Wouldn’t it be nice to know when a giant star is about to die in a cataclysmic supernova explosion? This is exactly what a team of astronomers did. If you see a giant red star surrounded by a thick shroud of matter, be careful, the star will probably explode in a few years.
When a massive star approaches the end of its life, it goes through several violent phases. Deep in the star’s core, it changes from the fusion of hydrogen to the fusion of heavier elements, starting with helium and working its way up to carbon, oxygen, magnesium and to silicon. At the end of the chain, the star eventually forms iron in its core. Because the iron saps the energy instead of releasing it, this spells the end of the star, and in less than a dozen minutes it flips over in a fantastic explosion called a supernova.
But despite all the turmoil going on in the stars’ hearts, from the outside it’s hard to tell exactly what’s going on. Of course, towards the end of their life, these giant stars reach extreme sizes. They also become intensely bright – up to tens of thousands of times brighter than the Sun. But because the surfaces of stars are so distended, their outer temperatures actually drop, making them appear like red giants.
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The most famous example of such a star near the terminal is Betelgeuse. If it was placed in our solar systemthis star – which is only 11 times more massive than the sun – would extend to the orbit of Jupiter. It will go supernova any day now, but “any day” to an astronomer could be a million years away. Even though we know these types of stars will eventually explode in a supernova, there’s no way to get a more accurate estimate than that. Or, at least, it was.
Now a team of astronomers has developed a way to spot supernovae that are likely to go off within a few years. They reported their results in an article published in the arXiv prepublication database 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. Unlike other supernovae, these explosions remain bright long after the initial explosion.
In a few instances, astronomers have consulted old catalogs and found images of 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 result from these types of supernovae are thought to be surrounded by dense envelopes of material before exploding. These shrouds are orders of magnitude denser than what is measured around Betelgeuse. It is the heating of this material from the initial shock wave that causes the luminosity to persist; there’s just more stuff lying around to keep shining after the first sign of the explosion.
This dense shroud also causes this type of supernova to become visible more quickly than its more exposed cousins. When the explosion initially occurs, the shock wave hits the material around the star, causing the shock wave to lose steam as it passes. While initially the energies of a supernova are sufficient to release high-energy radiation, such as X-rays and gamma rays, after the shock wave and surrounding matter mix, the radiation emitted is in optical wavelengths.
So it seems that these dense shrouds of matter around the stars are also a gift that a supernova is about to occur.
But how long does it take to form this shroud of matter? The researchers studied two models. In one model, the star blew high-velocity winds from its surface, which slowly tore off bits of itself and spread them out to form the shroud over decades. In the second model, the star suffered a violent pre-supernova explosion that sent gas weighing up to a tenth of the 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 builds 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 their extinction, 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 star has built a thick shroud of matter around itself, it’s likely to go supernova within a few years. So if you’re traveling through the cosmos and come across this exact scenario, consider yourself warned.
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