T Coronoa Borealis

The astronomy world is eagerly awaiting a new light in the sky. Or, well, actually it’s an old light that’s just going to get brighter for a few days. It does this from time to time, one of a very small number of systems of the type known as a recurrent nova. Its name is T Corona Borealis, its friends call it T CrB, and it’s going to go boom any day now.

So let’s get to know this system, and just what the heck a recurrent nova is anyway.

Nova vs. Recurrent Nova vs. Supernova

T CrB is a recurrent nova, which is different from a regular nova and different from a supernova, but you wouldn’t know it if you only know how they start.

Recurrent novae are binary star systems in which one member of the pair is a dead star. Specifically it’s a white dwarf, the core of what was once a relatively Sun-like star that has expired and left its center behind. The other member of the pair is a star that is still kicking, though it might be reaching the end stages of its life (as is the case with T CrB).

The action in a recurrent nova starts when the white dwarf begins to pull material off of the living star using its gravity. This material begins to accumulate on the surface of the dwarf. Of course, that’s exactly how a regular nova starts. It’s also how a certain kind of supernova, known as a Type 1a supernova starts. The difference comes once the accumulated mass on the white dwarf reaches a critical point. 

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With both regular ol’ novae and recurrent novae the mass that the dwarf sucks off of its companion sits above the dwarf’s surface as a sort of very dense atmosphere. This is important—the mass isn’t integrated into the dwarf itself, but is sitting on top of its surface. That distinction is going to become critical later.

White dwarfs are hot, with surface temps shooting up as high as 180,000F. That means the stuff sitting on top of it gets superheated over time. And once enough of it gets hot enough, it starts nuclear fusion. Nuclear fusion, if you didn’t know, is how stars burn. The stuff sitting on the white dwarf erupts, shooting off the surface of the dwarf even as it burns itself dazzlingly out, which can take anywhere from a few days to a few weeks.

For regular novae, this happens just once, or at least once over the time spans that humans have been staring at the sky. For recurrent novae like T CrB, as you might guess from the name, it’s a cycle happening over and over as the white dwarf expels its extra matter and then starts accumulating again.

An Even Bigger Explosion

So what’s a Type 1a supernova? This is when the extra mass isn’t just sitting on the surface of the white dwarf getting heated. It’s becoming part of the dwarf itself, increasing the dwarf’s mass. This is a problem, because white dwarfs can only get so massive before the proverbial stuff hits the fan.

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Above a certain mass, known as the Chandrasekhar limit after groundbreaking physicist 

Subrahmanyan Chandrasekhar, white dwarfs are no longer stable stellar corpses. The extra mass re-triggers the fusion process deep inside them. This starts a runaway effect inside the dwarf that produces an extreme amount of energy in a very small amount of space in a very short amount of time, which is a very dull and prolonged way of saying it makes a really huge explosion.

With both a regular and a recurrent nova, the white dwarf is still around after the excitement is done. When a Type 1a supernova happens, there is no white dwarf left afterwards. There’s no nothing left afterwards, not even the neutron star or black hole you might have left after other sorts of supernovae. This is total annihilation.

Back to T Corona Borealis

We know of less than a dozen recurrent novae in our skies, but T CrB is one of them. The span between eruptions for these novae is all over the place, from a few years to a few decades, but T CrB it averages a nova roughly every 80 years.

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We’re not sure, but we think the first recorded observation of this nova dates from 1217. Somebody definitely wrote down an account of a nova in that year, and it may have been our buddy T CrB. Certainly the timing would be about right. It also may have been observed in 1787. It was definitely observed in 1866, and then again in 1946. 

It’s the observations from this last eruption that lead us to believe the next one is imminent. Prior to the 1946 nova T CrB underwent a noticeable drop in brightness. It has now undergone this same sort of dimming, which is why astronomers started sounding the alarm about this object over the winter.

That said, these things aren’t quite a precise science. You might notice, for instance, that if it goes soon it will only be 78 years after the last nova, not 80. The rate at which material accumulates onto the surface of the white dwarf and then superheats isn’t exactly clockwork. If it wasn’t for the dimming, we might assume we had another couple of years to wait.

Astronomers seem fairly confident that the signs point to the eruption happening pretty imminently, even if we don’t know exactly when. So what’s it gonna look like anyway?

A New Star! Well, Briefly Anyway.

If you wanted to spot T CrB under normal circumstances, you’d need a telescope. Even binoculars might have trouble picking it out. It’s 3,000 light years away from us and usually the brightest thing about it is the companion, which is not bright at all.

When the nova goes though, it’s going to get about 1,500 times brighter than normal as the accumulated plasma on the white dwarf commences nuclear fusion. That sounds ridiculously bright, and it is, but it’s not going to light up our skies or anything.

What we will see is a new star in the constellation Corona Borealis, aka the Northern Crown. This new star will be as bright or possibly even brighter than the brightest star in that constellation (it’s a dim section of sky mostly), about the same brightness as Polaris, the North Star (which is not the brightest star in the sky—boy, would I love to get my hands on whoever started that rumor). 

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To find Corona Borealis I like to start with the Big Dipper. Which, yes, is nowhere near Corona Borealis, but it’s relevant, I swear. The handle of the Dipper is curved into an arc. If you follow the direction of the arc off the end of the handle and keep going until you hit a bright star, you will smack right into the bright star Arcturus, so astronomers call this the Arc to Arcturus.

Arcturus is among the brightest stars in the sky and is the brightest star in the constellation Boötes, the Herdsman (he’s a bear herdsman. Yes, I said “bear herdsman”. Don’t get me started). I like to call Boötes the Ice Cream Cone, seeing as it’s nice and high in the summertime and looks far more like an ice cream cone than a bear herdsman. Arcturus is the point of the cone.

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Once you’ve found Boötes look to its east for a fainter C shape. That C shape is Corona Borealis, the Northern Crown. The nova is going to be just outside of the C, to the east of the constellation’s brightest star, Alphecca. At this time of year Boötes and Corona Borealis will be well in the west when the Sun goes down, so don’t be looking high up in the sky for this. 

When the nova erupts, it’s expected that the brightness will last about a week before T CrB fades back to its usual lackluster magnitude and the white dwarf once gain begins the eight-decade process of accumulating matter.

Lights in the Sky

Once upon a time a new thing like this appearing would have heralded disaster. In many cultures the heavens were thought to be unchanging, and if something new did show up, it meant some deity or other was Not Happy. Now we know that we live in a universe that is changing all the time. Everything is moving. New solar systems are forming. Galaxies are colliding. Rocks and comets are flying around the solar system at ridiculous speeds (spoiler, I expect I’ll be covering comets in the coming weeks).

And sometimes a star just has to explode a little bit. After all, we all need to let off steam from time to time.