Scientists have discovered strong evidence that some massive stars end their existence with a whimper, not a bang, and sink into a black hole of their own making without the light and fury of a supernova.
To understand why this matters, we need to start with a crash course in stellar evolution. Stars generate energy through nuclear fusion processes in their core by which they transform hydrogen into helium. When stars with at least eight times the mass of our sun Running out of this supply of hydrogen, they start fusion reactions involving other elements instead – helium, carbon, oxygen, etc., until they are left with an inert iron core that requires more more energy for the fusion reaction than it can. produce. At this point, fusion reactions cease and the energy production that holds the star in place evaporates. Suddenly, gravity reigns free and causes the core to collapse, while the outer layers of the star bounce off the contracting core and explode outward, triggering a supernova that, for a few weeks, can sometimes shine brighter than a whole star. galaxy.
During this time, the collapsing core forms a compact object. This object is often an object that rotates neutron star called a pulsar — but, under certain conditions, it could be a black hole of stellar mass. This is the standard story from stellar timelines. However, astronomers are now beginning to accept the idea that some stars that produce black holes might do so. without a supernova explosion.
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Researchers have sometimes noticed cases of failed supernovas — stars that begin to brighten as if they were about to explode, but then fade and die. Elsewhere, studies of old photographic plates as part of the Objects that have disappeared and appeared during a century of observations (VASCO), led by Beatriz Villarroel, found dozens of stars on these old plates that we simply don’t see anymore; it’s as if they had disappeared without a trace.
Could these failed supernovas and disappearing stars be proof that stars are almost entirely pulled into the black hole they form before they have a chance to explode? Well, maybe, some scientists think.
“If one looked at a visible star completely collapsing, one could, at just the right moment, watch a star suddenly extinguish and disappear from the sky,” said Alejandro Vigna-Gómez of the Max Planck Institute . for astrophysics in Germany in a statement. “Astronomers have actually observed the sudden disappearance of bright stars in recent times.”
Although the idea is still only a theory, it now has solid evidence in the form of a strange binary system studied by Vigna-Gómez and his team. Designated VFTS 243, the system was discovered in 2022 and resides in the Tarantula Nebula, located in Large Magellanic Cloud; it contains a 25 solar mass star and a 10 solar mass black hole that must have been produced by a massive star that reached the end of its life relatively recently, in cosmic terms.
“VFTS 243 is an extraordinary system,” Vigna-Gómez said. “Although VFTS 243 contains a star that collapsed into a black hole, traces of an explosion are nowhere to be found.”
For example, the orbits of the star and black hole in VFTS 243, around their common center of mass, are still almost circular. However, supernova explosions are asymmetric, with slightly more energy produced in one direction than the other, which should give the forming compact object a “natal kick.” Such a kick would accelerate the compact object, causing its orbit to widen and lengthen. Typically, this kick is between 30 and 100 kilometers (19 and 62 miles) per second, but the VFTS 243 black hole was, at most, hit at just four kilometers (2.5 miles) per second .
The consequences of natal kicks have been observed before in pulsars, but never before in stellar-mass black holes. It is very possible that this will tell us something about how stellar-mass black holes form, and VFTS 243 is the clearest glimpse yet of the results of this process.
Natal kicks are the product of three things: the ejection of debris from the exploding star, an explosion of neutrinos of the collapsing core of the star, and gravitational waves. However, if there was no supernova there would be no debris, only neutrinos and gravitational waves would be left to provide a much weaker kick – which is exactly what we see in VFTS 243.
If correct, it means that many of the most massive stars in the universe, which shine so brightly, end their lives in silent darkness as they are dragged into the oblivion of a black hole. This could also be the final fate of the surviving VFTS 243 star when it reaches the end of its life.
There are also broader implications. A supernova explosion is an element factory. Not only are elements such as oxygen, carbon and nitrogen contained in the outer layers of a dying star thrown into space where they can be recycled into the next generation of stars and planets, the intense heat and energy of the supernova shock wave can cause even heavier elements to form in the supernova debris. For example, one of the reasons supernovas shine so long is that the radioactive decay of nickel isotopes produced during the explosion leads to the formation of cobalt and iron.
However, if some massive stars completely collapse into black holes without supernova explosions, then they cannot contribute to the creation and recycling of elements. Cosmochemists will therefore have to take this concept into account, if it is indeed true, in their models of the formation and propagation of elements in space. Only then can they begin to fully understand the chemical evolution of galaxies, including our own, and how quickly the elements needed to form planets like Earthperhaps even with their own life consisting of elements produced by the explosion of stars, can accumulate.
The results of VFTS 243 were published on May 9 in the journal Physical Examination Letters.