Sometime in the next few months, a spectacle may light up the northern sky.
There, in the constellation Corona Borealis, more than 2,500 light-years away, lurks a star called T Coronae Borealis that is building up to an explosion that will temporarily cause the star to become one of the brightest objects in the night sky.
Astronomers are waiting for this thing to explode, not only because it will be amazing, but also because of the wealth of data we will be able to gather about a type of stellar explosion called a classical nova.
The reason we know T Coronae Borealis (T CrB for short) will explode is because it has done so once every 80 years, for at least eight centuries.
That means it’s very close to a once-in-a-lifetime event – and that the technology we have to observe it now vastly surpasses what we had on its last trip in February 1946.
“There are a few recurring novae with very short cycles, but we don’t typically see recurring bursts in human lifetimes, and rarely this relatively close to our own system,” said astronomer Rebecca Hounsell of NASA’s Goddard Space Flight Center.
“It’s incredibly exciting to have that front row seat.”
Not to be confused with the near obliteration of stars in cataclysmic explosions known as supernovae, classical novae are smaller explosions that leave the star more or less intact. In fact, this is far from the first time this particular space object has gone through this experience.
The reason T CrB explodes repeatedly and on schedule is a quirk of the type of star it is. It’s a binary star system that contains the remnant collapsed core of a Sun-like star called a white dwarf and a fluffy red giant companion .
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White dwarfs are very small and very dense, between the size of the Earth and the Moon, packing in that size as much mass as 1.4 Suns. This means that they are quite gravitationally intensive; and if they have a binary companion in a close enough orbit, they tend to suck up material, mostly hydrogen.
Over time, this hydrogen accumulates on the surface of the white dwarf, compressed downward by the gravitational pull. Eventually, the pressure and heat on the bottom layer of hydrogen become so intense that the whole thing ignites in an uncontrollable thermonuclear explosion that violently ejects the excess hydrogen into space in spectacular style.
This is the new one; and for T CrB the duration of this process is about 80 years.
Over the past decade, astronomers have observed the binary system exhibiting behavior similar to the way it behaved leading up to the 1946 explosion; in particular, a decrease in brightness that portends the close approach of the eruption. Their analysis suggests that this could happen very soon – even before September 2024.
This means that astronomers are watching very closely a small patch of sky clustered with constellations – Lyra, Hercules, Volovar – and a small arc of stars sandwiched between them. This is the Corona Borealis.
We expect to hear about the new one almost as soon as it happens. It will bloom in the sky to become visible to the naked eye, then gradually fade from view over the course of a week. So you should have time to go out and check it out if that’s what you’re into.
Actually, if you could, that would be amazing. Citizen scientists are also called upon to collect data. The more eyes on T CrB, the better we will be able to understand its brilliant bursts.
And, of course, there will be so many telescopes that can be tuned, from the longest radio wavelengths to the most powerful X- and gamma radiation.
“Recurring novae are unpredictable and adversarial,” says astrophysicist Koji Mukai of NASA Goddard. “Just when you think there can be no reason for them to follow a certain pattern, they do – and as soon as you start counting on them repeating the same pattern, they completely deviate from it. We’ll see how T CrB behaves.”
