We sat down with Tony Prio, a theoretical astrophysicist at Carnegie Science Observatories, to talk about T Coronae Borealis, a once-every-80-year stellar explosion expected to occur in the coming months.

Q: We’ve seen some headlines about a star explosion that might happen in the near future. Can you tell us what it is?

Tony Piro: This is T Coronae Borealis. It’s a binary system consisting of a white dwarf and a red giant. A white dwarf is essentially what’s left after a star runs out of nuclear fuel and has a mass that’s almost 40% greater than our sun.

The binary system is in a 227-day orbit. The red giant is so large that its material is pulled away by the white dwarf’s gravity. This material forms a disk around the white dwarf. Then, over time, the disk transfers material to the white dwarf’s surface.

T Coronae Borealis is known as a “recursive nova” system. As material accumulates on the white dwarf, it becomes hotter and denser until it ignites a thermonuclear explosion. This burns the surface material, creating a bright event that can last for weeks to months. This is called a nova.

Sometimes it takes tens or even hundreds of thousands of years to build and ignite a nova. Most of the novae we see in our galaxy are like this. One reason T Cor Borealis is unique is that its novae appear much faster.

This is because the white dwarf is more massive than most, causing the accreting material to reach higher pressures and temperatures much faster. There are about 10 such recurring stars in our galaxy, but T Coronae Borealis is the closest.

When was the last time the T Coronae Borealis satellite exploded?

The last one was in 1946, and before that in 1866. So by simple math, it should have a nova every 80 years. But on top of that, for each of the previous novas, the white dwarf has been observed to go through different states as it approaches eruption.

First of all, it tends to be bright for about 10 years before it explodes. And we think that’s because the disk, as it accumulates more mass, becomes more active in funneling material onto the white dwarf. And over the last 10 years, again, we’ve seen T Coronae Borealis brighten up just as much as it did during the last two explosions.

Second, for each of the previous novae, the white dwarf began to dim about a year before the explosion. Similarly, since March 2023, T Coronae Borealis has also begun to dim. It is not clear why this is happening.

One idea is that the disk is sort of emptied and it’s finished accreting. And then there’s a cooling phase as the material compresses onto the white dwarf, reaching higher pressures, before it explodes.

So we don’t know exactly when this will happen, but since the glow started to fade in March 2023, we can expect an explosion in the next few months.

Why are repeating systems of new stars like T Coronae Borealis interesting to astronomers?

It’s interesting for a number of reasons. One is that it’s the closest of all the recurrent novae. It’s only about 26,000 light-years away. So it’s the one that we can study in the most detail. And it recurs relatively often, so we can predict it and prepare for the explosion.

Because the object is so close, we can track how material has been accreting onto the white dwarf over the past 80 years, giving us a better understanding of what conditions need to be met for the explosion to occur.

T Cor Bor is also interesting because its white dwarf is close to the maximum mass that white dwarfs can have. White dwarfs cannot be more massive than the so-called Chandrasekhar limit, which is about 1.4 times the mass of our sun.

Above this mass, the white dwarf begins to collapse and can potentially explode and produce a Type Ia supernova. When this happens, the entire white dwarf is destroyed, not just the surface layers, as in a nova. Type Ia supernovae are incredibly important, from synthesizing many of the heavy elements that are critical to life to serving as cosmic beacons that are used to measure the shape and size of the universe.

So novae like T Coronae Borealis are interesting in our ongoing efforts to understand how white dwarfs could reach such a limit and trigger a supernova explosion.

In addition, there has been a renaissance in nova research over the past decade, as new technologies and new telescopes have revealed that many of the high-energy gamma-ray emissions are actually coming from novae.

This was not at all expected, but gigavolt photons come from these events. We think that this is because the shocks are generated by the interaction of the exploding surface layers with the outflow of material from the red giant.

We really didn’t expect this until we had the technology to see it. For example, there are things like Cherenkov detectors that are on the ground that allow us to see cosmic rays—high-energy photons—as they hit the Earth’s atmosphere and create showers of particles. And by looking at that, we can reconstruct the energy of those photons and learn about the events that emitted them.

We’ve long known that gamma rays, supermassive black holes at the centers of galaxies, and other cosmic phenomena can produce high-energy emissions, but only recently have we learned that these novae can do so, too. So it will be very exciting to see one of the next novae studied in this way.

Will non-professional astronomers be able to enjoy this event?

Yes, absolutely. When the explosion is triggered, the white dwarf will be brighter for about five days, reaching a maximum brightness equal to the brightness of the North Star in the sky. So it will be that bright for a few hours and you will be able to see it with the naked eye. Then the brightness will decrease exponentially over the following weeks. So for maybe five days you will still be able to see it with good binoculars.

This reminds me of another interesting thing, which is that T Coronae Borealis is the only nova event that actually brightens again after about 100 days. Other repeating novae do not show this same feature, so this is something we want to investigate further.

The second brightening is too faint to be seen with the naked eye. But thanks to the powerful telescopes that astronomers use to do our work, we will be able to study it and find out why it happens.

Unfortunately, T Coronae Borealis is only visible in the Northern Hemisphere, and Carnegie Science’s Las Campanas Observatory is located in Chile, so we won’t be able to study the phenomenon at our own facilities. Interestingly, the 1946 eruption was observed at our famous Mt Wilson facility north of Pasadena.

What tools will be available to study T Cor Bor that were not available in the 1940s or earlier?

Most of the earlier detections were of course just optical light. We can now see objects and phenomena in so many different wavelengths. Apart from optical detectors, the most interesting tools will probably be things like X-ray detectors, which can reveal new information about high-energy emissions.

As I mentioned, Cherenkov detectors will allow us to better understand the shocks caused by a violent explosion on the surface of a white dwarf.

As a theoretician, are we able to test theories by studying T Cor Bor?

Because T Coronae Borealis is so close and can be observed even when it’s not exploding, we can get an inventory of how much mass has accumulated on the white dwarf. This can then be compared to the amount of material ejected in the explosion, which teaches us how white dwarfs can gain or lose mass over time. This is exciting because as white dwarfs gain mass, they can tend to become Type Ia supernovae.

And then the second thing I mentioned is that there’s a lot of interesting physics involved in accelerating particles of these high-energy emissions. Theorists are really excited about doing the math to explain how this acceleration works, and T Coronae Borealis is an amazing laboratory for testing these theories.

In addition, there are these various features, such as the dimming, which is speculated to be due to the disk clearing and getting a little bit cooler in the second before it triggers an eruption. All of the features of the dimming, the brightness, and then the secondary plateau, will be better characterized compared to theoretical models.

Is there anything else we haven’t talked about that you think is important to understand in the context of this particular phenomenon and others like it?

It’s interesting to think that since T Coronae Borealis is 2,600 light-years away and a nova appears about every 80 years, that means there have already been more than 30 such eruptions, and they’re all heading our way.

And so, you can almost imagine these shells in light of these explosions that have happened over the last 2,600 years, all heading our way through the Milky Way right now. So it’s not a matter of when the new ones will appear, but when they will finally get to us on Earth so we can learn about them.

Sometimes we talk about things that are quite abstract in astronomy, but this is something that you will be able to see with the naked eye and directly point to. It’s a unique opportunity to connect with the same kind of excitement that astronomers have felt for thousands of years when they discover something new in the sky. We don’t know exactly when that will happen, but it will be cool when it does.