Wouldn’t it be nice to know when a giant star will die in a cataclysmic supernova explosion? A team of astronomers did just that. If you see a huge red star surrounded by a thick shell of material, beware — the star is likely to explode within a few years.
As a massive star nears 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. Eventually, at the end of the chain, the star forms iron at its core. Since iron consumes energy instead of releasing it, the star is doomed, and in less than a dozen minutes it inverts itself in a fantastic explosion called a supernova.
But with all the excitement going on in the hearts of the stars, it’s hard to tell from the outside exactly what’s going on. Sure, toward the end of their lives, these giant stars swell to extreme sizes. They also become intensely bright – up to ten thousand times brighter than that Sun. But because the stars’ surfaces are so extended, their external temperatures actually drop, making them appear that way Red Giants.
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The best-known example of such a terminal star is Betelgeuse. If it were placed in ours solar systemthis star – which is only 11 times the mass of the Sun – would extend into the orbit of Jupiter. It will become a supernova any day now, but “any day” for an astronomer could be a million years away. While we know that these types of stars will eventually explode in a supernova, there’s no way to get a more accurate estimate. Or at least that’s how it used to be.
Ticking time bomb
Now a team of astronomers has developed a method to detect supernovae that are likely to start within a few years. They reported their results in a paper published in the preprint database arXiv and accepted for publication in the Monthly Notices of the Royal Astronomical Society.
They specifically studied a few dozen of a unique type of supernova known as a Type II-P supernova. Unlike other supernovae, these explosions remain bright long after the initial burst.
In some examples, astronomers have looked back in old catalogs and found images of the stars before they exploded, and they all appear to be red supergiants like Betelgeuse. This is a clear indication that these types of stars are supernova candidates, ready to explode at any moment.
The stars that give rise to these types of supernovae are believed to be surrounded by dense shells of material before they explode. These shrouds are orders of magnitude denser than what is measured around Betelgeuse. It is the heating of this material by the initial shock wave that causes the brightness to persist; There’s just more stuff lying around to give a good glow after the first signs of the explosion.
This dense envelope also causes this type of supernova to become visible faster than its more exposed cousins. In the initial explosion, the shock wave impacts material around the star, causing the shock wave to lose steam as it passes. While the energies of a supernova are initially sufficient to release high-energy radiation such as X-rays and gamma rays, once the shock wave and the surrounding material have mixed, the emitted radiation is in the optical wavelength range.
So it seems that these dense veils of material around the Stars are also an indication that a supernova is imminent.
Super cocoons
But how long does it take to form this shell of material? The researchers examined two models. In one model, the star blew off its surface in high-velocity winds that slowly shed parts of itself and scattered them over the decades to form the shroud. In the second model, the star suffered a violent explosion before the supernova, ejecting gas weighing up to a tenth mass of the sun into orbit in less than a year.
The researchers then modeled how all this material would affect our images of the star. In either case, once the star has built its envelope, it would be obscured enough for our current imaging technology to detect.
Because we have direct images of some of the pre-supernova stars taken less than 10 years before they exploded, astronomers concluded that the slow-and-steady model would not work. Otherwise the star would have been obscured.
All of this means that once a supergiant has built up a thick shell of matter around itself, it is likely to go supernova within a few years. So if you happen to be traveling the cosmos and come across this very scenario, be warned.
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