Very soon now, possibly in a few days, though more likely in the next few weeks, a new star will appear in our sky—except it’s really an old star. Called T Coronae Borealis (or T Cor Bor), it’s a binary system composed of a huge red giant star and a tiny white dwarf. Though small, white dwarfs are vicious: They pack much of a solar-type star’s mass into an approximately Earth-sized sphere. This makes them terrifically dense and hot, and they possess a fierce gravitational attraction.
The white dwarf in T Cor Bor is slowly drawing hydrogen and other gases off the red giant, piling up all that pilfered material on its surface. Eventually enough will accumulate that the immense gravity will fuse the hydrogen, detonating what is in essence a thermonuclear bomb the size of a planet. The explosion is so powerful that this ordinarily dim star, barely visible through binoculars, suddenly flares to naked-eye brightness despite being 3,000 light-years from Earth. In an instant it will blast out upwards of 100,000 times as much energy as the sun’s annual output. It’s a big deal.
Still, that cataclysm isn’t energetic enough to shatter the white dwarf to smithereens. Instead it will fade in the days and weeks after the explosion and eventually go back to its usual more modest luminosity. Over roughly 80 years the cycle repeats, making this a predictably recurrent event, which is why astronomers are excited about T Cor Bor imminently blowing its top (somewhat literally).
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Such an event is called a nova, shortened from the Latin stella nova, or “new star,” because it appears as if a new star has popped into existence where none was before. It’s a misnomer: There was already a star there—two stars, actually—and both are quite old. A white dwarf is what’s left after a star like the sun dies, and a red giant is a sunlike star in the process of dying. It takes billions of years for such stars to reach these evolutionary stages, so the “nova” nomenclature is somewhat ironic. The term is steeped in history, though, so we’re stuck with it.
But if a nova can recur, then it’s not really a star’s death. So how do stars die?
For “normal” stars—ones that create energy by fusing lighter elements together in their core to make heavier elements—death occurs when they run out of available fuel. A star like the sun takes roughly 12 billion years to fuse all its core hydrogen into helium. If the star has enough mass, it’ll squeeze that helium hard enough to fuse it into carbon, generating even more energy than it did before.
All that energy flows into the star’s gassy outer atmosphere, which starts to expand, like any gas does when soaking up energy. But then a funny thing happens: The surface area of the expanding star increases so much that the amount of energy it radiates per square centimeter actually gets lower. The temperature of the gas drops, changing the star’s color to vermillion. What we have then is a red giant.
Because the star gets so huge—up to 200 times the diameter of the sun!—the gravity at its surface weakens. Paired with the star’s surging luminosity, this means that gas will start to flow away from it, creating a “stellar wind” that can eventually eject more than half the star’s total mass. Ultimately all that’s left is the core: a small and unbearably hot but very faint stellar cinder, which we call a white dwarf.
For low- to medium-mass stars like the sun, that’s pretty much it; over the eons, a solitary white dwarf will gradually cool down, slowly fading to become a so-far-theoretical inert lump called a black dwarf. (The universe isn’t yet old enough for any white dwarfs to have gotten so cold, so we haven’t actually seen any yet.) If the star happens to be in a binary system like T Cor Bor, things get more interesting because it can go nova. But even then that’s not necessarily the end. Some novae don’t blow off all the accumulated material, so, given time, they actually increase in mass. Then things get a lot more interesting.
As material builds up, the gravitational force grows, causing the pressure inside a white dwarf to grow, too. That internal pressure can even grow high enough to reignite fusion reactions that slam together carbon nuclei to rapidly release a vast amount of energy. The temperature increases immensely, by billions of degrees, causing the white dwarf to explode.
This explosion is far bigger than the sweatiest apocalyptic dreams of humanity: it is 10 billion times brighter than the sun, luminous enough to outshine an entire galaxy! This event is called a supernova—specifically a Type Ia supernova.
There is another kind of star explosion called—unsurprisingly—a Type II supernova, which is triggered when a high-mass star dies. Stars with more than about eight times the mass of the sun have enough pressure in their core to fuse even heavier elements. Carbon is the last fusion product of sunlike stars, but much heftier stars can fuse carbon into neon, which can fuse into oxygen, then oxygen into silicon. At that point the star is in big trouble. The outward push of radiant energy from fusion at its core is what supports the star against the inward pull of its own gravity. But silicon only fuses to iron, and fusing iron actually requires more energy than it releases. Worse, all that newly forged iron soaks up electrons from the core that would otherwise help prop up the star.
So once iron fusion begins, all that stellar support vanishes, and the core collapses like a chair with its legs kicked out from underneath it. This releases an absurdly huge flood of energy (and subatomic particles called neutrinos)—like lighting a match in a dynamite factory times a zillion. The star’s voluminous outer layers absorb so much energy that they recoil outward, creating an immense Type II (or core-collapse) supernova. Kaboom!
In some of these explosions, the core completely disrupts, leaving behind only the rapidly expanding debris from the explosion. On the plus side, this material is enriched in heavy elements that then go on to form the next generation of stars—and planets, too. But sometimes the core can collapse to form an überdense neutron star or, more terrifyingly, a black hole. The details, unsurprisingly, are quite complex, and astrophysicists are still working to understand all the vagaries of these events.
Both Type Ia and Type II supernovae are so bright that they can be seen across billions of light-years, which is a significant swath of the observable cosmos. Our telescopes are now so sensitive that every year astronomers see tens of thousands of these stellar cataclysms somewhere in the universe. Of course, they’re also so explosive that you don’t want to be too close to one, either, lest the debris physically impact you.
And what of T Cor Bor, the recurrent nova that kicked off this whole topic? It appears to be the kind that gains mass slowly after the explosion, so it may eventually grow enough to go out with a bang as a Type Ia supernova. In that case, there will be a final blast from T Cor Bor before the really, truly final blast. That’s not likely to be for a very, very long time, though, so enjoy the show it’ll be putting on soon. It won’t give a repeat performance for about another 80 years!
