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00:00there's a killer lurking in our galaxy a star ready to explode into a supernova these are the
00:12most visually stunning events in the universe seen from earth it would have a terrible beauty
00:18but for us it could be fatal in a few seconds it can release as much energy as the sun will
00:25over its entire lifetime we're trying to hunt it down but it's lying low we haven't seen a supernova
00:33in the milky way in over 400 years it could be anywhere
00:38it is nearly impossible to predict where and when the next supernova will happen
00:47the hunt is on to find the next supernova before it finds us
00:55october 2019 one of the brightest stars in the sky looks dangerously unstable
01:20if you look at the constellation of orion one of the shoulders of orion is a star that is
01:28obviously red this is betelgeuse
01:32i could go into my backyard and see it you could clearly see that it was getting dimmer
01:43is this a warning is betelgeuse about to die in a massive cosmic explosion a supernova
01:53we've been studying this star for hundreds of years and one thing we're sure about is that it's big
02:02very big betelgeuse is a massive star maybe 15 or 20 times the mass of our sun and it's near the end of its life
02:12it is a massive enormous luminous star and one day it's gonna go boom
02:21betelgeuse is on our list of supernova candidates because of this massive size
02:30the bigger star they are actually the shorter the lifespan
02:35the lifespan of a star depends on a delicate balance between two competing forces
02:44gravity pulling in and heat and pressure pushing out stars exist because they're held up they're not
02:54held up by pillars they're held up by energy flowing out of the core toward the surface of the star
03:00that stops the gravitational contraction stars get their energy from nuclear fusion reactions right
03:07in the core and the most basic one is taking two hydrogen atoms and slamming them together to form
03:12a helium atom and you might think okay the more hydrogen you have the more stuff you have maybe
03:17the longer the star will live turns out it's exactly opposite the reason gravity the more mass a star has
03:26the stronger its gravity gravity that crushes its hydrogen atoms closer together as you crush things
03:35more and more the temperature gets hotter and hotter and hotter and the nuclear fusion reactions burn
03:40faster so bigger stars burn their fuel very very quickly and live short lives smaller stars burn their
03:47fuel much more slowly and live long protracted lives so when you are a big star you live fast and you die young
03:54beetlejuice burns brighter than 125 000 suns but now it's running out of its hydrogen fuel
04:09so it's burning whatever it has left just to stay alive
04:14stars are basically factories for burning hydrogen into helium and then once the helium is burned
04:24they start burning heavier and heavier elements like carbon and nitrogen and oxygen
04:29it's a little like you burn something you get ash but then if you crush the ash enough you can burn it again
04:37and then you crush it some more you can burn it yet again
04:41but this process can't go on forever
04:45as the size of the atomic nuclei being fused together grows the amount of energy released falls
04:54the fuel the star needs to resist the pull of gravity is running out
04:59unfortunately the amount of energy you can extract by putting two nuclei together
05:05gets smaller and smaller uh the bigger the nuclei are until you come to to making iron
05:12and iron it turns out is the last thing you can make that way
05:16the problem with iron is when you fuse it it doesn't make energy it takes it away
05:22so when the star builds up that iron core it's doomed
05:26it can no longer create energy in its core to flow out toward the surface
05:31strong enough to keep it from collapsing so collapse is what they do
05:37in a fraction of a second the star's core collapses down from the size of a planet
05:45to about the size of a small city
05:47a huge amount of energy is suddenly released which forces the collapsing layers back out
06:03the result an enormous explosion we call a supernova
06:13the shock wave from a supernova rips out at thousands of miles per second
06:17and for a brief period of time they're brighter than an entire galaxy
06:23a supernova could devastate life on earth
06:28and the evidence can be found at the bottom of our oceans
06:34there are layers and layers of silt that have built up and there seemed to be a layer about 2.6
06:39million years ago that was enriched in a very strange chemical element something called iron 60.
06:45iron 60 is a radioactive isotope of iron and it doesn't last very long just a few million years
06:51and the only place that we know of that can make iron 60 is a supernova in an exploding star
06:58that means there must have been a supernova close enough to the earth within the past couple of
07:11million years to have physically deposited material on our planet that freaks me out
07:19the sign of this shocking assault on our planet is a thin layer of this very rare type of iron we find
07:29it in the mud of every ocean floor and always at the same depth this interstellar dust must have drenched
07:37our world in one enormous burst 2.6 million years ago it was a terrible time a third of large animal
07:46species in the sea suddenly died out there were some pretty amazing fish probably the most amazing
07:53is the megalodon the giant shark teeth the size of dinner plates and so on but they went extinct
07:592.6 million years ago at the end of the pliocene what happened a lot of sea creatures died and a lot
08:09of them were in shallow waters whereas deep water animals tended to survive well that sounds kind of
08:15like a supernova that can do things that would affect our atmosphere would affect shallow water but not
08:21deeper water supernovas create huge amounts of cosmic rays when they crash into other atoms
08:31they break up and produce showers of dangerous shrapnel called muons these charged particles are
08:40similar to electrons only 200 times heavier so they penetrate more deeply and cause more damage
08:50they can pierce through our atmosphere pierce through our skin get into a cell and disrupt the dna
08:57they'll go right through a mouse but deposit in the body of a larger animal so the impact on an
09:02animal the size of a megalodon say could be pretty extreme
09:05muons can shatter dna causing mutations and cancer but their power weakens as they travel through water
09:16which may be why only deep sea creatures survived the extinction really tells us that we're not
09:24separate and apart from the universe and the goings-on up there right supernova going off and things like
09:29that okay it's a pretty light show no there's a direct impact to life on earth and us so are we in
09:37danger of extinction is beetlejuice about to explode
09:43when stars explode as supernovas they can devastate planets hundreds of light years away
10:02beetlejuice is about 550 light years from earth
10:06so when it dramatically dimmed in 2019 scientists were concerned
10:14but beetlejuice has dimmed before
10:18beetlejuice varies quite a lot over the years there are some cycles and sometimes these cycles
10:25come together and you get a deep minimum so dimming is part of the star's natural cycle as it nears the end
10:33of its life but to get a full picture we took beetlejuice's temperature
10:41if the star was dimming that would mean that the surface was cooling over time
10:45we actually made measurements of the temperature of beetlejuice and found out that wasn't happening
10:49it hardly cooled at all it cooled like like 50 or 100 degrees you might expect a much much
10:55more dramatic change in the surface temperature if it were about to explode
10:59so if beetlejuice wasn't cooling much what was making it dim
11:08to take a closer look we used the very large telescope and an exoplanet hunting instrument
11:16called sphere and came up with an extraordinary image
11:20when i first saw this image of beetlejuice it blew me away i almost gasped i i may have
11:29said a word i can't say on tv that was very exciting the image reveals that while the upper part of
11:38beetlejuice was still bright the lower part was noticeably dimmer we had images of beetlejuice from
11:46before and we were able to compare the new ones with it and so you could see that half of beetlejuice
11:52looked pretty much the same but the other half was significantly dimmer and what could make a star dim
11:59that quickly and remember how big this star is nothing happens on beetlejuice quickly so this must be
12:06something happening right on the surface as heavier material like silicon emerges from the surface of
12:14beetlejuice it cools and condenses it's kind of like sticking the hose in the wrong end of your vacuum cleaner
12:22instead of pulling stuff in it blows all this dust out into space
12:26beetlejuice has cosmic indigestion and is belching dust which makes the star seem dim but it's not over
12:39all through 2020 beetlejuice first brightened and then dimmed again
12:46so astronomers are watching this massive star with bated breath
12:50it's gonna explode the question is when it's probably sometime in the next hundred thousand years
12:58but it could be tomorrow it could have already exploded and we're just waiting to see the light
13:03with luck if beetlejuice blows all we'll see is a beautiful light show
13:10at a distance of 550 light years it's probably too far to do serious damage
13:16but is there another star we should worry about
13:26a closer star just 150 light years from earth could do us some major damage a star like i.k pegasi
13:37but it isn't this star which we can see in our night sky that's the threat
13:42the main star is only about 1.6 times the mass of the sun that's nowhere near enough mass to go supernova
13:50and yet we think it is the progenitor for a supernova how can that be the main star isn't alone
14:00it has a more dangerous accomplice
14:05there's another star there orbiting the larger star
14:08and this is what we call a binary system two stars orbiting each other
14:13right now the system is stable but things aren't always going to be the way they are now and sometime
14:18in the future things are going to change a lot
14:24i.k pegasi is really made up of i.k pegasi a a large white star and its accomplice
14:33a white dwarf called i.k pegasi b
14:40this tiny star is the real threat to earth
14:45you can think of a white dwarf as a zombie uh you know it's a dead star and they can eat living stars
14:53if there's a a normal star like the sun near a white dwarf the white dwarf has very very intense gravity
15:00it can literally pull material off that normal star and that material will then pile up on the
15:06surface of the white dwarf so it really is eating a living star
15:12these stars orbit each other just 18.5 million miles apart
15:18that's closer than mercury is to our sun
15:21but they're not interacting with each other
15:24yet the problem is sometime in the future that normal star is going to run out of fuel and when
15:31it does it's going to expand into a red giant
15:36when it gets to the end of its life i.k pegasi a will cool and swell up to become a red giant
15:43and that's it no big explosion it won't become a supernova but that's just when its accomplice
15:55i.k pegasi b will start to feed a lot of that material will gravitationally be attracted to the
16:03white dwarf and fall onto the surface as the white dwarf pulls material from its bloated red giant neighbor
16:11it gets more and more massive its gravitational pull increases so it feeds even faster
16:22eventually it can no longer support its own weight
16:27the core of the star is actually very dense in fact if you had like a teaspoon of material
16:31it would weigh about as much as an 18-wheel truck and it's basically right at the limit of normal matter
16:37being able to hold up at that density you dump more and more stuff onto it and eventually there's a
16:42limit that's reached and either collapses or more generally blows up
16:53when this happens i.k pegasi will be brighter than the full moon in our sky
16:59the sky because it's only 150 light years away
17:07having a supernova 150 light years sounds like a bad idea and it is that's close enough
17:13that it might have some physical effects on the earth
17:19right now i.k pegasi is about as far from earth
17:23as the supernova suspected of killing off the megalodon
17:29so how worried should we be the good news is the i.k peg system is moving away from the sun and the
17:38earth right now at a decent clip so if it's not going to blow up for a while that means it could
17:44be on the other side of the galaxy by the time it does by that time we'll be completely safe as an
17:50astronomer and an astronomer who has studied supernovas professionally having them far away is fine
17:56with me close enough that we can study them well but not so close that you know i can study them
18:01personally on a physical level on my own body yeah no
18:09a close supernova would be devastating for life on earth will there be any warning signs
18:17before one of our prime suspects is about to blow
18:40to find a supernova warning signal we need to know what's happening deep inside
18:45the core of an exploding star at the very beginning of a supernova explosion the core of a massive
18:53star is collapsing there's no more nuclear fusion going on and it is compressing to higher and higher
18:58densities the star's gravity crushes protons and electrons so close together they merge to form neutrons
19:09the star's core becomes one of the densest materials in the universe
19:13it's like a gigantic atomic nucleus roughly half a million earths compressed into the volume the size
19:23of a city that's really really dense stuff if you had about a teaspoon full of material
19:29that would be about as much mass as mount everest
19:37forcing protons and electrons together releases a huge amount of energy
19:42in the form of tiny elusive subatomic particles called neutrinos neutrinos are one of the most abundant
19:51particles in the universe but they don't interact with things very much at all
19:57neutrinos are often called ghost particles because they do what ghosts do they walk through walls but
20:03neutrinos walk through us they walk through the planet they walk through stars there are super ghosts
20:09at first these neutrinos can fly straight out of the core of the star but as the star collapses it
20:18gets so dense that some neutrinos get trapped and their energy turned into heat and that creates a
20:26shock wave that rips the star apart and the ensuing explosion is brighter than billions of stars all put
20:32together this light show may be spectacular but it's only one percent of the energy released in a supernova
20:43the rest is in the form of a massive burst of neutrinos so neutrinos could act as a supernova early warning system
20:53at least that's the idea
20:57on february 24th 1987 that idea was tested
21:05an astronomer was doing a routine survey of a dwarf galaxy close to ours
21:12he was taking pictures of it develops the pictures and says hey there's a star here that wasn't there
21:18you know yesterday he basically got up walked outside and looked and went oh there's that star
21:27and it turns out he had discovered a supernova
21:33because it was the first supernova spotted that year it was called supernova 1987a
21:411987a was an amazing event in the world of astronomy essentially a supernova went off
21:48in our own backyard it was very close to us occurring in a neighbor galaxy of the milky way and so it was
21:57the brightest thing seen in our skies since the invention of the telescope
22:05supernova 1987a blazed with the power of a hundred million suns but that wasn't the most exciting part
22:14for the first time we received an early warning that a supernova was about to appear three hours before it
22:22lit up our night sky neutrino observatories around the world saw a sudden surge in neutrinos from the same
22:31direction on the sky
22:32the neutrino's ability to zip across the galaxy slipping through stars and planets like ghosts
22:45gives them an unbeatable head start during a supernova
22:50the neutrinos are released in the very earliest moments of the supernova blast and they slip through
22:57the atmosphere of the star before it goes boom neutrinos can escape in as little as 10 seconds
23:09but it can take hours for the shock wave to travel right through the star and blast off the outer layers
23:15revealing the light the result is that we see neutrinos from a supernova explosion before we see the actual
23:22light
23:28so if we want to spot the next supernova explosion we've got to be paying attention to the neutrinos
23:38astronomers set up the supernova early warning system
23:41a network of neutrino detectors all around the world
23:49it should give astronomers several hours notice of an impending supernova
23:56but so far nothing no supernovas have occurred near enough for the system to detect
24:04neutrinos are like the friend that never comes we're sitting here waiting for them but we don't know when
24:10it's going to actually happen but when they do come we might be in trouble
24:18because some supernovas are armed with the most powerful weapon in the universe
24:25gamma rays
24:26our hunt for the milky way's next supernova has identified some potential suspects
24:45very massive lonely stars and stars with smaller sidekicks
24:51in 2018 astronomers found a system called apep 8 000 light years away with two very massive stars
25:04each one about as massive as beetlejuice
25:09these are giant stars nearing the end of their lives with massive outer layers of gas
25:17that continually contract and heat up again and again they become really huge and bloated and swollen
25:27and they're prone to huge outbursts
25:32these unstable stars are called wolf ray a stars
25:36they're very rare and so hot and bright they emit more radiation than a million sun-like stars
25:48this intense energy is blasting their outer layers off into space
25:54mass loss has been occurring from the star so much so that you've actually lost all the hydrogen
26:00that was uh wasn't burned into helium so now you have a star that's made uh entirely of helium and heavier elements
26:08with no hydrogen left these massive stars are running low on usable fuel
26:15they're like ticking time bombs made even more dangerous because they're spinning so fast
26:22it's spinning so quickly it's on the verge of ripping itself apart and this means that when this thing blows it's gonna blow hard
26:34when a star goes supernova its core collapses the smaller it gets the faster it spins
26:42some cores collapse into fast spinning neutron stars
26:46heavier ones like apep collapse into even denser and more mysterious objects black holes
26:58the immense gravity within apeps collapsing core will drag back some of the gas and dust into a spinning disk
27:09as the material falls onto the core it compresses and it speeds up
27:14the dying star spins faster and faster as it collapses
27:21in this incredible rotation drives the creation of massive magnetic fields that are capable of funneling
27:29material around and up and out in the form of huge beams of radiation
27:38so the energy from the supernova collapse instead of being emitted spherically in every direction
27:44comes at us in a tightly focused beam
27:48like a laser from the death star it is pointed in one direction
27:56this is a gamma ray burst
28:00it is the single scariest thing the universe has to offer this is an explosion so powerful that in a few
28:08seconds or minutes it can release as much energy as the sun will over its entire lifetime
28:16you do not want to get caught in a gamma ray burst let's just put it that way
28:21the impact of a nearby gamma ray burst on our home planet is almost too terrible to think about
28:34it would be a very bad day for earth
28:37earth's atmosphere could be partly blown away and there could be chemical reactions in the atmosphere
28:46that would form all kinds of noxious products
28:49a gamma ray burst from apet might last only 10 seconds but its impact would last for decades
28:59the generation of nitrogen oxide from a gamma ray burst would be disastrous in the upper atmosphere
29:06it would eat away at our ozone layer in the lower atmosphere it would come out as acid rain
29:12and the acid rain would destroy our crops nitrogen dioxide also filters out sunlight
29:22turning the skies dark and cooling the earth enough to trigger a new ice age
29:30any life on the land in the shallow parts of the sea or that live near the sea surface
29:36would be done in fact it would ultimately result in extinction
29:45blasted by ultraviolet radiation from our sun freezing cold and hungry humanity's future would be bleak
29:58so we really need to know when apep goes supernova and produces its deadly beam of gamma rays
30:07are we in its line of fire
30:10the good news is that we are probably not right in the direct firing line of apep
30:16the axis of rotation of the apep system is pointed 30 degrees away from us so if it does blow it's
30:25likely that the jets are going to miss us makes me feel better that this gamma ray burst isn't pointing
30:32at us but of course there are many other cosmic catastrophes potentially waiting to get us
30:38apep is on the edge of an enormous explosion its huge gravity and incredible spin should produce a
30:46a spectacular supernova but what if some stars are too big to blow
30:59galaxy ngc 6946 a local galaxy just 20 million light years away and well known to supernova detectives
31:22the fireworks galaxy because it has produced so many supernovas in the past century
31:29and they noticed that one star that they thought would become a supernova instead blinked out
31:34the star under investigation is n 6946 bh1 a cosmic heavyweight 25 times the mass of our sun
31:47that's way more than the eight solar masses we thought guaranteed a supernova this is a very massive
31:57very luminous star the prototype of what you expect to explode as a supernova
32:03and over the last couple of years its brightness has been changing maybe the star was beginning to
32:09go a bit unstable but then right in front of our eyes this star just completely disappeared
32:20this is a huge mystery why didn't this thing blow up how can a star just disappear there had to be
32:29something left behind so astronomers began a search for evidence and found a crucial clue
32:40when you look in the infrared you could still see some light there so there was something happening
32:45there but what we think the infrared light is heat coming off the debris of the dead star
32:53something is pulling it inwards something powerful but also invisible a black hole
33:06the outer stuff from the star is still falling onto that black hole
33:09and it's powering a little bit of light a little bit of the infrared light still gets out
33:13how can a giant star become a black hole without exploding into a supernova first
33:26the answer lies in how dying stars burn their fuel
33:32for stars that are about say 20 times the mass of the sun you're actually going to burn things
33:37convectively that means the gases inside the core are moving around a good analogy
33:43is water in a boiling pot of water you've got your potatoes up here you're trying to boil them
33:49you've got convective cells of water that are heated bring the heat up to the top get the potatoes hot
33:56and then those blobs of water cool down become denser and settle down to the bottom again where they're
34:01heated once more as fusion turns hydrogen to helium and then to carbon convection mixes the carbon so it
34:12burns up convection cells work inside of a star like massive elevators that take hot gas from the
34:24central regions bring it up to the surface allow it to cool and then pull that material back down
34:31they're constantly churning back and forth inside of a star
34:35but stars more massive than roughly 20 times the mass of the sun like n6946 bh1 don't burn carbon this way
34:50instead of mixing the heavier atoms created by the fusion reactions just start to pile up
34:57that means there's a layer of very dense material building up on just the surface of the core
35:02all of the stuff is just ready to collapse
35:06it's possible that if you have enough mass sitting around the collapse is so powerful that it actually
35:12collapses into a black hole before any supernova goes off that then is a failed supernova
35:19it's a star that pretty much directly collapses to form a black hole
35:22if many of the massive stars we expect to go supernova won't that's a problem
35:37we used to think we had the basics of supernovas cracked anytime you have a star more massive than
35:41eight times the mass of the sun it was destined to explode as a supernova and then along comes a star
35:47that screws everything up to make things worse we found no clear distinction between stars that go
35:55out with a bang and those that don't as many as 30 percent of massive stars could die without exploding
36:05our search for the next killer supernova is getting even harder stars blow up when we don't expect them
36:12to they don't blow up when we expect them to they can have several stars orbiting each other and the
36:18one that blows up isn't necessarily the one you think it will
36:24so right now we can't identify a prime suspect but the hunt continues
36:31as far as we know there are no life threatening stars out there but
36:35we haven't done a complete survey so please keep funding astronomy so we can keep looking
36:42supernovas destroy supernovas destroy but can they also create
36:52did a supernova spark humanity's rise to dominate our world and our solar system
37:03supernovas are spectacular
37:16devastating and frightening
37:22but without them we wouldn't exist the iron in your blood and the calcium in your bones was
37:29literally forged inside of a star that exploded billions of years ago as a supernova and i think
37:36this is one of the most beautiful and the most profound things that we've learned in astronomy that
37:41we're literally viscerally connected to the cosmos and the cosmos is connected to us
37:47with every breath we are inhaling oxygen that was created in a supernova explosion
37:59this is almost literally a cosmic cycle of life
38:03and the supernova may even be responsible for the dawning of our intelligence
38:15by causing lightning it might sound rather incredible but a supernova might actually influence
38:21directly weather right here on the earth the cosmic rays from a supernova will create charges in the lower
38:27atmosphere that energy will break apart molecules excite atoms and molecules and it will ionize them
38:35and an ionized atmosphere means that now it can conduct electricity so it probably increased lightning across the planet
38:47it's possible the same gamma ray burst that caused a mass extinction
38:512.6 million years ago also affected earth's atmosphere
38:58triggering tremendous bursts of lightning which caused
39:04forest fires we have evidence of widespread fires at this time so it could be that lightning was increased
39:13and that created more fires and those fires could have leveled forests and savannas creating grasslands
39:19so how could this change boost our intelligence
39:25with their forest homes burnt our ancestors early hominids had to adapt to life out in the open
39:32which meant standing up you're living in a savannah where there's lions and leopards and cheetahs
39:39and the savannah is mostly grassland it's a lot more efficient perhaps on two feet you can run
39:45and moving on two feet might have been the survival mechanism standing upright also triggered the most
39:53important change in our history walking around on two feet freed our hands to be able to start doing
40:01things and as you you know of course you can you can imagine as you start doing things that drives your
40:05brain to more complexity as you're trying to figure out how to manipulate things and this is perhaps the
40:10biggest evolutionary leap because without it we don't get tool use we don't get fire we don't get intelligence
40:19as our ancient ancestors adapted to their new habitat they took their first steps toward world domination
40:27at least that's the theory
40:29the idea presented here is this would be the dawn of modern humans as we see it and we would owe that
40:38to lightning created from a gamma ray burst that's nuts
40:46supernovas are extraordinary they launched our journey into the cosmos and in time a supernova may end it
40:55we're searching hard to spot which one it could be but for now the only way we'll know for sure is when
41:05it lights up our sky while a supernova might appear to be the death of a star the beauty of it is that
41:15it's really a story about beginnings as well
41:23supernovae giveth and they taketh away without supernovae the earth wouldn't exist and we wouldn't exist
41:32i actually do imagine standing out on a nice winter night looking up at beetlejuice
41:37and actually seeing the thing explode there would be this bright light i can imagine my face lighting up
41:43i would really lose it i would love to see a supernova up close right i mean what a light show
41:51but there's no way i would want to be that close because i don't want to die
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