'Black dwarf supernova': Physicist calculates when the last supernova
ever will happen
Date:
August 12, 2020
Source:
Illinois State University
Summary:
New theoretical research finds that many white dwarfs may explode
in supernova in the distant far future, long after everything else
in the universe has died and gone quiet.
FULL STORY ========================================================================== [Time and space abstract | Credit: (c) andrey_l / stock.adobe.com]
Time and space abstract concept (stock image).
Credit: (c) andrey_l / stock.adobe.com [Time and space abstract | Credit:
(c) andrey_l / stock.adobe.com] Time and space abstract concept (stock
image).
Credit: (c) andrey_l / stock.adobe.com Close The end of the universe as
we know it will not come with a bang. Most stars will very, very slowly
fizzle as their temperatures fade to zero.
==========================================================================
"It will be a bit of a sad, lonely, cold place," said theoretical
physicist Matt Caplan, who added no one will be around to witness this
long farewell happening in the far far future. Most believe all will
be dark as the universe comes to an end. "It's known as 'heat death,'
where the universe will be mostly black holes and burned-out stars,"
said Caplan, who imagined a slightly different picture when he calculated
how some of these dead stars might change over the eons.
Punctuating the darkness could be silent fireworks -- explosions of the remnants of stars that were never supposed to explode. New theoretical
work by Caplan, an assistant professor of physics at Illinois State
University, finds that many white dwarfs may explode in supernova in the distant far future, long after everything else in the universe has died
and gone quiet.
In the universe now, 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 like a poison,
triggering the star's collapse creating a supernova. But smaller stars
tend to die with a bit more dignity, shrinking and becoming 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," said Caplan. "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." Like white dwarfs today, they'll be made mostly of
light elements like carbon and oxygen and will be the size of the Earth
but contain about as much mass as the sun, their insides squeezed to
densities millions of times greater than anything on Earth.
But just because they're cold doesn't 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, Caplan said. "Fusion happens,
even at zero temperature, it just takes a really long time." He noted this
is the key for turning black dwarfs into iron and triggering a supernova.
Caplan's new work, accepted for publication by Monthly Notices of
the Royal Astronomical Society, calculates 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. If you wrote it out, it would take
up most of a page. It's mindbogglingly far in the future." Of course,
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 blow." Still, that
means as many as 1 percent of all stars that exist today, about a
billion trillion stars, can expect to die this way. As for the rest,
they'll remain black dwarfs. "Even with very slow nuclear reactions,
our sun still doesn't have enough mass to ever explode in a supernova,
even in the far far future.
You could turn the whole sun to iron and it still wouldn't pop."
Caplan calculates that the most massive black dwarfs will explode first, followed by progressively less massive stars, until there are no more
left to go off after about 1032000 years. At that point, the universe
may truly be 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." Even though he'll never see one, Caplan remains unbothered. "I
became a physicist for one reason. I wanted to think about the big
questions- why is the universe here, and how will it end?" When asked
what big question comes next, Caplan says, "Maybe we'll try simulating
some black dwarf supernova. If we can't see them in the sky then at
least we can see them on a computer."
========================================================================== Story Source: Materials provided by Illinois_State_University. Original
written by Rachel Hatch. Note: Content may be edited for style and length.
========================================================================== Journal Reference:
1. M E Caplan. Black Dwarf Supernova in the Far Future. Monthly
Notices of
the Royal Astronomical Society, 2020; DOI: 10.1093/mnras/staa2262 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2020/08/200812113354.htm
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