Astronomers have discovered in the constellation of the Great Bear extremely unusual zombie star, which “managed” to survive a full-fledged supernova explosion and explode a second time approximately 50 years after the first outbreak.
“This supernova breaks all the rules that we thought these objects live in. This is the biggest cosmic mystery I’ve had to solve in decades of watching the explosions of stars,” said Iair Arcavi of the University of California, Barbara (USA).
Life and death in space
Supernovae break out as a result of the gravitational collapse of massive stars, when the heavy nucleus of the star shrinks and creates a rarefied wave that throws out the light matter of the outer layers of the light into the open space. As a result, a glowing gas nebula is formed, which continues to expand for some time after the explosion.
Supernovae of the first type are formed as a result of the explosion of a double system of a white dwarf and a more massive star, and more common flares of the second type are caused by the explosion of giant stars. Such an explosion, as scientists previously believed, is an irreversible process, since the star must cease to exist or turn into another type of cosmic objects after the onset of the outbreak.
Two years ago, Archavi and his colleagues found, as they then thought, a fairly ordinary second-generation supernova in the constellation Ursa Major, iPTF14hls, flashed in one of the neighboring galaxies about 400 million light-years from the Earth.
As Archivi recalls, scientists expected her gas-dust cocoon to fade about 100 days after the outbreak was detected, but this did not happen six months or even a year after the discovery of iPTF14hls. Moreover, the spectrum, brightness and temperature of supernova remnants did not change for more than 600 days, which is an extremely uncharacteristic phenomenon for the shroud of the deceased star.
The fact is that supernova remnants are usually shone under the influence of two different factors – the decay of radioactive elements that originated during a thermonuclear explosion, and a shock wave that compresses and warms the discarded gas shells of the star. Both these factors, as the researchers note, can not physically make the nebula shine equally brightly for almost two years.
Dawn of the “living dead”
These searches opened two unusual things, pointing to the potential nature of the mysterious object that generated this anomalous supernova. First, scientists discovered in the vicinity of iPTF14hls traces of another supernova that exploded about 50-70 years ago and did not lead to the destruction of the star itself.
Secondly, astronomers managed to find photographs of this outbreak on archival photographs of 1954, which proved that the star that gave birth to it is “immortal”, since even a supernova explosion could not destroy it. This, according to Arkawi and his colleagues, indicates that iPTF14hls is an exotic and extremely rare object, the so-called pulsating pair-unstable supernova.
It is believed that pair-unstable supernovas appeared in the early stages of the life of the universe as a result of the explosions of the first luminaries, wholly consisting of hydrogen and helium. They were much heavier than modern stars-“heavyweights” – such luminaries are 200-300 times heavier than our Sun.
The unusual chemical composition of their subsoil caused a special scenario for their death. When the early stars ran out of hydrogen, a nucleus of oxygen ions formed at their center. At a sufficiently high temperature, oxygen atoms begin to absorb photons produced in the nucleus of the “elderly” light, and convert them into pairs of electrons and positrons.
Thanks to this, the total pressure of photons on the matter of the star (the force balancing the gravitational contraction of the light) drops sharply, as a result of which the nucleus begins to contract and become even warmer. This enhances the reaction of the formation of pairs of particles from photons, as a result of which the star turns into a huge thermonuclear bomb.
This star bomb, as the observations of iPTF14hls show, explodes not at once and does so very gradually, in the form of a series of powerful flares reminiscent of the strength and characteristics of supernova explosions of the second type. If this is so, then the “immortal” star can be a real giant, whose mass will be 95-130 times higher than the solar one.
To understand whether this is really so, while it is impossible – iPTF14hls continues to remain bright even three years after the flash opening. As scientists hope, further observations of it and the discovery of other similar objects will reveal the secrets of the survival of such cosmic “living dead”.