Astronomers have identified a new way for a star to die, in a veritable pileup at the heart of a galaxy, according to a study published in Nature. “Rather than a natural end, stars can die in a collision”, explains to Agence France-Presse the professor Andrew Levan, of the university of Radboud (Netherlands), first author of this recently published study.
Normally, the fate of a star depends on its mass. “Big stars end up in a supernova [a gigantic explosion], and low-mass ones, like the Sun, go out as a white dwarf,” recalls the astrophysicist. They can also, more rarely and in the case of neutron stars, merge with each other when they are in a binary system, like a pair of objects born too close to each other.
Not so here, with the observation in October 2019 of a gamma-ray burst, a jet of rays of colossal energy, coming from a distant galaxy located in the direction of the constellation Aquarius. Depending on its duration, ranging from less than two seconds to several minutes, such an event signifies respectively the merger of two neutron stars or the explosion of a large supernova star.
GRB191019 – the name of the burst –, with a duration beyond the minute, normally belonged to the second category. But astronomers have seen no supernova signals in the galaxy from which the jet originated, located about 2 billion light-years from Earth. Nothing surprising however, because this galaxy, very old, forms almost no more stars, and therefore certainly no longer the massive stars likely to end in supernova.
A long observation of the event gave the key to the mystery. It occurred at an extremely close distance from the core of the galaxy, less than a hundred light-years away. For comparison, our solar system is about 27,000 light-years from the center of the galaxy.
This galactic core “is a very dense region, which can contain tens of millions of stars, which can collide or scatter”, explains Andrew Levan. Especially since the “compact objects” that populate it, white dwarfs, neutron stars and small black holes, are subject to the gravitational force of the supermassive black hole, lurking in the center of the galaxy.
Which led the international team of researchers to conclude that the two celestial objects whose collision caused the gamma-ray burst “formed at different places from each other and met” in the heart of the galaxy, according to the astrophysicist.
Astronomers assume that such collisions, whose existence has been assumed theoretically, can occur routinely in such an environment. But that their observation is made very difficult because the hearts of galaxies are regions full of dust and gas.
