White dwarfs are small, dead stars with a dangerous side, and they are capable of exploding into supernovas and destroying planets; new research suggests these mysterious phenomena may be the key to understanding the universe itself.
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LearningTranscript
00:01White dwarfs. Small stars that pack a big punch.
00:07When white dwarfs were first discovered, astronomers' reaction was,
00:12no, no, no, no, no, no, no. That can't be real.
00:16What's going on inside these things can only be described as seriously weird.
00:20They're the cooling corpses of stars like our sun.
00:24But new research proves white dwarfs are one of the driving forces of our universe.
00:29They eat planets. They flare out in high energy light.
00:33They can really explode.
00:36And they can tell us literally about the nature of the universe itself.
00:40And there's a dirty secret at the heart of white dwarf science.
00:44We see dead stars exploding and we still don't understand why they're doing it.
00:48Have scientists finally discovered how these small stars could be such massive galactic players?
00:59We know that GSN 069 has a supermassive black hole in its center equal to about half a million times the mass of the sun.
01:12That's a big black hole. And it blasts out x-rays in a very, very steady pace every nine hours. Why?
01:25The flares are so energetic and regular, the supermassive black hole must be eating the mass of the planet Mercury three times a day.
01:44The big question is, what's feeding this black hole such a huge dinner?
01:51In March 2020, scientists found the answer.
01:58An unlucky star at the end of its life had wandered into the death zone of the black hole.
02:04A star getting too close to a supermassive black hole is like a glazed donut getting too close to me.
02:11That thing just is not going to make it.
02:14Stars that get too close to a black hole get torn apart.
02:18They sort of get attacked by the black hole.
02:21And some of that material is also getting launched off in very powerful winds and jets and streams getting out.
02:26Somehow, the star survives its close encounter with the supermassive black hole.
02:35Further investigation reveals it's a small, compact star, a white dwarf.
02:42So what makes this tiny star almost indestructible?
02:46The answer lies in how it's formed.
02:49We get a clue if we look at the life cycle of a star.
02:52It's burning hydrogen into helium that's causing nuclear fusion.
02:57And that causes a star to stay stable.
03:00There's this delicate balance between radiation pressure from that nuclear fusion pushing out
03:06and gravitational pressure pulling in.
03:09But when stars like our sun near the end of their life, they run out of hydrogen fuel.
03:15The sun-like star makes more and more helium, which builds up in its center.
03:20Gradually, the immense weight of the star's outer layers crushes the helium core.
03:27As the core ages, it gets smaller and hotter, which increases the rate of nuclear reactions.
03:34These nuclear fusion reactions produce more energy, which pushes the outer layer, or envelope, outwards.
03:41Because there's more energy flowing through the envelope, the envelope swells up.
03:47The star expands to around 100 times its original size.
03:52The yellow star becomes a red giant.
03:56Eventually, red giants shed their outer layers, forming stunning gas shells called planetary nebulas.
04:06Planetary nebulae are the most beautiful objects in space.
04:12They're all spectacular.
04:15A star that ends its life in one of these planetary nebulas leaves behind a white dwarf at the center.
04:21And this white dwarf is essentially a cinder, a stellar cinder.
04:25It's what's left after nuclear fusion is no longer possible for that particular star.
04:31All that remains, a glowing white dwarf, the leftover core of the dead star.
04:38But in galaxy GSN 069, the supermassive black hole turbocharged the process.
04:46It stripped off the outer layers of the red giant in a matter of days.
04:50The black hole has almost eaten all the juicy parts, all the easy to get at parts of the star.
04:56Leaving behind the sort of bone or the leftovers of the white dwarf.
05:01This white dwarf is just a fifth of the mass of the sun.
05:06How can such a small star survive being so close to a black hole?
05:11You might think that because a white dwarf is small, it's not going to last very long because there's not that much stuff there to eat.
05:18But it turns out it's quite the opposite.
05:22The pocket-sized white dwarf is packed full of matter.
05:26If it were a normal star, it would have been shredded long ago.
05:29But because it's such a dense, tight ball of matter, it survives.
05:35Imagine taking the sun and crushing it down to just about the size of the Earth.
05:40Same mass, but now packed way more tightly.
05:44So a basketball worth of this stuff would weigh as much as 35 blue whales.
05:51The white dwarf's extreme density protects it from the gravitational onslaught of the supermassive black hole.
05:59Its orbit takes it near that black hole every nine hours.
06:04And every time it encounters the black hole, some of its material gets sipped off.
06:09They're playing a game of interstellar tug-of-war with one another.
06:12The black hole is bigger, so it's going to win.
06:15But the white dwarf is very dense, so it's very tough, and it's able to hang in there for quite a long time.
06:20It's going to stay in orbit around a supermassive black hole for billions of years.
06:26Talk about David and Goliath.
06:29When astronomers first discovered white dwarfs, they thought they shouldn't exist.
06:34How could something have such an extreme density and not collapse under its own weight?
06:41Quantum mechanics, the science of atomic and subatomic particles, has the answer.
06:47We're used to the rules of physics up here in the macroscopic world.
06:52But when you zoom down into the subatomic world, things get weird.
06:57Here we have the electron, one of the tiniest particles in the universe.
07:03And it's these little electrons that are doing the work of supporting an entire star.
07:10Electrons really don't like being squashed into a small space.
07:14If you try to squash too many of them into too small a space, they'll push back really hard.
07:19And this is an effect called degeneracy pressure.
07:22These degenerate electrons stop white dwarfs from collapsing.
07:28But they give these stars strange qualities.
07:31White dwarfs behave very differently than normal matter.
07:35Take planets and stars.
07:37They become bigger when they gain mass.
07:40White dwarfs are the exact opposite.
07:42As they gain mass, they get smaller.
07:45The more massive a white dwarf, the tighter the electrons squeeze together.
07:50And the smaller and denser the star gets.
07:54The high density means the white dwarf's structure is also strange.
07:59It has an extremely thin atmosphere made of hydrogen or occasionally helium gas.
08:05If you were to take an earth skyscraper and put it on a white dwarf star,
08:10if you climb to the top of that skyscraper, you'd be outside of the white dwarf's atmosphere.
08:15You'd actually be in space.
08:17Beneath the thin atmosphere lies a surface of dense helium around 30 miles thick.
08:24It surrounds an interior made of superheated liquid carbon and oxygen.
08:30A white dwarf at its surface can be a half a million degrees.
08:34It's even hotter in the interior.
08:36And so that kind of material, it's not going to behave the way normal matter does.
08:42Eventually, over billions of years, the center of the white dwarf cools down into a solid.
08:49As the carbon and oxygen atoms cool down, they form a crystal.
08:53Diamonds are actually crystals of carbon.
08:55So at the center of these cool white dwarfs could be a diamond the size of the earth.
08:59White dwarfs gradually give off their remaining energy,
09:03until there's just a cold dead ball of matter.
09:07A black dwarf.
09:09We've never seen what we call a black dwarf.
09:11And there's a simple reason for that.
09:13It takes a tremendous amount of time, many tens of billions of years,
09:17longer than the age of the universe to reach that point.
09:19This is the dark destiny of most mid-sized stars, including our sun.
09:25This long, slow death may make white dwarfs seem ordinary.
09:31But these tiny stars could answer some big questions about our universe.
09:37They might be small and they might be dim, but they are essential for our understanding of physics.
09:45New research into white dwarfs may answer one of the biggest questions of all.
09:51Can life survive the death of its star?
10:06In the past, we've underestimated white dwarfs.
10:10But now, they're causing a buzz among astronomers.
10:14One of the big questions over the last decade is,
10:17could a planet survive around a white dwarf?
10:21The logical answer would be no.
10:23On their way to becoming white dwarfs, stars evolve through a red giant phase.
10:27They expand to become very huge.
10:36So we figured any planets around these stars might just get eaten.
10:43In December of 2019, evidence from the constellation of Cancer turned that idea on its head.
10:51Astronomers spotted a strange-looking white dwarf about 1,500 light years from Earth.
10:56Subtle variations in light from the star revealed a mystery.
11:05The elements oxygen and sulfur in amounts never before seen on the surface of a white dwarf.
11:12We know what the chemical signature of a white dwarf is, and this stuck out like a sore thumb.
11:17Normally, hydrogen and helium make up the outer layers of a white dwarf.
11:22Oxygen and sulfur are heavier than hydrogen and helium,
11:25and they should have sunk down, but we still see them there.
11:28So they must have gotten there recently.
11:31Using ESO's Very Large Telescope in Chile, astronomers took a closer look.
11:37They discovered a small Earth-sized white dwarf, surrounded by a huge gas disk roughly ten times the width of the sun.
11:46The disk contained hydrogen, oxygen, and sulfur.
11:50A system like this had never been seen before.
11:52And so the next step was to look at a profile of these elements and figure out where we'd seen something similar.
11:59And the amazing thing is, we've seen these elements in the deeper layers of the ice giants of our solar system, Uranus and Neptune.
12:13Hidden in the gas ring is a giant, Neptune-like icy planet.
12:18It's twice as large as the star, but the fierce 50,000-degree heat from the white dwarf is slowly evaporating this orbiting planet.
12:28The white dwarf is bombarding the planet with high-energy radiation, X-rays, UV rays.
12:33It's pulverizing the ice molecules in its atmosphere and blowing them out into space.
12:38And the ice molecules are streaming behind the planet like the tail of a comet.
12:43The icy planet loses mass at a rate of over 500,000 tons per second.
12:49That's the equivalent of 300 aircraft carriers every minute.
12:54It sounds like that could be curtains for the planet.
12:57But remember, the planet is large and the star is cooling down.
13:01As it cools, it will stop blasting the planet so intently and that stream of gas will cease.
13:07The planet will probably end up losing only a few percent of its total mass.
13:12So the planet should survive and continue orbiting the white dwarf.
13:17But a mystery remains.
13:19Why didn't the closely orbiting planet die when the star swelled to a red giant?
13:26It had to have started farther out and moved inwards.
13:30Our best guess is that other ice giants were probably lurking somewhere in the outer regions of the system
13:37and knocked that planet inwards towards the white dwarf sometime after the red giant phase
13:43in some kind of cosmic pool game, if you will.
13:47This isn't the only white dwarf with evidence of planets.
13:51About 570 light years from Earth, there's a white dwarf star called WD 1145 plus 017.
14:02After studying the star for five years, researchers report that the white dwarf is ripping apart and eating a mini rocky planet.
14:10So as the planet is being torn up, we see this huge cloud of dust blocking out 50% of the light of the star
14:17and huge chunks of rock passing in front of the star.
14:19It's exciting to see this planet being torn apart because it's not often that we get to see an event.
14:27We get to see something in the process that we can observe and we can learn from.
14:34There's more and more evidence that planetary systems can survive the death of their star and the formation of a white dwarf.
14:42It just depends on the planet's composition and location.
14:48The distance from the planet to the star is a critical factor because as you move farther and farther out from a star,
14:56the intensity of that solar radiation decreases.
15:00So the farther you go out, the less heat you have, the less high energy particles are reaching the surface of that planet.
15:06Also, rocky planets can survive better than gas giants because rocky planets can hold onto their stuff better,
15:13whereas gas can be blown away much more easily.
15:15These new discoveries raise questions about habitability around stars.
15:23Could white dwarf systems support life?
15:27If we limit ourselves to only looking for life on planets orbiting stars like our sun,
15:32we would be doing ourselves a huge disservice.
15:35Far more important is to look for, around whatever star, the habitable zone, the Goldilocks zone,
15:43the region around a star where a planet could support life.
15:49When it comes to supporting life, white dwarfs have some surprising advantages.
15:54Even though there's no fusion happening, they have all of this internal energy stored up
15:58that they release that warms the nearby planets.
16:02Life might even prefer hanging out around a white dwarf because it doesn't change much over the course of billions of years.
16:11With something like our sun, there are flares and coronal mass ejections and eventually it's going to die and we have to deal with that.
16:19That's not a problem with a white dwarf.
16:21So if life can gain a foothold, it has a nice stable home.
16:26We now think 25 to 50 percent of white dwarfs have planetary systems.
16:34Perhaps one day we'll find one with an Earth-like planet and maybe even life.
16:42But not all of these tough little stars are good hosts.
16:46White dwarfs have a volatile nature.
16:50They can explode in some of the biggest bangs in the cosmos.
16:53White dwarfs are the dead remains of stars like the sun.
17:11Most of these zombie stars slowly cool down over billions of years.
17:18Most, but not all.
17:22Some go out in a spectacular explosion known as a Type 1A supernova.
17:29A Type 1A supernova is one of the most violent, powerful, energetic events in the universe.
17:39We are talking about a star exploding.
17:42They can outshine entire galaxies.
17:44They can create devastation over hundreds and hundreds of light years.
17:48They're a big deal.
17:49We'd seen the aftermath of these cosmic fireworks, but for over 60 years we had little direct evidence they came from white dwarfs.
18:00Then, students from University College London, UK got lucky.
18:06While taking routine photographs, they spotted a supernova explosion in our own cosmic neighborhood.
18:13M82, the cigar galaxy, is actually really close to us on cosmic terms.
18:20It's only about 12 million light years away.
18:23This makes it one of the closest galaxies in the sky.
18:26The blast called Supernova 2014 J was the closest Type 1A supernova for over 20 years.
18:33Its proximity allowed us to look for the signature of a white dwarf supernova.
18:40A blast of gamma rays.
18:43Gamma rays are a type of light that's incredibly energetic.
18:48They're the most energetic type of rays or photons on the electromagnetic spectrum.
18:53White dwarfs should release gamma rays when they explode.
18:57But dust in interstellar space soaks up the rays.
19:02So unless an explosion is close by, they're hard to detect.
19:07For years, astronomers had been looking for the gamma rays that should be emitted by a Type 1A supernova.
19:13But no one had found them.
19:16Now, scientists had their chance, and the technology, to see the elusive rays.
19:21Using ESA's integral satellite, they sifted through the shock wave sent out by the explosion in M82.
19:29It was tough, but finally, they got a reading.
19:33The telltale signal of gamma rays.
19:36It's the best evidence yet for white dwarfs exploding in Type 1A supernovas.
19:42The reason Supernova 2014 J was so cool is that this observation gave scientists evidence.
19:48It's white dwarfs that explode to create the specific type of supernova.
19:53So which white dwarfs fade out, and which ones go out with a bang?
20:01A survey of stars revealed around 30% of white dwarfs live in binary systems.
20:08But white dwarfs are not good neighbors.
20:11A white dwarf in a binary system is like a zombie.
20:14It's the corpse of a star that used to be alive, but now it is eating the material from a star that is still alive.
20:21They very literally suck the material and suck the life out of that star by swallowing up all of its outer layers.
20:29The white dwarf's zombie tendencies can backfire.
20:32Adding mass to a white dwarf is like this.
20:37We keep adding mass from that companion star, a little bit of hydrogen at a time, building up that atmosphere.
20:48And for a long time, everything's fine until you add too much mass and you reach that critical threshold.
20:55The real-world consequences of reaching the threshold are devastating.
21:06The extra weight of gas stolen from the companion star compresses carbon deep in the core of the white dwarf.
21:13When the white dwarf reaches 1.4 times the mass of our sun, it hits a tipping point known as the Chandrasekhar limit.
21:23You add up the mass little by little by little till you get to that Chandrasekhar limit and then, blam, there's a supernova.
21:30In a flash, carbon undergoes nuclear fusion, releasing a tremendous amount of energy.
21:35If the white dwarf explodes at the Chandrasekhar limit, it's a little bit like fireworks that all have the same amount of gunpowder.
21:46They'll all go off in the same way. They'll be equally loud.
21:50Well, the supernovas will be equally bright.
21:53This equal brightness of all Type Ia supernovas is vital to our understanding of space.
21:58Type Ia's are known as standard candles and are useful tools for calculating vast cosmic distances.
22:08They were the key to the Nobel Prize winning discovery that the expansion of our universe is accelerating.
22:15But what kind of companion star triggers Type Ia supernovas?
22:20For decades, the number one suspect was red giant stars.
22:25A red giant's a good candidate because it's a very big, puffy star.
22:31That material becomes easy pickings for the white dwarf to siphon off until it gets big enough to explode.
22:38To prove the theory, we needed to find evidence in the debris left behind after a supernova.
22:44Stars are surprisingly hardy objects. They can survive an explosion of a nearby star.
22:50Some of these companion stars should still be there. A lot of them will be, you know, worse for the wear, but they'll still exist.
22:58Scientists searched through the remains of 70 Type Ia supernovas.
23:03Only one blast zone contained the glowing remains of a red giant.
23:08The fact that we've only found maybe this one example suggests that actually they're not quite the serial killers we thought.
23:17It's probably likely that this is the minority of these types of supernova explosions.
23:23Indeed, we now think that only a small fraction of these white dwarf supernovas involve a red giant.
23:30Despite the fact that in the standard textbooks for decades, that was the preferred explanation.
23:37If red giants don't cause the majority of Type Ia supernovas, what does?
23:43New evidence suggests colliding white dwarfs.
23:48Star mergers that could exceed the Chandrasekhar limit.
23:52Producing explosions with different brightness.
23:54But if the explosions vary in brightness, can they still be used as standard candles?
24:01If we don't really know what a Type Ia supernova is, then when we use them to map out the universe and the way the universe is expanding, we just can't be sure any longer what it is we're looking at.
24:13If we're wrong about that, then we're wrong about so many other things that our whole model of the universe falls apart.
24:18Is our understanding of the cosmos completely wrong?
24:23White dwarfs explode in spectacular Type Ia supernovas.
24:40They're a crucial tool for measuring the universe.
24:43But there is a problem.
24:46The standard model says that white dwarfs gradually steal mass from a red giant star.
24:54Until they reach a tipping point called the Chandrasekhar limit.
25:01But recent observations prove this doesn't explain how most Type Ia supernovas occur.
25:07The majority of Type Ia explosions remain a mystery.
25:11We call the explosions from white dwarfs standard candles, but they're really not that standard.
25:16We actually think there's different types of explosions.
25:19It may be imperative to our understanding of the entire universe that we really get this straight.
25:25Because the reason we think the expansion rate of the universe is accelerating, it's based on the brightness of Type Ia supernovas all being the same.
25:32And maybe that's not the case.
25:33Researchers suspect that a theoretical type of merger could be responsible for more Type Ia supernovas.
25:42The result of two white dwarfs crashing together.
25:46But this messes with the math.
25:49The Chandrasekhar limit says white dwarfs should explode when they reach 1.4 times the mass of our sun.
25:55Two white dwarfs colliding can exceed this mass, and more mass means a bigger bang and a brighter explosion.
26:06You're not adding gas little by little, you're adding a whole other white dwarf.
26:12That will go off. It will look like a Type Ia supernova, but it won't be the standard candle. It'll be brighter than we expect.
26:18But no white dwarf mergers have been found.
26:22Because detecting one after it happens is virtually impossible.
26:27If two white dwarfs merge together, it's almost impossible to tell because the DNA of the two systems is all mixed together and it's all identical.
26:36You can't tell that there was a separate companion in the first place.
26:40So we can't just look at when there's a bright flash, we have to go look for the ticking time bombs in the galaxy.
26:48Astronomers investigating a strange shaped cloud of gas made a breakthrough.
26:54Using ESO's Very Large Telescope, they focused in on a planetary nebula called Hennise 2-428.
27:03Planetary nebulas are normally symmetric because red giants shed their outer layers evenly as they become white dwarfs.
27:12But this one is lopsided.
27:14We think in this case there might be the presence of a companion star that shapes and twists and sculpts that planetary nebula.
27:26Researchers peeled back the gaseous layers and discovered something shocking.
27:30A two-star system made up of the most massive orbiting white dwarf pair ever discovered.
27:38Each star is 90% as massive as our sun.
27:43And they're so close together they take just four hours to orbit each other.
27:48And they're getting closer.
27:49If you've ever seen a car crash about to happen, you know that sense of inevitability as you witness that.
28:00That's what we're seeing in this system.
28:02We see these two massive white dwarfs spiraling closer and closer and closer.
28:08And we know that disaster is coming.
28:11In around 700 million years, these stars will merge and explode in a Type 1A supernova.
28:17Now, thanks to the discovery of more systems like Hany's 2428, we think white dwarf collisions could be responsible for the majority of Type 1A supernovas.
28:32Two white dwarfs can merge together, and if the sum of their masses is greater than 1.4 solar masses, then you can get a Super Chandra Type 1A.
28:46We've now observed nine Super Chandra explosions.
28:50And to complicate matters further, we've spotted another form of white dwarf supernovas, Sub Chandra Type 1As.
28:58These mysterious white dwarfs that we don't quite understand die off much quicker than regular white dwarf supernovas.
29:08The explosions are less violent than normal Type 1A supernovas and fade away faster.
29:15But we don't know why.
29:19Maybe it has something to do with the properties of the star or the rotation, but the Chandra Sekhar limit may not be so exact.
29:25It's kind of a Chandra Sekhar range.
29:28The physics textbooks are now being sort of rewritten, or at least modified, because we know that not all Type 1A supernovas come from Chandra mass white dwarfs.
29:38There's actually a variety of Type 1A supernovas, a variety of white dwarf masses and configurations that can explode.
29:47These new discoveries mean researchers now study the chemistry and duration of Type 1A supernovas, not just their brightness.
29:57The deeper we investigate, the more mysteries we uncover, like rogue white dwarfs streaking across the galaxy, and tiny stars that explode over and over again.
30:13Can these odd white dwarfs shed more light on the mystery of Type 1A supernovas?
30:20White dwarfs are surprisingly difficult to understand.
30:34They behave in completely unexpected ways.
30:41But these oddballs may help answer the remaining questions about Type 1A supernovas.
30:46These are white dwarfs, but not as we know them.
30:512017. Astronomers spot a rebellious star raising hell in the Little Dipper constellation.
30:59It's like a zombie, but this isn't one shambling down the road. It runs like Usain Bolt.
31:05This thing is screaming through the galaxy at a much higher speed than you'd expect for a star like it.
31:11The white dwarf called LP 40365 is moving incredibly fast towards the edge of the Milky Way.
31:18It's not the only star behaving oddly. In 2019, we spotted three more white dwarfs racing across the galaxy.
31:29Finding one white dwarf blasting its way through space is weird enough.
31:33But to find three more, that's telling you that something is going on and whatever it is that's going on happens a lot.
31:40So what sent these renegades racing across the galaxy?
31:43LP 40365 and these other weird white dwarfs could be the results of failed supernovas.
31:52People have theorized that maybe these things didn't finish exploding.
31:56And if so, we should find some unburnt fractions wandering around the galaxy.
32:02In the last 20 years, we've spotted some unusually dim supernovas that could have sent LP 40365 and friends flying.
32:11So what looks like happened is that in a binary pair, there was stuff dumping onto a white dwarf and we were about to have a type 1 supernova.
32:20But the type 1 supernova didn't go off symmetrically. Some of it actually exploded and some of it didn't.
32:26That energy didn't go out in all directions.
32:30And one of the things that occurred is that these stars got sent hurling across space at these incredible speeds.
32:35We call them type 1A supernovas.
32:43They could make up between 10 and 30% of type 1A supernovas.
32:48Many could throw out a runaway star.
32:52But we still don't know why the supernova fails.
32:55A funny thing about science is, things that fail still teach you what's going on.
33:02Why are these ones different? Were they not massive enough? Were they too massive?
33:06Was the companion star not feeding them the material the right way?
33:09Something happened there to make these stars not basically blow themselves to bits.
33:15And that's telling us something about the way type 1As do explode.
33:19It seems that life in a binary star system can be rough for white dwarfs.
33:27But for some lucky stars, their lives can be more mellow.
33:32Just because a white dwarf has a normal star companion that it's stealing material from,
33:38does not spell a death sentence for that white dwarf.
33:40February 2013. Astronomers discover a star in the Andromeda galaxy that flashes over and over and over again.
33:51With each flare, it shines a million times brighter than our sun, before dimming to its normal state.
33:59It's called M31N 2018-12A.
34:03This is not a supernova. It's its little sibling, a nova.
34:11But what's weird about this one is that it happens every year.
34:15Astronomers have known for a long time that there are these cases of these nova that go off, you know, somewhat regularly,
34:23every ten years, every hundred years. But finding one that goes off every year is a remarkable discovery.
34:29Much like supernovas, novas occur in a close binary system where a white dwarf and another star orbit each other.
34:38The white dwarf pulls in hydrogen from the companion star.
34:44The gas falls onto its surface.
34:47And so as that hydrogen piles up, eventually it gets to the point where it can fuse into helium and goes bang.
34:54In supernovas, fusion happens deep inside the star's core.
35:01But in novas, fusion only occurs on the surface.
35:05An explosion flares across the white dwarf's exterior, hurling unburned hydrogen out into space.
35:13The result? An object called a remnant.
35:16The remnant from Nova M31n is 400 light years wide.
35:24This particular remnant is much bigger than even supernova remnants.
35:29It's much larger, much denser and brighter than most normal remnants are.
35:32But that makes sense if this star flares up so often.
35:35Think about this star flaring away for millions of years.
35:38You build up a gigantic nova remnant.
35:43The repeating flares explain the huge size of the remnant.
35:46But why does the nova explode so frequently?
35:49Classically, we thought that when a nova went off on the surface of a white dwarf star,
35:56that the white dwarf star's mass didn't change very much.
36:00Or maybe it got a little smaller.
36:01Now we think that after a nova, the white dwarf gains a bit of mass.
36:08Recurrent novas, like M31n, steal more mass from their companion star than they blow off in each explosion.
36:17Some gain more and more mass, exploding more frequently until they reach the Chandra Sekhar limit
36:24and go full-on supernova.
36:26M31n may very well be the missing link that shows us that some nova systems eventually become supernova systems.
36:37Working out how novas become supernovas and why some supernovas fail
36:43might help us understand what makes white dwarfs explode.
36:47But just when we think we get a break, white dwarfs hit us with another bombshell.
36:56Death rays.
36:58White dwarfs can explode in violent supernovas.
37:13But that's not their only deadly trick.
37:19They might also create the most magnetic and terrifying beast in the universe.
37:25A magnetar.
37:27Magnetars are scary. They just are.
37:31I mean, it's even in the name, the word magnetar sounds scary.
37:35They're the reigning champion of the largest magnetic field in the universe.
37:38The magnetic fields around magnetars are so strong that they can stretch and distort individual atoms.
37:49They can turn an atom into a long, thin pencil shape.
37:53Once you start stretching atoms out into this shape, they can't bond together in the usual ways anymore.
37:59And so you can just throw out every chemistry textbook in the world.
38:04If an astronaut were unlucky enough to get close to a magnetar, say within 600, 700 miles,
38:10the whole body of the astronaut would be completely obliterated.
38:14They would more or less dissolve.
38:15The origin of these fearsome creatures is a mystery.
38:19But it must be something very violent.
38:22We think they send out a clue as they form.
38:25Powerful blasts of energy shooting across the cosmos.
38:29In the past few decades, we've noticed these very odd, very confusing and very brief flashes of intense radio energy.
38:39They're known as fast radio bursts, or FRBs.
38:44Some FRBs don't repeat. They're one and done.
38:48So you're talking about an incredible amount of energy released in less than a second, then it's over.
38:53Because these non-repeating FRBs are so powerful, we think they could come from a huge collision.
38:59The heavier and denser the objects colliding, the bigger the bang.
39:06New research suggests a white dwarf star hitting a dense, heavy neutron star could be enough to birth a magnetar.
39:16Sending out FRBs in the process.
39:20A neutron star is like a white dwarf, even more so.
39:24It is the leftover core of a giant star.
39:29They're effectively giant balls of neutrons squeezed together into things about the size of a city.
39:35You have a neutron star, an incredibly nasty, complicated, exotic object.
39:40And a white dwarf, an incredibly nasty, ugly, complicated object, crashing headlong into each other.
39:47As the two stars orbit more closely, the neutron star strips gas from the white dwarf.
39:54This material spirals onto the neutron star, causing it to spin faster and faster.
40:03The rapid rotation amplifies its magnetic fields until the two stars collide, creating a very magnetic monster, a magnetar.
40:17It's a turbulent situation. You can think of it as a newborn baby coming into the world, kicking and screaming.
40:22The turbulence produces a powerful blast of electromagnetic radiation.
40:27It races out of the collision site at the speed of light until we detect it as a fast radio burst.
40:37We can hear the screams of agony from millions of light years away.
40:44And those screams are the fast radio bursts.
40:48This could be the most difficult childbirth in the cosmos.
40:50Few suspected that white dwarfs could create something as violent as a magnetar.
41:04White dwarfs are emerging from out of the shadows and taking their rightful place as one of the most fascinating objects in the universe.
41:13When we first observed white dwarfs, they were weird, they were curious, but just like a sideshow.
41:20But now white dwarfs are showing us what they're truly capable of.
41:24White dwarfs can sort of be seen as these underdogs of the universe.
41:28But it's really become an exciting and cutting edge area of research.
41:31Now we think these objects may have a lot of exciting science to deliver.
41:37Things like, will the universe expand forever?
41:39What is the ultimate fate of the universe?
41:41All of that may be waiting for us inside a white dwarf.
41:45Discount these things at your own risk, because honestly, they're one of the driving forces in the universe.
41:51Just because it's little, don't mean it ain't bad. Don't underestimate a white dwarf.