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 it. If you see a red giant star surrounded by a thick layer of material, be careful: the star is likely to explode within a few years.
As a massive star approaches the end of its life, it goes through several violent phases. Deep in the star’s core, it goes from fusing hydrogen to fusing heavier elements, starting with helium and moving through carbon, oxygen, magnesium, and silicon. At the end of the chain, the star ends up forming iron in its core. Since iron absorbs energy rather than releasing it, this spells the end of the star, and in less than a dozen minutes it spins up in a fantastic explosion called supernova.
But despite all the commotion going on in the hearts of the stars, from the outside, it’s hard to tell exactly what’s going on. Of course, toward 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 ground. But because the stars’ surfaces are so spread out, their outer temperatures actually drop, making them appear red giants.
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The most famous example of a near-terminal star is Betelgeuse. If it were placed inside 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” for an astronomer could be a million years away. While we know that these types of stars will eventually go supernova, there is no way to get a more precise estimate than this. Or at least that was the case.
Now, a team of astronomers has developed a way to detect supernovae that are likely to occur within a few years. They reported their results in a paper published in the arXiv preprint 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 stay bright long after the initial burst.
In some examples, astronomers have looked back in 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 go off at any moment.
Stars that give rise to this type of supernova are thought to have dense blankets of material surrounding them before they explode. 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 glow to persist; There’s simply more stuff around to continue glowing after the first sign of the explosion.
This dense shroud also makes this type of supernova 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 travels through it. While initially the energies of a supernova are sufficient to release high-energy radiation, such as X-rays and gamma rays, after the mixing of the shock wave and the surrounding material, the emitted radiation is in wavelengths optical waves.
So it looks like these dense material sails around stars they are also a gift that a supernova is about to happen.
But how long does it take for this layer of material to form? The researchers studied two models. In one model, the star blew high-speed winds from its surface, which slowly detached pieces of itself and spread them to make the shroud over decades. In the second model, the star underwent 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 this material would affect our images of the star. In any case, once the star builds its shroud, it would become very dim 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 out, the astronomers concluded that the slow-and-steady model wouldn’t work. Otherwise, the star would have been dimmed.
All of this means that once a supergiant star builds up a thick layer of material around itself, it is likely to go supernova within a few years. So, if you’re traveling through the cosmos and encounter this exact scenario, consider yourself warned.
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