Astrophysicists may have solved the mystery of disappearing stars

Instead of dying dramatically in a massive supernova explosion, some big stars can die quietly and without fanfare.

This could explain the mysterious and sudden disappearance of certain stars from the night sky that astronomers have observed over the years, according to a new paper in the journal Physical Review Letters.

These massive stars can completely collapse in on themselves and become black holes without supernova, long thought to be a necessary part of the death of a large star.

Stars “burn up” through a process called nuclear fusion, which involves fusing lighter hydrogen atoms into heavier helium, releasing enormous amounts of energy in the form of light and heat. When this hydrogen fuel runs out, the star enters the later stages of its life and begins to die.

Stars can die in various ways depending on their mass. When stars run out of hydrogen, the star’s core contracts and heats, causing the outer layers to expand and cool, and the star becomes a red giant. Smaller stars—up to about 8 times the mass of our sun—will eventually shed these outer layers and the core will become a white dwarf. Larger stars, on the other hand, go through several stages of nuclear fusion, creating heavier and heavier elements until iron is made, at which point nuclear fusion stops, the star collapses under its own gravity, and then explodes in a huge burst of energy known as a supernova.

If the remaining stellar core is between about 1.4 and 3 solar masses, it will collapse into a neutron star, an incredibly dense object composed mostly of neutrons, while if the core is larger than about 3 solar masses, it will collapse into a black hole, a point in space with by gravity so strong that even light cannot escape. from that.

However, this new research found strong evidence that massive stars can die and form a black hole without any supernova, in a process known as “total collapse”.

“We believe that the star’s core can collapse under its own weight, as happens with massive stars in the final stages of their lives. But instead of the contraction culminating in a brilliant supernova explosion that would outshine its own galaxy, as expected for stars more than eight times more massive than the Sun, the collapse continues until the star becomes a black hole,” study co-author Alejandro Vigna-Gómez, a researcher at the Max Planck Institute for Astrophysics, who was a postdoctoral fellow at the University of Copenhagen’s Niels Bohr Institute during the study, said in a statement.

According to the researchers, this discovery could also explain the phenomenon of stars quietly disappearing from the sky without an explosive supernova explosion.

“If one were to stand looking at a visible star undergoing complete collapse, it might, at just the right time, be like watching a star suddenly extinguish and disappear from the sky. The collapse is so complete that no explosion takes place, nothing escapes and no bright supernova would be seen in the night sky. Astronomers have recently observed the sudden disappearance of bright stars. We cannot be sure of the connection, but the results we obtained from the analysis of VFTS 243 brought us much closer to a plausible explanation,” said Vigna. – Gomez.

This discovery was prompted by the observation of a binary star system called VFTS 243 in a small neighboring galaxy known as the Large Magellanic Cloud, which consists of a large star and a black hole with a mass about 10 times that of our Sun.

This system should have traces of a black hole star that went supernova in the past, but astronomers could find no such evidence.

“Typically, supernova events in star systems can be measured in a variety of ways after they occur. But despite the fact that VFTS 243 contains a star that collapsed into a black hole, traces of the explosion are nowhere to be found. VFTS 243 is an extraordinary system orbiting barely changed from the collapse of the star into a black hole,” said Vigna-Gómez.

Astronomers would expect to see signs of a “natal shock”, which is when a neutron star or black hole formed in a supernova is accelerated to between 100-1000 km per second by the immense forces of the supernova explosion. However, the black hole in the VFTS 243 system appears to be accelerated to only about 4 km/s. In addition, supernova forces usually affect the orbital symmetry of a binary star system, but such effects were not observed in the VFTS 243 system.

“The orbit of the VFTS is almost circular and our analysis shows that there are no signs of large asymmetries during the collapse. This again points to the absence of an explosion,” said Vigna Gomez.

Therefore, the black hole in the VFTS 243 system appears to have been born without a supernova.

“Our analysis points unequivocally to the fact that the black hole in VFTS 243 most likely formed immediately, and the energy was mostly lost via neutrinos,” co-author Irene Tamborra, also a researcher at the Niels Bohr Institute, said in a statement

We hope that this discovery will open new doors to the study of how stars die and how black holes are born.

“Our results highlight VFTS 243 as the best observable case so far for the theory of total collapse stellar black holes, where a supernova explosion fails, and which our models have shown is possible. This is an important reality check for these models. And we certainly expect that system will serve as a key benchmark for future studies of stellar evolution and collapse,” Tamborra said.

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