Sometime in the coming months, a spectacle could 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, building up to an explosion that will temporarily cause the star to become one of the brightest objects at night. sky.
Astronomers are eagerly waiting for this thing to explode, not only because it will be amazing, but also because of the wealth of data we can gather about a type of stellar explosion called a classical nova.
The reason we know that T Coronae Borealis (T CrB for short) is going to explode is because it happens once every 80 years, for at least eight centuries.
This means that it is very close to a once-in-a-lifetime event – and that the technology we have to observe it is now much better than what we had on the last excursion, in February 1946.
“There are a few recurring novae with very short cycles, but typically we don’t see a repeat outburst very often in a lifetime, and rarely one that is so relatively close to our own system,” says astronomer Rebekah Hounsell of NASA’s Goddard Space Flight. Centre.
“It’s incredibly exciting to have this front row seat.”
Not to be confused with the near-destruction of stars in the cataclysmic explosions known as supernovas, classical novas are smaller explosions that leave the star more or less intact. In fact, this is far from the first time this particular cosmic object has had this experience.
The reason that T CrB explodes repeatedly and on schedule is a quirk of the kind of star it is. It is a binary star system that contains the remaining collapsed core of a Sun-like star called a white dwarf and a puffy red giant companion.
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White dwarfs are very small and very compact, about the size of the Earth and the moon, and at that size they have a mass of as much as 1.4 suns. That means they are quite gravity intensive; and if they have a close enough orbiting binary star companion, they tend to suck off material, mainly hydrogen.
Over time, this hydrogen accumulates on the white dwarf’s surface, compressed due to gravity. Eventually, the pressure and heat on the bottom layer of hydrogen become so intense that the whole thing ignites in a runaway thermonuclear explosion that spectacularly violently expels the excess hydrogen into space.
That’s the nova; and for T CrB the length of time this process takes is approximately 80 years.
Over the past decade, astronomers have observed the binary system exhibiting behavior similar to how it behaved leading up to the 1946 explosion; specifically, a dip in brightness that heralds the close approach of the eruption. Their analysis suggests this could happen very soon – as soon as September 2024.
This means astronomers are keeping a close eye on a small patch of sky, clustered with constellations – Lyra, Hercules, Boötes – and a small arc of stars in between. That’s Corona Borealis.
We expect we’ll hear about the nova pretty much as soon as it happens. It will bloom into the sky to become visible to the naked eye, then gradually disappear from view over the course of a week. So you should have the time to go out and watch it, if you feel like it.
In fact, if you can, that would be great. Citizen scientists are being called upon to also collect data. The more eyes on T CrB, the better we will be able to understand its flashy outbursts.
And of course there will be as many telescopes tuning as possible, from the longest radio wavelengths to the most powerful X- and gamma-rays.
“Recurring novae are unpredictable and unruly,” says astrophysicist Koji Mukai of NASA Goddard. “If you think there can’t possibly be a reason why they follow a certain pattern, they do it – and as soon as you start to trust them to repeat the same pattern, they deviate from it completely. We will see how T CrB behaves.”