How the Universe Works - S09E05 - Curse of the White Dwarf
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00:00White dwarfs, small stars that pack a big punch.
00:06When white dwarfs were first discovered, astronomers' reaction was,
00:11no, no, no, no, no, no, no, that can't be real.
00:15What's going on inside these things can only be described as seriously weird.
00:19They're the cooling corpses of stars like our sun.
00:23But 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?
00:59December, 2018
01:03Astronomers spot strange flares coming from a galaxy 250 million light-years from Earth.
01:10GSN 069
01:14We know that GSN 069 has a supermassive black hole in its center,
01:18equal to about half a million times the mass of the sun.
01:22That's a big black hole.
01:24And it blasts out x-rays in a very, very steady pace every nine hours.
01:32Why?
01:34The flares are so energetic and regular,
01:37the supermassive black hole must be eating the mass of the planet Mercury three times a day.
01:43The big question is, what's feeding this black hole such a huge dinner?
01:47In March 2020,
01:49scientists found the answer.
01:52An unlucky star at the end of its life had wandered into the death zone of the black hole.
01:58A star getting too close to a supermassive black hole is like a glazed donut getting too close to me.
02:04That thing just is not going to make it.
02:08Stars that get too close to a black hole get torn apart.
02:11They sort of get destroyed.
02:14Stars that get too close to a black hole get torn apart.
02:18They sort of get attacked by the black hole,
02:20and some of that material is also getting launched off in very powerful winds and jets and streams getting out.
02:29Somehow, the star survives its close encounter with the supermassive black hole.
02:34Further 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.
02:54That's causing nuclear fusion, and 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.
03:14The 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.
03:42Because there's more energy flowing through the envelope, the envelope swells up.
03:48The star expands to around 100 times its original size.
03:53The yellow star becomes a red giant.
03:57Eventually, red giants shed their outer layers,
04:01forming stunning gas shells called planetary nebulas.
04:08Planetary nebulae are the most beautiful objects in space. They're all spectacular.
04:14A star that ends its life in one of these planetary nebulas leaves behind a white dwarf at the center.
04:20And 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,
04:55leaving 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.
06:03And 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.
06:14But the white dwarf is very dense, so it's very tough and it's able to hang in there for quite a long time.
06:21It's going to stay in orbit around a supermassive black hole for billions of years.
06:25Talk about David and Goliath.
06:28When 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?
06:40Quantum mechanics, the science of atomic and subatomic particles, has the answer.
06:46We're used to the rules of physics up here in the macroscopic world.
06:51But 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.
07:23These degenerate electrons stop white dwarfs from collapsing.
07:28But they give these stars strange qualities.
07:32White 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.
07:55The 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,
08:09if you climbed to the top of that skyscraper, you'd be outside of the white dwarf's atmosphere.
08:14You'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.
08:31A 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.
09:00White dwarfs gradually give off their remaining energy until 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, longer than the age of the universe to reach that point.
09:21This is the dark destiny of most mid-sized stars, including our sun.
09:27This long, slow death may make white dwarfs seem ordinary.
09:32But these tiny stars could answer some big questions about our universe.
09:38They might be small and they might be dim, but they are essential for our understanding of physics.
09:46New research into white dwarfs may answer one of the biggest questions of all.
09:50Can life survive the death of its star?
10:01In the past, we've underestimated white dwarfs.
10:05But now, they're causing a buzz among astronomers.
10:09One of the big questions over the last decade is, could a planet survive around a white dwarf?
10:16The logical answer would be no.
10:18On their way to becoming white dwarfs, stars evolve through a red giant phase.
10:26They expand to become very huge.
10:30So we figured any planets around these stars might just get eaten.
10:35In December of 2019, evidence from the constellation of Cancer turned that idea on its head.
10:47Astronomers spotted a strange-looking white dwarf about 1,500 light-years from Earth.
10:56Subtle variations in light from the star revealed a mystery.
11:00The elements oxygen and sulfur in amounts never before seen on the surface of a white dwarf.
11:08We know what the chemical signature of a white dwarf is, and this stuck out like a sore thumb.
11:13Normally, hydrogen and helium make up the outer layers of a white dwarf.
11:18Oxygen and sulfur are heavier than hydrogen and helium, and they should have sunk down, but we still see them there.
11:23So they must have gotten there recently.
11:26Using ESO's Very Large Telescope in Chile, astronomers took a closer look.
11:33They discovered a small, Earth-sized white dwarf, surrounded by a huge gas disk roughly ten times the width of the Sun.
11:41The disk contained hydrogen, oxygen, and sulfur.
11:45A system like this had never been seen before.
11:48And so the next step was to look at a profile of these elements and figure out where we'd seen something similar.
11:56And the amazing thing is, we have.
11:59We've seen these elements in the deeper layers of the ice giants of our solar system, Uranus and Neptune.
12:08Hidden in the gas ring is a giant, Neptune-like icy planet.
12:13It's twice as large as the star.
12:16But the fierce 50,000-degree heat from the white dwarf is slowly evaporating this orbiting planet.
12:22The white dwarf is bombarding the planet with high-energy radiation, X-rays, UV rays.
12:28It's pulverizing the ice molecules in its atmosphere and blowing them out into space.
12:33And the ice molecules are streaming behind the planet like the tail of a comet.
12:38The icy planet loses mass at a rate of over 500,000 tons per second.
12:44That's the equivalent of 300 aircraft carriers every minute.
12:48It sounds like that could be curtains for the planet.
12:51But remember, the planet is large, and the star is cooling down.
12:55As it cools, it will stop blasting the planet so intently, and that stream of gas will cease.
13:01The planet will probably end up losing only a few percent of its total mass.
13:05So the planet should survive and continue orbiting the white dwarf.
13:11But a mystery remains.
13:12Why didn't the closely orbiting planet die when the star swelled to a red giant?
13:19It had to have started farther out and moved inwards.
13:23Our best guess is that other ice giants were probably lurking somewhere in the outer regions of the system
13:30and knocked that planet inwards towards the white dwarf sometime after the red giant phase,
13:36in some kind of cosmic pool game, if you will.
13:38This isn't the only white dwarf with evidence of planets.
13:42About 570 light years from Earth, there's a white dwarf star called WD 1145 plus 017.
13:52After studying the star for five years,
13:55researchers report that the white dwarf is ripping apart and eating a mini rocky planet.
14:01So as the planet is becoming smaller and smaller,
14:03we see this huge cloud of dust blocking out 50% of the light of the star
14:07and huge chunks of rock passing in front of the star.
14:10It's exciting to see this planet being torn apart
14:14because it's not often that we get to see an event.
14:18We get to see something in the process that we can observe and we can learn from.
14:25There's more and more evidence that planetary systems have evolved
14:29There's more and more evidence that planetary systems can survive the death of their star
14:34and the formation of a white dwarf.
14:37It just depends on the planet's composition and location.
14:42The distance from the planet to the star is a critical factor
14:46because as you move farther and farther out from a star,
14:50the intensity of that solar radiation decreases.
14:53So the farther you go out, the less heat you have,
14:57the less high energy particles are reaching the surface of that planet.
15:01Also, rocky planets can survive better than gas giants
15:05because rocky planets can hold onto their stuff better,
15:08whereas gas can be blown away much more easily.
15:11These new discoveries raise questions about habitability around stars.
15:17Could white dwarf systems support life?
15:20If we limit ourselves to only looking for life on planets orbiting stars like our sun,
15:25we would be doing ourselves a huge disservice.
15:28Far more important is to look for, around whatever star,
15:32the habitable zone, the Goldilocks zone,
15:36the region around a star where a planet could support life.
15:41When it comes to supporting life, white dwarfs have some surprising advantages.
15:46Even though there's no fusion happening, they have all of this internal energy stored up
15:50that they release that warms the nearby planets.
15:53Life might even prefer hanging out around a white dwarf
15:58because it doesn't change much over the course of billions of years.
16:03With something like our sun, there are flares and coronal mass ejections
16:07and eventually it's going to die and we have to deal with that.
16:11It's not a problem with a white dwarf.
16:13So if life can gain a foothold, it has a nice stable home.
16:20We now think 25 to 50 percent of white dwarfs have planetary systems.
16:26Perhaps one day we'll find one with an Earth-like planet, and maybe even life.
16:35White dwarfs are the dead remains of stars like the sun.
16:39Most of these zombie stars slowly cool down over billions of years.
16:46Most, but not all.
16:51Some go out in a spectacular explosion known as a Type Ia supernova.
16:57A Type Ia supernova is one of the most violent, powerful, energetic events in the universe.
17:03We are talking about a star exploding.
17:06They can outshine entire galaxies.
17:09They can create devastation over hundreds and hundreds of light years.
17:13They're a big deal.
17:17We'd seen the aftermath of these cosmic fireworks.
17:20But for over 60 years, we had little direct evidence they came from white dwarfs.
17:26Then, students and researchers came up with an idea.
17:30Then, students from University College London, UK, got lucky.
17:36While taking routine photographs, they spotted a supernova explosion in our own cosmic neighborhood.
17:44M82, the cigar galaxy, is actually really close to us on cosmic terms.
17:49It's only about 12 million light years away.
17:52This makes it one of the closest galaxies in the sky.
17:54The blast called Supernova 2014-J was the closest Type Ia supernova for over 20 years.
18:02Its proximity allowed us to look for the signature of a white dwarf supernova,
18:07a blast of gamma rays.
18:10Gamma rays are a type of light that's incredibly energetic.
18:15They're the most energetic type of rays or photons on the electromagnetic spectrum.
18:19White dwarfs should release gamma rays when they explode.
18:23But dust in interstellar space soaks up the rays.
18:27So, unless an explosion is close by, they're hard to detect.
18:32For years, astronomers had been looking for the gamma rays that should be emitted by a Type Ia supernova.
18:38But no one had found them.
18:42Now, scientists had their chance, and they were able to.
18:46Now, scientists had their chance and the technology to see the elusive rays.
18:53Using ESA's Integral Satellite, they sifted through the shockwaves sent out by the explosion in M82.
19:00It was tough, but finally, they got a reading.
19:03The telltale signal of gamma rays.
19:06It's the best evidence yet for white dwarfs exploding in Type Ia supernovas.
19:12The reason Supernova 2014J was so cool is that this observation gave scientists evidence.
19:19It's white dwarfs that explode to create the specific type of supernova.
19:23So, which white dwarfs fade out, and which ones go out with a bang?
19:31A survey of stars revealed around 30% of white dwarfs live in binary systems.
19:38But white dwarfs are not good neighbors.
19:40A white dwarf in a binary system, it's like a zombie.
19:44It'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.
19:51They very literally suck the material and suck the life out of that star by swallowing up all of its outer layers.
19:59The white dwarf's zombie tendencies can backfire.
20:03Adding mass to a white dwarf is like this.
20:06We keep adding mass from that companion star.
20:11A little bit of hydrogen at a time, building up that atmosphere.
20:17And for a long time, everything's fine.
20:21Until you add too much mass and you reach that critical threshold.
20:30The real world consequences of reaching the threshold are devastating.
20:34The extra weight of gas stolen from the companion star compresses carbon deep in the core of the white dwarf.
20:43When the white dwarf reaches 1.4 times the mass of our sun, it hits a tipping point known as the Chandrasekhar limit.
20:52You add up the mass little by little by little until you get to that Chandrasekhar limit and then, blam, there's a supernova.
20:57In a flash, carbon undergoes nuclear fusion, releasing a tremendous amount of energy.
21:05If the white dwarf explodes at the Chandrasekhar limit, it's a little bit like fireworks that all have the same amount of gunpowder.
21:13They'll all go off in the same way. They'll be equally loud.
21:17Well, the supernovas will be equally bright.
21:19This equal brightness of all Type Ia supernovas is vital to our understanding of space.
21:25Type Ia's are known as standard candles and are useful tools for calculating vast cosmic distances.
21:33They were the key to the Nobel Prize winning discovery that the expansion of our universe is accelerating.
21:40But what kind of companion star triggers Type Ia?
21:44For decades, the number one suspect was red giant stars.
21:49A red giant's a good candidate because it's a very big, puffy star.
21:54That material becomes easy pickings for the white dwarf to siphon off until it gets big enough to explode.
22:01To prove the theory, we needed to find evidence in the debris left behind after a supernova.
22:06Stars are surprisingly hardy objects. They can survive an explosion of a nearby star.
22:12Some of these companion stars should still be there.
22:15A lot of them will be, you know, worse for their wear, but they'll still exist.
22:19Scientists searched through the remains of 70 Type Ia supernovas.
22:25Only one blast zone.
22:28The fact that we've only found maybe this one example suggests that actually,
22:33they're not quite the serial killers we thought.
22:36It's probably likely that this is the minority of these types of supernova explosions.
22:42Indeed, we now think that only a small fraction of these types of supernova explosions
22:49were actually caused by a red giant.
22:51If red giants don't cause the majority of Type Ia supernovas, what does?
22:56New evidence suggests colliding white dwarfs.
23:00Star mergers that could exceed the Chandrasekhar Red Giant.
23:05But what does that mean?
23:08It means that there's a possibility that the supernova explosion was caused by a red giant.
23:13But what does that mean?
23:16It means that there's a possibility that the supernova explosion was caused by a red giant.
23:21That's our limit. Producing explosions with different brightness.
23:26But if the explosions vary in brightness, can they still be used as standard candles?
23:32If we don't really know what a Type Ia supernova is,
23:36then when we use them to map out the universe and the way the universe is expanding,
23:40we just can't be sure any longer what it is we're looking at.
23:44If we're wrong about that, then we're wrong about so many other things
23:48that our whole model of the universe falls apart.
23:51Is our understanding of the cosmos completely wrong?
24:01White dwarfs explode in spectacular Type Ia supernovas.
24:06They're a crucial tool for measuring the universe.
24:10But there is a problem.
24:13The standard model says that white dwarfs gradually steal mass from a red giant star
24:18until they reach a tipping point called the Chandrasekhar limit.
24:25But recent observations prove this doesn't explain how most Type Ia supernovas occur.
24:32The majority of Type Ia explosions remain a mystery.
24:37We call the explosions from white dwarfs standard candles, but they're really not that standard.
24:41We actually think there's different types of explosions.
24:44It may be imperative to our understanding of the entire universe that we really get this straight.
24:49Because the reason we think the expansion rate of the universe is accelerating,
24:53it's based on the brightness of Type Ia supernovas all being the same.
24:57And maybe that's not the case.
24:59Researchers suspect that a theoretical type of merger could be responsible for more Type Ia supernovas,
25:06the result of two white dwarfs crashing together.
25:10But this messes with the math.
25:13The Chandrasekhar limit says white dwarfs should explode when they reach 1.4 times the mass of our Sun.
25:21Two white dwarfs colliding can exceed this mass,
25:25and more mass means a bigger bang and a brighter explosion.
25:32You're not adding gas little by little, you're adding a whole other white dwarf.
25:36That will go off, it will look like a Type I supernova,
25:38but it won't be the standard candle, it'll be brighter than we expect.
25:42But no white dwarf mergers have been found,
25:46because detecting one after it happens is virtually impossible.
25:51If two white dwarfs merge together, it's almost impossible to tell
25:55because the DNA of the two systems is all mixed together and it's all identical.
26:00You can't tell that there was a separate companion in the first place.
26:04So we can't just look at when there's a bright flash,
26:06we have to go look for the ticking time bombs in the galaxy.
26:11Astronomers investigating a strange-shaped cloud of gas made a breakthrough.
26:17Using ESO's Very Large Telescope,
26:21they focused in on a planetary nebula called Henyes 2428.
26:27Planetary nebulas are normally symmetric,
26:30because red giants shed their outer layers evenly as they become white dwarfs.
26:35But this one is lopsided.
26:38We think in this case there might be the presence of a companion star
26:43that shapes and twists and sculpts that planetary nebula.
26:49Researchers peeled back the gaseous layers and discovered something shocking.
26:55A two-star system made up of the most massive orbiting white dwarf pair ever discovered.
27:01Each star is 90% as massive as our sun.
27:05And they're so close together, they take just four hours to orbit each other.
27:10And they're getting closer.
27:13If you've ever seen a car crash about to happen,
27:17you know that sense of inevitability as you witness that.
27:22That's what we're seeing in this system.
27:25We see these two massive white dwarfs spiraling closer and closer.
27:29And we know that disaster is coming.
27:33In around 700 million years, these stars will merge and explode in a Type Ia supernova.
27:44Now, thanks to the discovery of more systems like Henyes 2428,
27:49we think white dwarf collisions could be responsible for the majority of Type Ia supernovas.
28:00Two white dwarfs can merge together.
28:03And if the sum of their masses is greater than 1.4 solar masses,
28:07then you can get a superchandra Type Ia.
28:10We've now observed nine superchandra explosions.
28:15And to complicate matters further, we've spotted another form of white dwarf supernovas,
28:21subchandra Type Ia's.
28:24These mysterious white dwarfs that we don't quite understand
28:27die off much quicker than regular white dwarf supernovas.
28:32The explosions are less violent than normal Type Ia supernovas and fade away faster.
28:38But we don't know why.
28:42Maybe it has something to do with the properties of the star or the rotation.
28:46But the Chandrasekhar limit may not be so exact.
28:49It's kind of a Chandrasekhar range.
28:52The physics textbooks are now being sort of rewritten or at least modified
28:55because we know that not all Type Ia supernovas come from Chandramass white dwarfs.
29:02There's actually a variety of Type Ia supernovas,
29:06a variety of white dwarf masses and configurations that can explode.
29:13These new discoveries mean researchers now study the chemistry and duration of Type Ia supernovas,
29:20not just their brightness.
29:25But mysteries remain.
29:27We still don't know what triggers the actual explosion.
29:31Something has to set off a supernova.
29:34It's like a sort of tinderbox.
29:36This star is all ready to be set alight,
29:38but something has actually got to set it alight in the first place.
29:42And the question is, what exactly is doing that?
29:48Our best hope of understanding white dwarf supernovas is to run computer simulations,
29:52experimenting with ways to trigger fusion in the heart of the star.
29:57Turns out you need a spark to get that fusion going.
30:00Otherwise, the usual failure in computer models of supernovae is,
30:04oop, never saw anything.
30:07We do not understand what creates the explosion.
30:11We don't know what the mechanism for that is yet.
30:14While computer simulations are getting closer,
30:16we still don't fully understand Type Ia supernovas.
30:22The deeper we investigate, the more mysteries we uncover.
30:26Like rogue white dwarfs streaking across the galaxy,
30:30and tiny stars that explode over and over again.
30:35Can these odd white dwarfs shed more light on the mystery of Type Ia supernovas?
30:44White dwarfs are surprisingly difficult to understand.
30:47They behave in completely unexpected ways.
30:53But these oddballs may help answer the remaining questions about Type Ia supernovas.
30:59These are white dwarfs, but not as we know them.
31:032017.
31:05Astronomers spot a rebellious star raising hell in the Little Dipper constellation.
31:11It's like a zombie, but this isn't one shambling down the road.
31:14It runs like Usain Bolt.
31:16This thing is screaming through the galaxy at a much higher speed than you'd expect for a star like it.
31:23The white dwarf called LP40365 is moving incredibly fast towards the edge of the Milky Way.
31:31It's not the only star behaving oddly.
31:34In 2019, we spotted three more white dwarfs racing across the galaxy.
31:39Finding one white dwarf blasting its way through space is weird enough,
31:43but to find three more, that's telling you that something is going on,
31:47and whatever it is that's going on happens a lot.
31:50So what sent these renegades racing across the galaxy?
31:55LP40365 and these other weird white dwarfs could be the results of failed supernovas.
32:02People have theorized that maybe these things didn't finish exploding,
32:05and if so, we should find some unburnt fractions wandering around the galaxy.
32:11In the last 20 years, we've spotted some unusually dim supernovas
32:16that could have sent LP40365 and friends flying.
32:22So what looks like happened is that in a binary pair,
32:26there was stuff dumping onto a white dwarf, and we were about to have a type 1 supernova.
32:30But the type 1 supernova didn't go off.
32:32Some of it actually exploded, and some of it didn't.
32:35That energy didn't go out in all directions.
32:39And one of the things that occurred is that these stars got sent hurling across space at these incredible speeds.
32:47Stars could be sent flying so fast that they're no longer bound by the gravitational pull of their home galaxy.
32:54LP40365 is known as a hypervelocity star,
32:59and it's moving so fast, it's definitely headed out of the Milky Way.
33:07We call them type 1AX supernovas.
33:11They could make up between 10 and 30 percent of type 1A supernovas.
33:15Many could throw out a runaway star.
33:18But we still don't know why the supernova fails.
33:22A funny thing about science is, things that fail still teach you what's going on.
33:28Why are these ones different?
33:30Were they not massive enough?
33:32Were they too massive?
33:34Was the companion star not feeding them the material the right way?
33:37Something happened there to make the supernova fail.
33:40Was the companion star not feeding them the material the right way?
33:43Something happened there to make these stars not basically blow themselves to bits.
33:49And that's telling us something about the way type 1As do explode.
33:55It seems that life in a binary star system can be rough for white dwarfs.
34:00But for some lucky stars, their lives can be more… mellow.
34:05Just because a white dwarf has a normal star companion that it's stealing material from,
34:11does not spell a death sentence for that white dwarf.
34:15February 2013.
34:17Astronomers discover a star in the Andromeda galaxy that flashes over and over and over again.
34:25With each flare, it shines a million times brighter than our sun,
34:30before dimming to its normal state.
34:32It's called M31n 2018-12a.
34:40This is not a supernova.
34:42It's its little sibling, a nova.
34:45But what's weird about this one is that it happens every year.
34:49Astronomers have known for a long time that there are these cases of these nova
34:54that go off, you know, somewhat regularly, every 10 years, every 100 years.
34:58But finding one that goes off every year is a remarkable discovery.
35:04Much like supernovas, novas occur in a close binary system
35:08where a white dwarf and another star orbit each other.
35:13The white dwarf pulls in hydrogen from the companion star.
35:17The gas falls onto its surface.
35:20And so as that hydrogen piles up, eventually it gets to the point where
35:24it can fuse into helium and goes bang.
35:29In supernovas, fusion happens deep inside the star's core.
35:35But in novas, fusion only occurs on the surface.
35:39An explosion flares across the white dwarf's exterior,
35:43hurling unburned hydrogen out into space.
35:47The result? An object called a remnant.
35:50The remnant from Nova M31n is 400 light-years wide.
35:55This particular remnant is much bigger than even supernova remnants.
36:00It's much larger, much denser and brighter than most normal remnants are.
36:04But that makes sense if this star flares up so often.
36:07Think about this star flaring away for millions of years.
36:11You build up a gigantic nova remnant.
36:15The repeating flares explain the huge size of the remnant.
36:18But why does the nova explode so frequently?
36:21Classically, we thought that when a nova went off on the surface of a white dwarf star,
36:28that the white dwarf star's mass didn't change very much.
36:32Or maybe it got a little smaller.
36:34Now we think that after a nova, the white dwarf gains a bit of mass.
36:41Recurrent novas, like M31n, steal more mass from their companion star.
36:46They steal more mass from their companion star than they blow off in each explosion.
36:51Some gain more and more mass, exploding more frequently,
36:55until they reach the Chandrasekhar limit and go full-on supernova.
37:01M31n may very well be the missing link that shows us
37:06that some nova systems eventually become supernova systems.
37:11Working out how novas become supernovas, and why some supernovas fail,
37:18might help us understand what makes white dwarfs explode.
37:24But just when we think we get a break, white dwarfs hit us with another bombshell.
37:30Death rays.
37:40White dwarfs can explode in violent supernovas.
37:46But that's not their only deadly trick.
37:49They might also create the most magnetic and terrifying beast in the universe.
37:55A magnetar.
37:58Magnetars are scary. They just are.
38:01I mean, it's even in the name. The word magnetar sounds scary.
38:05They're the reigning champion of the largest magnetic field in the universe.
38:11The magnetic fields around magnetars are so strong
38:16that they can stretch and distort individual atoms.
38:20They can turn an atom into a long, thin pencil shape.
38:24Once you start stretching atoms out into this shape,
38:27they can't bond together in the usual ways anymore,
38:31and so you can just throw out every chemistry textbook in the world.
38:35If an astronaut were unlucky enough to get close to a magnetar,
38:38say within 600, 700 miles,
38:41the whole body of the astronaut would be completely obliterated.
38:44They would more or less dissolve.
38:46The origin of these fearsome creatures is a mystery.
38:50But it must be something very violent.
38:53We think they send out a clue as they form.
38:56Powerful blasts of energy shooting across the cosmos.
39:00In the past few decades, we've noticed these very odd,
39:03very confusing and very brief flashes of intense radio energy.
39:10They're known as fast radio bursts, or FRBs.
39:14Some FRBs don't repeat. They're one and done.
39:18So you're talking about an incredible amount of energy released in less than a second,
39:22then it's over.
39:24Because these non-repeating FRBs are so powerful,
39:27we think they could come from a huge collision.
39:30The heavier and denser the objects colliding,
39:34the bigger the bang.
39:37New research suggests a white dwarf star hitting a dense, heavy neutron star
39:43could be enough to birth a magnetar,
39:46sending out FRBs in the process.
39:49A neutron star is like a white dwarf even more so.
39:55It is the leftover core of a giant star.
39:58They're effectively giant balls of neutrons squeezed together into things about the size of a city.
40:05You have a neutron star, an incredibly nasty, complicated, exotic object,
40:10and a white dwarf, an incredibly nasty, ugly, complicated object,
40:14crashing headlong into each other.
40:17As the two stars orbit more closely,
40:20the neutron star strips gas from the white dwarf.
40:24This material spirals onto the neutron star
40:28causing it to spin faster and faster.
40:33The rapid rotation amplifies its magnetic fields
40:38until the two stars collide,
40:41creating a very magnetic monster, a magnetar.
40:46It's a turbulent situation.
40:49You could think of it as a newborn baby coming into the world, kicking and screaming.
40:53The turbulence produces a powerful blast of electromagnetic radiation
40:59It races out of the collision site at the speed of light
41:04until we detect it as a fast radio burst.
41:09We can hear the screams of agony from millions of light years away,
41:14and those screams are the fast radio bursts.
41:18This could be the most difficult childbirth in the cosmos.
41:21Few suspected that white dwarfs could create something as violent as a magnetar.
41:32But in 2015, astronomers found yet another strange magnetic white dwarf
41:38in a binary system called AR Scorpion.
41:42We don't really know why white dwarfs become magnetic,
41:46but what we do know is that the ones that tend to be the most magnetic
41:49are often the heaviest.
41:52One possibility is there's a big star that has come to the end of its life
41:57and shrunk down into a white dwarf.
42:00And if that star had its own magnetic field,
42:03as it shrinks down it actually has a kind of concentrating effect.
42:07And the magnetic field gets even stronger as that happens,
42:11so that it's incredibly strong.
42:14As well as being very magnetic,
42:16AR Scorpio's white dwarf is spinning very fast.
42:20When you combine a fast spin with a strong magnetic field,
42:25what you get is some really crazy physics.
42:29The white dwarf starts to act a bit like a lighthouse,
42:33sending out intense radiation into the cosmos.
42:37As the white dwarf spins,
42:40this beam of radiation hits its red dwarf neighbor, making it glow.
42:43This is highly unusual.
42:46No other system glows like this.
42:50White dwarfs like this can help reveal some of the mysteries of magnetism.
42:55When you have a magnetic field that is this strong,
42:59it's something like 100,000 times stronger than any magnetic field
43:03that we can create here on Earth.
43:06It means that by studying what's going on in this very remote space,
43:09it means that by studying what's going on in this very remote system,
43:14you're actually learning something about physics
43:17that you could never learn here on Earth.
43:20White dwarfs are emerging from out of the shadows
43:23and taking their rightful place
43:26as one of the most fascinating objects in the universe.
43:29When we first observed white dwarfs, they were weird,
43:32they were curious, but just like a sideshow.
43:35But now white dwarfs are showing us what they're truly capable of.
43:39White dwarfs can sort of be seen as these underdogs of the universe,
43:43but it's really become an exciting and cutting-edge area of research.
43:48Now we think these objects may have a lot of exciting science to deliver.
43:52Things like, will the universe expand forever?
43:55What is the ultimate fate of the universe?
43:57All of that may be waiting for us inside a white dwarf.
44:01Discount these things at your own risk,
44:04because honestly, they're one of the driving forces in the universe.
44:06Just because it's little, don't mean it ain't bad.
44:09Don't underestimate a white dwarf.