Wouldn't it be nice to know when a giant star is about to die in a cataclysmic supernova explosion? A team of astronomers has done just that. If you see a giant red star surrounded by a thick shroud of material, watch out — the star will likely explode within a few years.
Supernovas
leave behind dramatic bubbles of gas. (Image credit: X-ray:
NASA/CXC/MIT/L.Lopez et al; Infrared: Palomar; Radio: NSF/NRAO/VLA) |
When a massive star approaches the end of its life, it goes
through several violent phases. Deep in the star's core, it shifts from fusing
hydrogen to fusing heavier elements, starting with helium and moving up to
carbon, oxygen, magnesium and silicon. At the end of the chain, the star
eventually forms iron in its core. Because iron saps energy rather than
releasing it, this spells the end for the star, and in less than a dozen
minutes, it turns itself inside out in a fantastic explosion called a
supernova.
But for all the commotion that goes on in the stars' hearts,
from the outside, it's hard to tell exactly what's going on. Sure, near the end
of their lives, these giant stars swell to extreme sizes. They also become
intensely bright — up to tens of thousands of times brighter than the sun. But
because the stars' surfaces are so distended, their outer temperatures actually
drop, making them appear as red giants.
The most famous example of such a near-terminal star is
Betelgeuse. If it were placed within our solar system, this star — which is
only 11 times more massive than the sun — would stretch to the orbit of
Jupiter. It will go supernova any day now, but "any day" for an
astronomer could be a million years away. Even though we know that these kinds
of stars will eventually detonate in a supernova, there's no way to get a more
precise estimate than that. Or, at least, that used to be the case.
Ticking time bomb
Now, a team of astronomers has developed a way to spot
supernovas that are likely to go off within a few years. They reported their
results in a paper published to the preprint database arXiv and accepted for
publication in the journal Monthly Notices of the Royal Astronomical Society.
They specifically studied a few dozen of a unique type of
supernova known as Type II-P supernovae. In contrast to other supernovas, these
explosions remain bright long after the initial outburst.
In a few examples, astronomers have looked back at old
catalogs and found images of the stars before they exploded, and they all seem
to be red supergiants like Betelgeuse. That's a clear indication that those
kinds of stars are supernova candidates, ready to go off at a moment's notice.
The stars that result in these kinds of supernovas are
thought to have dense shrouds of material surrounding them before they explode.
These shrouds are orders of magnitude denser than what's measured around
Betelgeuse. It's the heating of that material from the initial shock wave that
causes the brightness to linger; there's simply more stuff lying around to keep
glowing well after the first sign of the explosion.
That dense shroud also causes this kind of supernova to
become visible more rapidly than its more exposed cousins. When the explosion
initially happens, the shock wave hits the material around the star, which
causes the shock wave to lose steam as it passes through. While initially the
energies from a supernova are enough to release high-energy radiation, like
X-rays and gamma-rays, after the mixing of the shock wave and the surrounding
material, the radiation given off is in optical wavelengths.
So it seems that these dense shrouds of material around the
stars are also a giveaway that a supernova is about to happen.
Super cocoons
But how long does it take to form that shroud of material?
The researchers studied two models. In one model, the star blew high-velocity
winds from its surface, which slowly detached pieces of itself and spread it
around to make the shroud over the course of decades. In the second model, the
star suffered a violent pre-supernova explosion that sent gas weighing up to
one-tenth the mass of the sun into orbit in less than a year.
The researchers then modeled how all that material would
affect our images of the star. In either case, once the star built its shroud,
it would be heavily obscured in a way that our current imaging technology could
detect.
Because we have direct images of some of the pre-supernova
stars taken less than 10 years before they went off, the astronomers concluded
that the slow-and-steady model wouldn't work. Otherwise, the star would have
been obscured.
All this means that, once a supergiant star builds a thick shroud of material around itself, it is likely to go supernova within a few years. So, if you happen to be traveling through the cosmos and you come across this exact scenario, consider yourself warned.
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