At the end of the universe, long after the last shining starts flicker out, there might be one final array of explosions to eclipse all those that came before them. This will be called the black dwarf supernovae, these dazzling blasts will herald in the everlasting darkness as the universe sinks into nothing, a new study implies.
These newly proposed supernovas are a unique breed that have not yet happened anywhere in our universe. Black dwarf supernovas might be the last events that happen in our universe, which by then will be a largely empty place where the temperature approaches absolute zero.
Stars’ lives and deaths are determined by their mass. Large ones 10 or more times the mass of the sun explode as supernovas and can become deadly black holes. But often smaller ones, which usually do not produce heavier elements through the nuclear fusion at their cores, end their lives as a small dense husk of starts commonly referred to as white dwarfs. Over trillions of years, they dim and turn into frozen, lightless objects known as black dwarfs. A new study describes how these black dwarfs might release the final flashes of light in the universe as they explode as supernovas.
The black dwarf supernovas would form through a quantum process called pycnonuclear fusion. Stars are normally fuelled by a thermonuclear fusion, where high temperatures and pressures overcome atomic nuclei’s natural electric repulsion, enabling atoms to combine into fresh, heavier elements. But in pycnonuclear fusion, quantum tunnelling allows atomic nuclei to get closer to one another than they usually would. Pycnonuclear fusion can thus incredibly slowly change the elements in the white dwarf into iron – the final element that can be created by fusion.
By comparison, the sun fuses more than 10^38 protons per second. To convert a black dwarf into iron by pycnonuclear fusion would take a insane 10^1,100 and 10^32,000 years. If you wrote out all the zeros in numbers, they would take up the entire length of a paragraph to a whole book chapter. Once the black dwarf was mostly iron, it would be crushed by its own mass. This runaway collapse – the supernova – would trigger a massive implosion that ejects the outer layers of the leftover black dwarf. In larger starts today, this iron pileup is also what leads to the more common core-collapse supernovas.
Black dwarf supernovae, however, would only occur in black dwarf stars with masses between 1.16 and 1.35 times that of the sun. Those black dwarf stars are in turn created from normal stars that start off with six to 10 times the mass of the sun.
It is not exactly a rare population, but also not the most common. In fact, these stars make up about 1% of all stars today, and scientists estimate there will be about a billion trillion (10^21) of these supernovae before the end of the universe. Since the black dwarfs have fairly low masses, the black dwarf supernovas would probably be a bit smaller than ones occurring in the present universe, but still an amazing show in an otherwise pitch-black universe.
After these final last gasps of light, nothing left in the universe will be able to explode or shine. So, while the universe will seemingly end in ice, it will all go out with a bang. Perhaps a better understanding of this pycnonuclear process could inform energy sector developments in the future? With a better understanding of how the universe works, our best and brightest can utilize that knowledge for the better of humanity.