I do love a good space explosion.

Supernovae were one of my first astronomical loves. Stars explode! Like, completely. And they blast off as much energy as billions of normal stars, sometimes shining as brightly as their host galaxy.

Then I learned about other types of catastrophic cosmic events. Solar flares. Gamma-ray bursts. Magnetars. Planet collisions. Galaxy collisions.

They're all amazing and cool and happily (generally) very far away. I've read about them, written about them (literally, I wrote the book about them), and even done some scientific research into them.

But then I read about an event recently … and after twenty minutes of leaning forward over my monitor scrutinizing several research papers, I fell back into my chair, eyes wide, neck hair standing up, and under my breath I muttered, "Holy &%^#%$@."

I'm talking about M31N 2008-12a. It's what's called a recurrent nova, a cyclically repeating epic blast. A nova is a seriously powerful explosion, but this one … well.

I've given an overview of how a nova works before, but I'll recap. You start with a white dwarf: a super-compressed ball of extremely hot and dense matter. These are actually the cores of stars like the Sun, once it gets up in age a bit. As it ages, the Sun will swell into a red giant, blow off its outer layers, and reveal its super-hot über-dense core. This'll have roughly half the mass of the Sun, but only be the size of the Earth. That's small.

If the white dwarf is part of a binary system, orbiting a normal star, it can siphon material off that other star. Either the second star has swollen up into a red giant itself and dumps material onto the white dwarf, or it blows a dense wind of material (like a super-solar wind) that falls on the white dwarf. Either way, material, usually mostly hydrogen, piles up.

Zoom In Artist's drawing of the RS Ophiuchi system, a symbiotic star and recurring nova, where a white dwarf is accumulating matter from a star orbiting it. Credit: David Hardy & PPARC

Mind you, the white dwarf is massive and small. That means its surface gravity is crushingly strong, as much as half a million times stronger than Earth's gravity! So the material piling up on the surface is getting hellaciously squeezed. If enough piles up, the atoms can get so compressed that they will fuse together. This can happen all over the surface of the white dwarf all at once, essentially a thermonuclear bomb that releases as much energy as 100,000 times what the Sun does!

They are so bright they can be seen at great distances, appearing like a new star in the sky. Hence the term nova, Latin for "new” (and short for the old-fashioned term "stella nova” — "new star”).

Once the material blows away, things settle down, and the process can start up again. Matter piles up, BANG, material blows off, things settle, matter piles up, lather, rinse, repeat. If it takes less than about 100 years for the event to repeat, we call this a recurring nova. Quite a few are known in our Milky Way galaxy. We see them in M31, the Andromeda Galaxy, too.

Video of White Dwarfs & Planetary Nebulae: Crash Course Astronomy #30

And now we can talk about M31N 2008-12a. The name means it's a nova ("N”) in M31, and the first one ("a”) seen in December 2008. It was also found to be a recurrent nova, but not like any ever seen before: Instead of taking centuries or even decades between blasts, M31N 2008-12a explodes every year.

Every. Year. Literally, every 340 days or so. It's a white dwarf with a red giant companion, and the wind from the giant blows material onto the dwarf more rapidly than most other systems. It piles up more quickly, getting to critical mass in less than a year. That's incredible.

But there's more. Oh yes, there's more.

It's long been theorized that there should be a big cloud of gas surrounding recurrent novae. Some novae do have small nebulae surrounding them, the expanding gas blasting away from the eruption, but in this case we're talking much bigger. A normal nova might have something a light year or three across around it — in general that's about as far as it can get before ramming into the gas between the stars slows it to a stop.

But a recurrent nova is repeatedly blasting stuff off, continuously pumping more material into the cloud around it. This will give the nebula expansion more energy, allowing it to plow up more material, and get bigger.

A nebula this big, however, has never been seen.

Zoom In The super-remnant around the recurring nova M31N-2008-12a. The outline of the remnant (left) indicates its size (the nova is offset from the center, indicated by the black line). A Hubble image shows more structure (middle), and zooming in on some the structure reveals filaments separated by just a few light years (1 parsec (pc) = 3.26 light years). Credit: Darnley et al.

Until M31N 2008-12a. Surrounding this episodically epic eructator is a bubble, a cavity carved out by previous eruptions. It's elliptical in shape, like a Tic Tac or a rugby ball, and it's slightly bigger than usual.

It's 450 x 300 light years across. At least.

Holy &%^#%$@.

That's huge. Vast. Analysis of the bubble indicates that very little of it is actually from the eruptive events; the vast majority is swept-up interstellar material. And there's a lot of it: probably several hundred thousand times the mass of the Sun.

Holy &%^#%$@!

No wonder astronomers are calling this a super-remnant.

But there's one more thing. Given the size, expansion speed, and mass of the bubble, the astronomers figure that the nova has been doing this routine for a while now. When they do the math they find that it likely has been exploding like this for the past million years.

Holy &%^#%$@!

Yeah, getting to that part of the paper is pretty much when my brain had had enough. That's violence on a scale that's nearly impossible to grasp; this nova explodes with the fury of 100,000 Suns every year and has done it for a million years. Wow.

If this object were in our own galaxy, the Milky Way, it would probably be one of the most celebrated objects in the sky! But it's in Andromeda, removed by 2.5 million light years, so faint that you need a pretty good telescope to see it at all. That's why it wasn't discovered until 2008.

And oh, there's one more thing.

The white dwarf that powers this ridiculously over-the-top event is very close to the upper limit of how big one can get. If they grow too massive, all the subatomic particles inside feel so much squeezing that they will fuse together. In some kinds of white dwarfs they'll collapse to form an even more dense neutron star. In a different kind, the elements inside the dwarf will fuse all at once, all of them, and the entire star detonates like a nuclear bomb the size of our planet. The release of energy is so profound it tears the star apart. It explodes, creating a supernova.

M31N 2008-12a is likely the later kind, and is already very near this upper limit. At the rate it's collecting material, it'll go supernova in something less than a million years. Compared to a human lifespan that's a long time, but to an astronomer, that's soon.

We won't be in any danger from it; it's far too far to hurt us. But it'll be quite a show; the star will go from being invisible to easily visible to the naked eye. And that'll put an end to its episodic nature; once it blows, it blows. It'll be gone. Nothing left except a huge cloud of debris expanding at a decent fraction of the speed of light.

And there's one more one-more-thing. The region around the white dwarf has been swept fairly clear of material, snowplowed up by the repeated explosions. In that vacuum, the debris will expand pretty freely. After a few centuries it'll slam into all that stuff previously pushed out, and when it does it'll energize the heck out of it. It'll blast out energy across the electromagnetic spectrum, and light the nebula up like a Christmas tree.

And when it does, our great-great-great-greatnth descendants will see it, and when they do, there's only one thing they can do.

Say, "Holy &%^#%$@!”