Universe may end with fireworks of stellar remnants called black dwarfs: Study
The analysis reveals that the biggest black dwarfs will be the first to explode, with smaller ones taking longer to reach this stage until there are no more left to go off
Towards the end of the universe, most stars will slowly fizzle as their temperatures will fade to zero, according to a new study. Many believe all will be dark as the universe comes to an end. But before everything goes dark and silent forever, stellar remnants known as black dwarfs will provide the final fireworks, says theoretical physicist Matt Caplan. According to Caplan, an assistant professor of physics at Illinois State University, the universe could end with a "black dwarf supernova" trillions and trillions of years from now. A supernova is the explosion of a star and it happens where there is a change in the core or center of a star.
The study is based on the idea that the universe will eventually run out of energy in what is known as "heat death". "It will be a bit of a sad, lonely, cold place. It’s known as ‘heat death,’ where the universe will be mostly black holes and burned-out stars," describes Caplan in the analysis which has been accepted for publication by Monthly Notices of the Royal Astronomical Society. He calculated how some of these dead stars might change. "Punctuating the darkness could be silent fireworks — explosions of the remnants of stars that were never supposed to explode. Many white dwarfs may explode in supernova in the distant far future, long after everything else in the universe has died and gone quiet," adds Caplan.
While it is now expected that all stars will evolve toward degenerate remnants such as neutron stars, white dwarfs or black holes, the fate of these objects in the far future is an open question. The current study explores what will happen to white dwarfs long into the future.
Currently, the dramatic death of massive stars in supernova explosions comes when internal nuclear reactions produce iron in the core. Iron cannot be burnt by stars, it accumulates triggering the star’s collapse, creating a supernova. In the case of smaller stars, they shrink and become white dwarfs at the end of their lives. "Stars less than about 10 times the mass of the sun do not have the gravity or density to produce iron in their cores the way massive stars do, so they can’t explode in a supernova right now. As white dwarfs cool down over the next few trillion years, they’ll grow dimmer, eventually freeze solid, and become ‘black dwarf’ stars that no longer shine," explains Caplan.
However, just because the stars are cold does not mean nuclear reactions stop. "Stars shine because of thermonuclear fusion. They’re hot enough to smash small nuclei together to make larger nuclei, which releases energy. White dwarfs are ash, they’re burnt out, but fusion reactions can still happen because of quantum tunneling, only much slower. Fusion happens, even at zero temperature, it just takes a long time, says Caplan, emphasizing that this is the key for turning black dwarfs into iron and triggering a supernova.
The scientist calculated how long these nuclear reactions take to produce iron, and how much iron black dwarfs of different sizes need to explode. He calls his theoretical explosions “black dwarf supernova” and calculates that the first one will occur in about “10 to the 1100th years". "In years, it’s like saying the word ‘trillion’ almost a hundred times,” he explains.
But not all black dwarfs will explode. Only the most massive black dwarfs, about 1.2 to 1.4 times the mass of the sun, will explode, implying that as many as “1% of all the stars that exist today, about a billion trillion stars,” can expect to die this way. The rest will remain black dwarfs.
The analysis reveals that the biggest black dwarfs will be first to explode, with smaller ones taking longer to reach this stage “until there are no more left to go off after about 10^32000 years". After these final explosions, the universe may become "dead and silent". "It’s hard to imagine anything coming after that, black dwarf supernova might be the last interesting thing to happen in the universe. They may be the last supernova ever. By the time the first black dwarfs explode, the universe will already be unrecognizable. Galaxies will have dispersed, black holes will have evaporated, and the expansion of the universe will have pulled all remaining objects so far apart that none will ever see any of the others explode. It won’t even be physically possible for light to travel that far," writes Caplan.