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Back in 2008, astronomers spotted a weird thing in our neighboring galaxy Andromeda.

Without warning, a star suddenly flashed incredibly brightly!

But then, it faded back to normal over about fifteen days.

And then, a year later, it did it again!

Year after year, it kept on bursting into brightness and then settling back down.

Scientists named it M31N 2008-12a, which we'll call 12a for short.

And it turns out, this thing is something really special.

It's a rare phenomenon called a recurrent nova.

And it may hold the key to understanding the lives and cataclysmic deaths of massive stars.

In 2008, though, scientists didn't realize how cool this thing was.

They believed that the first flash of 12a was a plain old nova.

If you've heard of supernovas, the violent explosions that happen at the end of some stars' lives,

well, these are totally different.

So you can thank astronomers for that.

Novas are much less destructive.

They originate from a very specific kind of binary star system:

a system with one normal star, and one small, dense white dwarf.

The white dwarf has a pretty intense gravitational pull, so over time,

it steals hydrogen gas from the outer layer of its neighboring star.

Over the years, that gas builds up on the white dwarf's surface,

where it gets hotter, and hotter, and hotter.

And when it reaches about twenty million degrees Celsius,

nuclear fusion is kick-started within that outer layer.

The stolen hydrogen is converted into heavier elements, and that reaction

releases a huge amount of energy, starting a process called thermonuclear runaway.

In this process, the fusion generates energy, which heats up the hydrogen more,

which boosts fusion,which generates more energy and heat, and more fusion, and you get the idea.

Ultimately, that sudden release of runaway energy is enough to blast what's left of

the stolen hydrogen and the fusion products outwards, away from the star.

And you end up with a really bright shell of material,

lit up by the white dwarf and companion star within.

From Earth, we see novas as sudden pulses of bright light that slowly fade over time,

as their shells expand and dissipate into space.

And generally, they're not that uncommon.

Astronomers see about 10 in the Milky Way every year, and at least 25 in Andromeda.

But 12a was still special, because it wasn't just a regular, run-of-the-mill nova.

It was a recurrent nova.

In other words, after it blasted away all that gas,

it kept stealing hydrogen from its companion star

until it went nova again, and again, and again.

Once a year, every year, practically right on cue.

This wasn't the first recurrent nova scientists had ever seen,

but it does seem to be the one that explodes the most often,

since most objects like this only go off about once every decade or few decades.

And this behavior doesn't seem to be new, either.

In fact, based on how big the shell of material floating around 12a is,

scientists think this nova has been regularly erupting for millions of years. As for why?

Well, scientists think it happens because 12a's white dwarf is likely massive.

With more mass, it has stronger gravity,

so it can pull hydrogen from its companion more rapidly,

and begin thermonuclear runaway in less time.

This recurrent nova is cool partly for that weird factor,

like, a star that partially explodes every year? Yes please!

But it might also have something important to teach astronomers.

12a is so massive and is collecting so much hydrogen that, eventually,

scientists believe it will reach what's called the Chandrasekhar Limit.

That's the maximum mass a white dwarf can be and still be stable,

so after that point, it will transform into some other kind of object.

There are a couple of ways this could happen,

but researchers think that it's very likely to explode as a Type Ia supernova.

These supernovas are famous for their consistency.

Basically every Ia that goes off is known to have the same absolute brightness,

and this is so well-documented that these supernovas

are part of a group nicknamed standard candles.

Scientists use them all the time to calculate how far away distant galaxies are,

since their actual brightness goes down the farther away they are.

This is incredibly useful, because it helps us measure

the structure and expansion of the universe. You know, big questions!

But here's the catch: We still don't exactly know how type Ia supernovas happen.

Our models can't quite explain it.

So assuming 12a will go supernova someday, we have a unique opportunity

to study a system that's potentially on the brink of destruction.

Seeing how it evolves could give astronomers precious information about standard candle supernovas,

and by extension, our tools for measuring the universe.

The only problem is that some researchers think

it'll take another 40,000 years before 12a goes supernova.

So we might have a little while to wait!

But in the meantime, our new understanding of recurrent novas

and the giant remnant shells that they leave behind will help us find more of these things.

And maybe we'll even catch one in the act of going supernova.

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