A mismatched pair of stars 3,000 light years away may be gearing up for an explosion in the next few months. T Coronae Borealis (T CrB to its friends) is normally too dim to see with the unaided eye, but roughly every 80 years, it flares brightly enough to appear as a “new” star, or nova, in Earth’s northern sky. Its last event was in 1946, and astronomers say it’s likely to go off again this summer.
Twinkle, twinkle, little — boom
T CrB is actually a pair of stars, trapped in each other’s orbit — and in an extremely messy relationship that sometimes literally explodes.
One member of the pair is an aging red giant, which has burned up all its hydrogen fuel and is now fusing helium atoms together at its core. Helium fusion produces a tremendous amount of heat and radiation pressure, so the aging star’s outer layers have swelled outward to many times its original size (this is the same fate that will eventually befall our Sun in about 5 billion years). The other half of the dynamic duo is a white dwarf: the burned-out remains of a star’s core (this is what our Sun will look like sometime after its red giant phase).
About every 80 years, the red giant shrugs off its outermost layers of gas, and the white dwarf’s gravity grabs them. White dwarfs are nowhere near as dense as neutron stars, but they're still pretty dense, being the remnants of stellar cores — and it doesn't take much to ignite one again, at least temporarily. When the gas discarded by the red giant falls onto the surface of the white dwarf, it puts just enough pressure on the inner layers of the white dwarf to kickstart nuclear fusion.
That sudden burst of fusion triggers a chain reaction that eventually engulfs the outer layers of the star in what's called a runaway thermonuclear reaction. The envelope of gas around the white dwarf (the same envelope it just snatched away from the nearby red giant), heated by the runaway nuclear fusion, explodes outward at roughly 3,700 miles per second. In other words, KABOOM.
And 3,000 light years away, people on Earth will be able to see what looks like a new star in the night sky.
How to see the Nova in T. Coronae Borealis
You can’t see the T Coronae Borealis system with the unaided eye right now, because it’s too dim and too far away. When the nova happens, that will change; the now-invisible star will suddenly appear, blazing brightly for about a week. But you’ll need to know where to look.
This occasionally explosive pair of stars is located in a very small, C-shaped constellation called Coronae Borealis (or the Northern Crown if you’re not a fan of conversational Latin). The Northern Crown lies between the big kite-shaped constellation Boötes and the smaller constellation Hercules. Hercules makes a good landmark, because it’s shaped a little like the more familiar Orion, and it’s almost directly overhead after sunset.
If you’re not an experienced stargazer with a clear view of the night sky, though, your best bet might be to download a star-chart app that can help you pinpoint objects in the sky. It’s a good idea to play with the app and find out where T Coronae Borealis is – the better to appreciate its sudden appearance when the nova finally goes off.
How do we know when T. Coronae Borealis will go supernova?
For the last several centuries, astronomers have watched this pair of stars flare up fairly regularly: about once every 80 years. And the last time this happened, in 1946, astronomers had advanced enough equipment to measure changes in the stars’ brightness in specific wavelengths of light, especially during the months and years leading up to the nova.
“Its behavior over the past decade appears strikingly similar to the observed behavior in a similar timeframe leading up to the 1946 eruption,” writes NASA in a recent press release. “If this pattern continues, some researchers say, the nova event could occur by September 2024.”
But there are no guarantees, and predicting a nova in a star system 3,000 light years away is far from an exact science, especially since astrophysicists still don’t understand the mechanics of these explosions in much detail (which is something they’re hoping this year’s nova can shed more light on).
“Recurrent novae are unpredictable and contrarian,” says NASA astrophysicist Koji Mukai in a recent statement. “We’ll see how T Coronae Borealis behaves.”
What will we learn from the Nova?
The handful of stars in our galaxy that regularly flare up in bright novae are even farther away than T Coronae Borealis (which is actually pretty close by, in stellar terms). That makes it difficult for astronomers to see much detail about exactly how the runaway thermonuclear reaction kicks off and how it finally burns itself out. If predictions are correct, though, T Coronae Borealis should give astronomers an excellent view of all the gory details this time around.
Amateur astronomers all over the world are watching T Coronae Borealis’s dark spot in the night sky right now so they can alert scientists the moment it explodes. When that happens, electronic eyes all over the planet – and in orbit – will pivot toward the star system to record what happens. Teams of astronomers plan to watch the fireworks show with JWST, the Fermi Gamma-Ray Space Telescope, the Swift Observatory, the Very Large Array, and a host of other telescopes on Earth and in space. Those observations should cover pretty much the whole spectrum of radiation, from long, slow radio waves to short, energetic gamma rays.
Much of that information will be new; scientists didn’t have anything like the Fermi Gamma-Ray observatory back in 1946, so they have no idea how a nova looks in gamma rays. Another new addition is NASA’s Imaging Polarimetry Explorer, or IXPE, with its ability to study how closely the light waves in a given beam of x-rays line up with one another; that can reveal all sorts of detail about the x-rays’ origins and the space they’ve traveled through.
What about Betelgeuse?
It won’t take T CrB’s white dwarf very long to burn up all its hydrogen fuel and settle back down: It’ll only be visible in the sky for a few days, and the runaway thermonuclear reaction itself will last just a fraction of that time. When Betelgeuse eventually explodes, however, the glow of its death throes will hang in the sky for several months. What’s the difference?
The kind of explosion that lights up T CrB once every 40 years is called a nova, and it happens when a white dwarf gets briefly reignited in a runaway nuclear reaction. Imagine throwing lighter fluid and a match onto a pile of burned-up charcoal. It will catch fire and burn for a moment, but it won’t last.
When Betelgeuse dies, however, it will go out in a blaze of glory called a supernova. A supernova happens when an extremely massive star runs out of fuel for its nuclear reactions; the outer layers collapse, and the pressure of the collapse triggers an enormous explosion. Think of it as blowing up the grill: It’s going to be bigger and brighter than what happened when you lit up the charcoal, and you’re only going to be able to do it once.
Novae like T CrB, though, are the explosive cosmic gifts that just keep on exploding.
