We might witness a star explode soon — and it’s not Betelgeuse.
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 year.
This illustration shows the red giant in T CrB with a disk of discarded gas around it. The white dwarf (the bright spot in the disk at the beginning of the animation) eventually gobbles up so much material that its surface explodes in a runaway thermonuclear reaction.
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 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 even more mass gets piled on the surface, it adds just enough heat and pressure for the white dwarf to start fusing hydrogen and helium in its inner layers.
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, 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.
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.
