How the Universe Works - S05E05 - Stars That Kill
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00:00When the sun goes down, the monsters come out to play.
00:10Some of those stars you see are actual psychos, and they'll kill you.
00:15In the last decade, astronomers have uncovered a sinister side to our universe, killer stars
00:24with the power to destroy on a cosmic scale.
00:27We hear about death stars in movies, but they actually exist in real life.
00:32Solar systems torn to shreds, living worlds vaporized in an instant.
00:41Somewhere in the universe, dozens of worlds just like ours are being annihilated by killer
00:48stars.
00:51Scared of the dark?
00:54You should be.
01:12In June 2015, a small robotic telescope scans the night sky over Chile, South America.
01:23The All-Sky Automated Survey for Supernovas, or ASSASSIN for short, is programmed to spot
01:30the bright flashes of light that mark the death of giant stars.
01:37That night, the telescope found a faint glow in the sky that nobody had seen before.
01:44At first, astronomers think it's a nearby supernova, but when they analyze the light,
01:50they discover something extraordinary.
01:53The exploding star was unimaginably far away, nearly four billion light years.
02:00To be visible from that mind-bending distance, the flash of light had to be a record breaker,
02:07the brightest supernova in recorded history.
02:11It radiated more energy than the sun will radiate in its entire 10 billion year lifetime,
02:15but it did that in a month.
02:18This is at the absolute edge.
02:20This is the brightest we think a supernova can possibly be.
02:31Astronomers named the record-breaking blast of light ASSASSIN-15LH an appropriate name
02:38because this superluminous supernova was a mass murderer.
02:44This supernova isn't just going to destroy life on the planets that orbit that star.
02:48It's going to destroy life on millions of planets.
02:51That's millions of apocalypse events.
02:58The destructive power of 15LH had little to do with explosive force.
03:04This deadly assassin's weapon was light.
03:07To understand how brightness can cause devastation on a galactic scale, planetary scientist Nina
03:14Lanza is supersizing a familiar backyard experiment.
03:19All the light entering this giant lens is being concentrated into a point right there,
03:23which is maybe, you know, half an inch to an inch in diameter.
03:27Look, we're already catching wood on fire, so that's amazing.
03:30That was only a few seconds.
03:33Light is made up of tiny packets of energy called photons, and the more concentrated
03:39these photons are, the greater their destructive effect.
03:44So right here, we have more photons, so you could call this brighter.
03:49It's much brighter in that little spot than it is outside of the lens.
03:53So brightness is catching this wood on fire.
03:58Nina's backyard death ray is thousands of times brighter than the sun, but the superluminous
04:04supernova 15LH?
04:07That's shown hundreds of billions of times brighter.
04:11An onslaught of photons so concentrated it would have vaporized the surfaces of nearby
04:17planets and stripped away the atmospheres of more distant worlds.
04:23A real-life, mass-murdering planet killer.
04:28And who knows, maybe some of the millions of worlds destroyed by 15LH could have had
04:35civilizations just like ours.
04:39For a normal supernova, the kill radius is about 30 light years.
04:43We think the intense UV radiation from a supernova will destroy the ozone on Earth if the supernova
04:49happens within 30 light years.
04:51A superluminous supernova like 2015LH is so much more luminous than a normal supernova
04:57that the kill radius is much larger.
05:00Maybe 500 light years or even out to about 1,000 light years.
05:05Imagine a volume of space stretching 1,000 light years in all directions.
05:11It holds hundreds of millions of stars and perhaps billions of living worlds.
05:19Just one superluminous supernova in the center of this space is all it would take to wipe
05:26this vast region completely clean of life.
05:30It's violent enough when a single star blows up and destroys its solar system, but these
05:34actually might be the true mass murderers of the universe.
05:41So how do you turn a giant star into a mass killer like 15LH?
05:48Astronomers have observed only a few dozen superluminous supernovas, but they think the
05:53secret to their formation is spin.
05:58Superluminous supernovas start life as oversized, bright-burning stars known as blue supergiants.
06:06These blue supergiants live fast and die young, burning through their fuel supply in just
06:1210 million years.
06:15As they die, their cores collapse to form a super-dense object called a neutron star.
06:22If this neutron star is spinning fast enough, it can develop intense magnetic fields, transforming
06:30into something new and altogether more extreme, a magnetar.
06:36All neutron stars have very, very intense magnetic fields, but sometimes a true monster
06:41is created.
06:42There really is a limit to how powerful a magnetic field can be before it starts to
06:46rip apart space and time itself.
06:48And right on the edge of that is a magnetar.
06:54Magnetars are like neutron stars on steroids.
06:57Their intense magnetic fields reach out into the expanding outer gas layers of the dying
07:02star, raising temperatures and releasing an intense burst of light.
07:10But to get the kind of brightness produced by 15LH, you need a very special type of magnetar,
07:17the most powerful, fastest spinning magnetar we have ever seen.
07:24It pushes the magnetar model to the absolute limits because you need the magnetar to be
07:29rotating with about a one millisecond spin period.
07:32That means the neutron star has to be spinning a thousand times per second.
07:36And then over the course of the month of this explosion, you need to take all of that rotational
07:42energy that's inside the neutron star and blast it outwards into the surrounding star
07:47to make the light show that we see billions of light years away.
07:5115LH was the brightest supernova scientists have ever seen.
07:58And almost like a perfect storm, it could be the brightest supernova we'll ever see.
08:04There's a theoretical upper limit to how much energy a supernova can generate.
08:09And this thing was right at the edge of it.
08:14Unfortunately, superluminous supernovas are also super rare.
08:19So we're unlikely to have one explode in our neighborhood any time soon.
08:24But the galaxy is a big place and there are plenty more killers out there.
08:30And as powerful as light can be, the power of dark can be just as deadly.
08:36Supernovae may have many ways to kill you.
08:38A superluminous supernova might kill you in a death by fire, but in an unnova, death
08:44by ice.
08:57Some of the brightest lights in the universe are created by young blue stars as they die
09:03in bright explosions.
09:08Superluminous supernovas are the ultimate example.
09:11But other types of stars can go supernova too.
09:16Yellow stars, like the sun, swell up as they age, transforming into oversized monsters
09:22known as red giants.
09:26The biggest of these bloated stars are called red supergiants.
09:30And astronomers often see them explode in bright, violent supernovas.
09:37The bigger the red giant, the bigger the bang.
09:40But there's a problem.
09:43Nobody has ever witnessed the flash of the very biggest red supergiants in our galaxy.
09:49These most massive of bloated old stars have to be dying.
09:54But if not in a flash of light, then how?
09:59Now scientists have come up with an extraordinary theory.
10:03Instead of exploding in a bright supernova, the biggest of the red supergiants are simply
10:09blinking out of existence.
10:13Scientists dubbed these weird, disappearing deaths unnovas.
10:17And new evidence suggests these unnovas could be cold-blooded planet killers.
10:25You're looking up into the sky.
10:26The sun is shining.
10:27Then all of a sudden, it just turns out.
10:30That's what an unnova would look like.
10:33It would be the biggest catastrophe in the history of the planet.
10:36Life, as we know it, would not be able to survive.
10:40Scientists believe the key to the biggest red supergiants disappearing is a super-efficient
10:45transformation from a giant burning ball of gas to a tiny, dense black hole.
10:54Something has to be perfectly tuned to get an unnova.
10:57The star can't be rotating very quickly, and the outer layers can't expand much.
11:01When all the conditions are right, it just collapses into a black hole.
11:05They basically just become a black hole.
11:10It's not unusual for red supergiant stars to form black holes when they die.
11:15But most do it after they've released the violent flash of light we see as a supernova.
11:22The biggest supergiants have so much mass and so much gravity in their cores that when
11:28they collapse, not a single photon of light escapes from the newly formed black hole.
11:36To an observer, the star simply disappears.
11:39The death of a star without the flash, an unnova.
11:44It's almost like the star has fallen in and has forgotten to come out.
11:49More realistically, what's going on is when it falls in, it just can't come out.
11:53It's created so much gravity around itself that even an explosion doesn't allow it to escape.
11:59It doesn't get to explode.
12:00It just falls right into a black hole.
12:03So what makes unnovas planet killers?
12:07The biggest red supergiants age relatively quickly, dying after just 10 million years.
12:13But astronomer David Kipping believes that just might be enough time for these giant
12:17stars to create potentially habitable worlds.
12:23Planets can form pretty quickly.
12:24They can form within a million years around these stars.
12:27So there should be time for these massive stars to form planets.
12:30And in fact, when we look at the remnants of massive stars, we indeed find rocky planets
12:35around them.
12:36So as far as we can tell, these stars really should have worlds orbiting them.
12:44Imagine a lone rocky world warmed by the far distant light of a red supergiant star.
12:52Simple life clings to shallow rock pools on the young planet's surface, but their warm,
12:58comfortable existence is doomed.
13:01High in the sky, the far distant supergiant sun is burning through the last of its hydrogen fuel.
13:09The force of gravity pushing in overcomes the force of fusion pushing out.
13:15And 20,000 trillion, trillion tons of hot, burning hydrogen gas collapses down into a
13:22single point in space, a black hole.
13:29Surprisingly, the black hole that makes the unnova so dark doesn't put the planet in immediate
13:36danger.
13:37This is one of the biggest misconceptions of movies, is that suddenly, if a star becomes
13:42a black hole, then the planet is going to be sucked into it.
13:45The gravity of the black hole is exactly the same as the gravity of the star, as long as
13:48it hasn't lost any mass.
13:50From a distance, if you're orbiting the star, it's the same as orbiting the black hole.
13:55Nothing would change.
13:57As the supergiant star collapses, the view from the far distant planet would be surreal.
14:04What you're going to see is, there's your star in the sky, and then a minute later,
14:08it's gone.
14:09You will actually see it collapse, forming a black hole, and the whole thing just falls
14:12into it, and that's that.
14:15What would happen next would be a long, slow, cold death.
14:19You're basically turning off your star.
14:22A fleeting moment, but the beginning of a winter that would never end.
14:28If you suddenly turned off the light from the sun, life wouldn't actually be immediately
14:32extinguished.
14:34It would just be like the night.
14:35We would be a little bit cooler than normal, but eventually, over time, over weeks, over
14:40months, over years, the planet would begin to freeze over.
14:46After a hundred years, a global ice age engulfs the planet.
14:51First the land, then the oceans.
14:55The oceans would freeze over into a thick crust.
14:58Maybe a little bit of liquid water would still be there at the bottom of the ocean, warmed
15:01by volcanic vents.
15:04Actually, what you're left with are things that don't depend on sunlight to live.
15:08Maybe there are tube worms living in vents, cracks, hydrothermal vents in the bottoms
15:13of the oceans, and that sort of thing.
15:15But even those can't possibly live forever.
15:19The internal heat of the planet continues to radiate out into space.
15:23The surface temperature drops below minus 350 degrees Fahrenheit, and the atmosphere
15:30collapses onto the surface as snow.
15:33The core of the planet can no longer support active geology.
15:39All life is gone.
15:41The planet is dead.
15:45So these habitable worlds will eventually end up being just these spheres of ice.
15:53Whether killed by the light of a supernova or by the darkness of an un-nova, planets
16:00are in the firing line from killer stars.
16:03But research, recently released, suggests that stars, too, can fall victim to murder.
16:09And some of these killings are straight out of a horror movie.
16:14How can an old star, that's on the way to die, get more mass so it can become young
16:22again?
16:23Well, it can do exactly what a vampire does.
16:25It can suck life from something else.
16:332012, the Hubble Space Telescope makes a gruesome discovery.
16:50It finds killer stars sucking the life from their neighbors.
16:56These vampire killers are found lurking inside tightly packed groups of stars known as clusters
17:03of stars.
17:05Stars are born in giant clouds of dust and gas.
17:08And these clouds have enough material to make dozens or even hundreds of stars.
17:12We call these family of stars star clusters.
17:15And we think they're all roughly the same age.
17:21Young clusters shine like jewels with an array of bright colors, blues, yellows and reds.
17:29But this starry rainbow changes over time.
17:32The blue stars disappear first.
17:36These are the biggest stars in the cluster, burning brightly and dying young after millions
17:41of years.
17:43Next to go are the yellow stars.
17:46These medium-sized stars age over billions of years, gradually turning red like ripening
17:52fruit.
17:53After 10 billion years, the entire cluster matures to a deep red.
17:59This gradual shift in color is useful to astronomers because it allows them to judge
18:05just how old a cluster is.
18:09When you look at a population of stars in one place, if you see a lot of blue stars,
18:14you can be pretty confident that that must have young stars.
18:18They couldn't have been born too long ago because blue stars are massive and massive
18:23stars go through their fuel quickly and live very short lives.
18:28But in the 1950s, astronomers spotted something seriously weird in an ancient cluster.
18:34Tucked amongst the old red stars, they found a handful of brightly shining young blue stars.
18:41New stars don't usually form inside mature clusters, so how did they get there?
18:47The only explanation?
18:49Somehow, the old stars were getting younger.
18:53Astronomers dubbed these age-defying stars blue stragglers.
19:00In some clusters, we see these blue stars that appear younger than they should.
19:05In some ways, they're kind of straggling behind the natural aging of the cluster.
19:09Something must be actively rejuvenating a star, but what could do that?
19:13If you imagine these blue straggler stars were people in a crowd, these stars would
19:17look like they had been given a facelift.
19:20They're masquerading as younger stars when really they're just as old as everybody else
19:24in the room.
19:27Astrophysicist Natalie Gosnell believed the blue stars were being rejuvenated by a fresh
19:32supply of hydrogen fuel.
19:35But where was it coming from?
19:37In 2015, Natalie took a closer look at the Hubble images of the blue straggler cluster.
19:45She discovered that most of the young-looking stars were in binary partnerships with the
19:50corpses of dead stars that appear to have had their gas sucked away from them.
19:56So in movies, vampires are perpetually youthful because they are sucking blood from humans.
20:06And so in this case, we have stars that are sucking gas and material from other stars,
20:11keeping them looking young.
20:15Gas is the fuel that allows all stars to burn.
20:19And with new gas, a star is revitalized.
20:23But for a star to turn into a gas-sucking vampire, scientists believe it needs to start
20:28its life in a close-orbiting binary pair.
20:33If you have two stars in orbit about each other, which is a very common thing in the
20:37universe, then they're not going to be the same mass in most cases.
20:40The one that's more massive will evolve more quickly.
20:44And as it ages and evolves, it will swell up.
20:47And it will get so large that its outer surface can come in contact with the gravitational
20:53region of influence of its partner star.
20:57The smaller star becomes a vampire.
21:00It sucks the bloated outer layer of gas from its bigger partner.
21:05And as the vampire feasts, it burns hotter and hotter, turning a brilliant, youthful
21:11blue.
21:13The vampire's victim is sucked dry, reduced to a lifeless, stellar core known by astronomers
21:19as a white dwarf.
21:22But like any good horror movie, this murderous tale has a twist, exploding zombies.
21:30There's all sorts of stories about zombies.
21:32What if the dead could actually come back and take revenge on the people who killed
21:36them?
21:37Well, something similar really does happen with stars.
21:40As the blue straggler ages, it swells so much, the dead white dwarf starts to steal some
21:46of its gas back from the vampire.
21:49The dead star rises again to become an exploding zombie.
21:58As that material piles up, it gets hotter and hotter.
22:01And you're basically piling up tremendous amounts of hydrogen.
22:05And if it gets hot enough and the pressure gets enough, basically you have created a
22:09hydrogen bomb the size of a planet.
22:12That star explodes.
22:19These stars can get revenge.
22:21Once the zombie explodes, it takes out the vampire that sucked its life away.
22:26For decades, astronomers have been puzzled by the number of tiny, dead white dwarf stars
22:31they see exploding in the night sky.
22:34But here, finally, could be an explanation.
22:38Our galaxy is filled with vampires and exploding zombies.
22:44What does that say about the rest of our galaxy?
22:46A huge chunk of the stars in our galaxy are sort of stealing life from their friends to
22:51stay forever young.
22:55The birth and death of vampires could be the reason we see blue stragglers today.
23:00But amazingly, it could also explain why we're here too.
23:05There's no way in this universe to get life without death.
23:08You can't possibly have materials to build planets or people or anything around us without
23:13supernovae.
23:14So these vampires and these zombies, well, actually, they're our parents.
23:19You and I, we could be the result of these vampire stars transferring gas back and forth
23:23in these binary systems, leading to stellar explosions that blew out the building blocks
23:29of life into the universe.
23:33Our home star, the sun, is not in a binary pair.
23:39But if you think that makes us safe from cosmic vampires, think again.
23:44New observations suggest some vampires can fly.
23:50And a close pass by our solar system is all it takes to finish off the Earth for good.
24:14The universe seems to run like clockwork.
24:22Moons orbit planets.
24:24Planets orbit stars.
24:27And the stars themselves revolve around the center of our galaxy.
24:32Everything seems to be in the right place, ordered and stable.
24:37But some killer stars don't follow the rules.
24:41And once in a while, you find something careening across the sky in exactly the wrong direction.
24:46How did that rogue star get there?
24:50In the last decade, scientists have spotted hundreds of lone stars hurtling through our
24:55galaxy like ballistic missiles.
25:00Scientists name these rogues runaway stars because they can travel at incredible speeds.
25:07We're talking about stars that are going a thousand times faster than a rocket.
25:13Hypervelocity anything is dangerous.
25:15A hypervelocity star is incredibly dangerous.
25:19A ball of hot gas a million miles across careening through our cosmic backyard.
25:27Runaway stars don't even need to score a direct hit to inflict damage.
25:33And grazing the outer limits of our solar system could be enough to destroy the Earth.
25:40You don't want any star getting too close to us under any circumstances because that
25:43could disrupt the orbits of the planets.
25:46A star passing by wouldn't even have to get all that close to us to wreak a huge amount
25:50of havoc.
25:51We have the giant Oort cloud of comets that extends as much as two light years away from
25:55the sun.
25:56If a star passes anywhere close to there, we could be rained on by destructive comets.
26:03All of the planets in that solar system would either be disrupted or fired into the star.
26:12And I would expect no solar system to remain afterwards.
26:16Astronomers traced the paths of these runaway stars and found that many came from the galactic
26:23center.
26:24In the middle of our galaxy, we see stars that are trapped in orbit around the central
26:30black hole.
26:31They're actually kind of buzzing around like a hive of angry bees.
26:35Well these stars interact gravitationally with each other and sometimes they can fling
26:39each other clear across the galaxy.
26:43Most runaway stars are harmless to us.
26:46They shoot straight out into space from the galactic center.
26:50But from time to time, we find runaway stars a little closer to home.
26:56In 2016, astronomers turned their attention to a rogue star just 3,000 light years away
27:02from the Earth.
27:05The star's trajectory and its composition didn't seem to make any sense.
27:11There's a star, SDSS J1128.
27:14And it's weird.
27:16It's weird because first of all, it is moving extremely rapidly through the galaxy, way
27:20faster than it could possibly be moving if it's just simply in orbit.
27:24Something gave it a huge kick.
27:26But it's got something else unusual about it as well.
27:29It's a star, much like the sun, but it seems to have a lot of carbon in it.
27:34And that's unusual.
27:36Sun-like stars only produce carbon at the end of their life cycle, once they've swollen
27:41up to form red giants.
27:44But here was a star that was burning through its hydrogen in the regular part of its lifetime
27:49covered with carbon.
27:51What happened there?
27:54Scientists now believe the carbon-rich runaway star must have once been a vampire, locked
28:01in a very close, very fast binary partnership with a much larger supergiant star.
28:08The vampire sucked carbon-rich gas from its bloated giant partner.
28:12But the red supergiant was a reluctant victim and exploded in a vast supernova.
28:20The force of the blast should have taken the vampire out.
28:24But its orbital speed was so great, this carbon-stained star was flung away into space.
28:34These two stars are going around each other quite rapidly.
28:37When one of them blows up, it loses most of its mass.
28:40It loses a lot of its gravity.
28:42It doesn't have enough gravity to hold on to the lower-mass star.
28:46So they're spinning around.
28:47This one blows up, and suddenly this one finds itself slingshot out into the galaxy.
28:52And that explains everything.
28:53It got the carbon from the high-mass star.
28:55The high-mass star blew up and flung that lower-mass star out at very high velocity.
29:02So what are the chances of our planet getting fried by a flying vampire star?
29:08In the vastness of the galaxy, our solar system presents a mercifully tiny target.
29:16It is so rare for any two stars to get close enough together for this to happen, even over
29:21the billions of years that a star can live, that it's almost never going to happen in
29:25the lifetime of any given star.
29:32Flying vampires may be unlikely to destroy the Earth.
29:37But there's another type of killer star out there that offers a clear and present danger.
29:43And we know it could hit us, because it's done it before.
30:07In March 2008, stargazers watched open-mouthed as a faint light in the sky blinked into life
30:15and less than a minute later faded away.
30:19A rare treat for the stargazers, but a stark reminder that our planet is in the firing
30:24line from the most powerful type of killer star in the universe, a cosmic superweapon
30:32known as a gamma-ray burst.
30:35Gamma-ray bursts, these cosmic ray guns, are the most powerful, the most deadly weapon
30:40that the universe has come up with.
30:42The jets of a gamma-ray burst actually don't last very long.
30:45They go off in really just a couple of minutes.
30:47But in that time, the energy released is equivalent to a hundred trillion nuclear weapons going
30:53off every second for a hundred billion years.
31:00Gamma-ray bursts are super-concentrated beams of high-energy light, and they pack enough
31:06punch to reduce nearby planets to vapor.
31:12They're a kind of supernova, but it's more like a super-supernova, the idea being that
31:17instead of blowing up a star and letting the debris expand in every direction, what if
31:22somehow instead of blowing up in every direction, it blew up in one direction, that it was somehow
31:28focusing all of that material and it was being shot out like a beam from the explosion
31:33site?
31:35The key insight here is that we're not talking about something that's exploding like a sphere
31:40and spreading its energy out in all direction.
31:43A gamma-ray burst is beamed, so it's taking all of its energy but pointing it right at
31:48us and firing it directly towards us.
31:51That's why it's so powerful.
31:56The 2008 gamma-ray burst landed a direct hit on the Earth.
32:00But it had come from so far away, this super-weapon appeared to have lost its punch.
32:07Astronomers quickly calculated where the beam had come from, and they pinpointed a location
32:12on the far side of the universe.
32:15Incredibly, this cosmic sniper's bullet had been traveling through space for seven billion
32:22years, far longer than the age of our solar system.
32:26We're talking about something that was so energetic that had you been looking at it
32:30with your naked eye, you could see it, even though it was more than seven billion light
32:35years away.
32:36Can you imagine something that energetic, something that violent, appearing as a gentle
32:41little star going on and off in the sky?
32:44Think about that for a second.
32:45You've got a thing that's so bright that you could see it by naked eye, even though it's
32:49halfway across the observable universe.
32:53In 2008, we got lucky.
32:57If the same gamma-ray burst had gone off within a few thousand light years, the Earth's atmosphere
33:02would have been turned to plasma.
33:06But we're safe now, right?
33:09These things are happening somewhere in the universe every day, okay?
33:14Every day.
33:15I am not misspeaking here.
33:16Not every year, not every century, not every millennium.
33:19Every day, somewhere in the universe, in the hundreds of billions of galaxies making
33:23up our cosmos, there is a supermassive star that is creating these jets and frying everything
33:30within a few hundred light years of itself.
33:34So where do these planet-melting jets come from?
33:38And how worried should we be?
33:40So what you do is you take a massive star that's rapidly rotating.
33:43A massive star that's rapidly rotating burns through all of its nuclear fuel and eventually
33:49produces an iron core at its center.
33:52That iron core becomes unstable and collapses to a neutron star.
33:56If that neutron star is rapidly rotating, that means if it's spinning about a thousand
34:01times a second, it will have a huge store of rotational energy.
34:08This energy is concentrated as the star shrinks, constrained by the neutron star's powerful
34:13magnetic fields.
34:17Eventually, with nowhere else to go, the pent-up energy bursts from the poles of the neutron
34:23star.
34:24It can make a jet, almost like squeezing a tube of toothpaste.
34:30The jet can go punching through the star in each direction.
34:33It essentially sets off a magnetized bomb that then produces these jets that just go
34:38ripping through the star in one direction, out each pole, the north and the south pole.
34:46And the amount of energy that is packed into these beams make them death rays.
34:51These are the single most energetic events going on in the modern universe.
34:59So what's the likelihood of a direct hit on Earth?
35:03The simple answer?
35:05We just don't know.
35:07The problem with gamma-ray bursts is that they're so dangerous from so far away.
35:11There actually may be stars out there about to go gamma-ray burst that are pointed towards
35:15us we don't even know about.
35:16We've made a survey of the sky, we've identified which stars we think are dangerous, and we
35:20don't appear to be looking right down the gun barrel.
35:23So maybe for now, we're safe.
35:25We don't have to stay up late at night worrying about being killed by a gamma-ray burst.
35:30But the universe is big and old, and that means weird things happen all the time.
35:36Our solar system could well be taken out by a gamma-ray burst in the future.
35:42But it's a big universe, we're a small target, and that stacks the odds in our favor.
35:49In the meantime, astronomers have caught another killer in the act, and they think that this
35:55sinister star type will almost certainly be the same killer that finally destroys the
36:01Earth.
36:12We now know that our universe is filled with killer stars, and that right now, some planet
36:25somewhere is about to be toast.
36:30But until very recently, the evidence we had for this mass planetary slaughter was indirect.
36:37We've seen that there are a lot of death stars out there that are frying planets all the
36:40time.
36:42But we've never seen any of these planets actually get destroyed by these stars, except
36:47once.
36:48And it turns out that was a planet that may have once been much like Earth, and it was
36:51orbiting a star that was once much like the Sun.
36:562015.
36:59Astronomer David Kipping is poring over data from Kepler, a space telescope designed to
37:05spot alien planets as they pass in front of their host stars.
37:10A white dwarf star with a very unusual signature catches his eye.
37:16Instead of being dimmed by a neat, round planet, the tiny star appeared to be surrounded by
37:23vast chunks of disintegrating rock and dust.
37:28What we're seeing from this white dwarf are multiple dips in its brightness, sometimes
37:32as much as 40% of the light being blocked.
37:35And they're all on slightly different periods, but all roughly about five hours, which means
37:40we have something orbiting very close in, but it's in clumps.
37:46What could these clumps be?
37:48They were too tightly packed for a swarm of asteroids, and too close to their host star
37:54to be icy comets.
37:56The best explanation we had was that this wasn't just one planet going around this white
38:01dwarf star.
38:02There were very small planetesimals that were probably the product of a disintegrating planet.
38:09The white dwarf star had been caught in the act of murdering its own planet.
38:15So a planet which is so close to that white dwarf star that the tidal forces, the gravity
38:21of that white dwarf has ripped apart the planet, and now we are seeing the fragments fall into
38:27the surface of the white dwarf.
38:28It was a beautiful discovery.
38:30I mean, we had suspected this was happening, but to actually see the direct evidence with
38:36almost your own eyes, you know, seeing this light curve, seeing a planet disintegrating
38:40right before you was confirmation of something we had suspected for a long time.
38:47We've only seen one star killing one planet, but scientists suspect this same act of murder
38:54could be happening all over the cosmos.
39:00If white dwarfs can destroy planets, then that means that 90% of stars are actually
39:05capable of destroying planets, so there's ultimately no safe haven for planets.
39:11It's also a chilling look into the future.
39:14In a few billion years' time, our sun will begin its transformation into a white dwarf.
39:21At first, it'll swell up, engulfing the inner rocky planets, Mercury and Venus.
39:28Now that means that the planets will be orbiting around inside the sun.
39:32Usually we orbit through empty space, there's nothing really to change the way we move around
39:36the star, but inside the sun there's a lot of gas, we'll be dragging against that, and
39:41that will take energy away from the planets, and they will spiral in closer and closer
39:45to the star.
39:50Our bloated star blows off its outer layers completely, leaving behind a tiny, dense white
39:56dwarf, and revealing the scorched Earth, now on an ever-decreasing death spiral into
40:04the sun.
40:05If you're getting closer and closer to the white dwarf, you're doomed, because eventually
40:10your rocky planet will be ripped apart.
40:12The intense gravity of our white dwarf sun pulls on the charred remains of the inner
40:17planets, and piece by piece, they break apart.
40:22So if the planets do survive, being in the sun's envelope, and they're now orbiting close
40:28to the surface of this white dwarf star, first Mercury is going to go, it's going to be ripped
40:33apart, then Venus is going to be ripped apart, and finally, the Earth will cease to exist.
40:41So maybe in a few billion years, an alien civilization will be looking at the sun, and
40:46the sun is no longer this big, luminous star, it is now a dead white dwarf, and they will
40:51notice a planet about the same size of the Earth going around it.
40:55But that planet, just like the case with the Kepler data, will be disintegrating in front
41:00of their eyes, and they will wonder about whether in the past there had been a civilization
41:04or life on this Earth-sized planet.
41:07The star that brings us light and warmth today may one day rip our planet to shreds.
41:16Our own sun will become a killer star.
41:21But in time, the atoms that make up our planet, and everything around us, will be returned
41:27to the cosmos as our dead star gradually fades away, and perhaps new stars will be born from
41:35those pieces, and the cycle of life and death in the universe can begin again.
41:43It's sad to think that a star might destroy planets, because after all, stars are life-giving.
41:49Our sun gives us life, and we're here because of it.
41:53But eventually our sun is going to kill us too.
41:55Stars giveth and taketh away.
41:58Killer stars may sound like something evil, but there's no life without death.
42:02You have to turn the cycle of the universe.
42:04It really is the way the universe works.