WASHINGTON, April 3 : A supernova – the explosive death of a star – is always violent, destroying material in space while normally leaving a compact stellar remnant like a neutron star or a black hole. But some supernovae that affect the most massive stars in the universe may be so powerful that they leave nothing behind.
Scientists since the 1960s have the hypothesis of the existence of supernovas very powerful, and now they have presented evidence for them – although indirect – in studies involving black holes and waves of time called gravitational waves.
Such supernovas are predicted to occur in the most massive stars – those with a mass of about 140 to 260 times that of the sun, according to Hui Tong, a doctoral student in astronomy at Monash University in Australia and lead author of a study published Wednesday in the journal Nature.
“Despite their huge mass, they live a very short life, about a few million years. In comparison, the sun will live for about 10 billion years, so these stars burn up about a thousand times faster – like a huge fire that burns a lot and for a while before it explodes,” Tong said.
The explosion of massive stars of a certain mass leaves behind a neutron star, a collapsed star cluster. Some even more massive stars, when they explode, leave behind a black hole, an exceptionally dense object with such a strong gravitational pull that even light cannot escape. The black hole retains part of the mass of the original star, and the rest is blown into space.
In this study, the researchers combined the data of 153 pairs of black holes, determined their mass based on the gravitational waves they produced, and then classified the black holes that were created by the merger of two small black holes.
What the researchers found at the time was the absence of black holes between about 44 and 116 times the mass of the sun, which they called the “barred factor.”
They said, that absence can be explained well if the most massive stars, which can be expected to leave black holes in that mass region, instead, were erased at the end of their life in an unusual explosion called “pair-instability supernova”, leaving behind remnants.
“An erratic supernova is one of the most powerful forms of stellar death,” said astronomer and co-author Maya Fishbach of the University of Toronto’s Canadian Institute for Theoretical Astrophysics.
“Especially, massive stars form black holes. The more massive the star, the more massive the black hole is,” Fishbach said, until the stars reach a certain mass limit beyond which the physics of their explosion means that there are no stellar remnants left.
These massive stars evolve in the same way as other massive stars in the beginning, burning hydrogen and helium and building a core made mostly of carbon and oxygen. In order for the core to remain stable, there needs to be a balance between the internal pressure of gravity and the release of energy from the outside – in the case of these stars high-energy photons, the particles that make up light.
But in the extreme heat present in these stars, some of the photons become pairs of small atomic particles called electrons and positrons, thus weakening the external pressure that helped maintain the stability of the core. These pairs of particles and the instability they cause define the name of this supernova group.
“The core becomes unstable, leading to a massive collapse and a massive nuclear explosion that rips the star apart,” Tong said.
Although these supernovas were predicted six decades ago, Fishbach said, “they are rare and difficult to find and identify.”
Scientists have observed another type of stellar explosion called a superluminous supernova which is a candidate for an unstable supernova. These explosions can be more than 10 billion times brighter than the sun. But for now, the evidence presented in this study may be the best indication of the existence of unstable supernovas.
“We’re actually using an invisible substance, black holes, as a record of the most luminous explosions in the universe,” Tong said.
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