappearance of brightly shining stars in recent times. We cannot be sure of a connection. but the results we obtained from analyzing VFTS 243 have brought us much closer to a credible explanation.”
When a star more massive than about 8 times the mass of the Sun goes supernova, things are extremely messy. The outer layers – most of the star’s mass – are thrown explosively into space around the star, where they form a huge, expanding cloud of dust and gas that lingers for hundreds of thousands to millions of years.
Meanwhile, the star’s core, no longer supported by the external pressure of nuclear fusion, collapses under gravity, forming an ultra-dense object, a neutron star or a black hole, depending on the mass of the original star.
These collapsed cores do not always stay in place; if the supernova explosion is crooked, it can shoot the nucleus into space in a birth kick. We can also sometimes trace the trajectory of the core to the cloud of material it spewed out when it died, but if enough time has passed the material may have disappeared. But the signs of the birthing kick can last much longer.
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VFTS 243 is a very interesting system. It consists of a massive star that is about 7.4 million years old and about 25 times the mass of the Sun, and a black hole that is about 10 times the mass of the Sun.
Although we can’t see the black hole directly, we can measure it based on the orbital motion of its companion star – and of course we can infer other things about the system as well.
One interesting thing is the shape of the track. It is almost circular. This, along with the system’s motion in space, suggests the black hole didn’t get a huge kick from a supernova. The researchers who discovered the black hole in 2022 suspected the same thing; now the work of Vigna-Gómez and his colleagues has confirmed this.
Mounting evidence suggests that massive stars can sometimes collapse directly into black holes, without passing through a supernova or collecting 200 space dollars. VFTS 243 represents the best evidence we have for this scenario to date.
“Our results highlight VFTS 243 as the best observable case yet for the theory of stellar black holes formed by total collapse, where the supernova explosion fails and which our models have shown is possible,” says astrophysicist Irene Tamborra from the Niels Bohr Institute. .
“It’s an important reality check for these models. And we certainly expect the system to serve as a crucial benchmark for future research into stellar evolution and collapse.”
The research was published in Physical Assessment Letters.