How the Universe Works - S08E04 - Death of the Last Stars
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00:00Imagine a universe with no stars. A dark, endless night. This is not some sci-fi nightmare.
00:17This is our future.
00:21There will definitely be a point in the future when you look up, you will no longer be able
00:25to see stars. Things really will get darker and darker, until there will be almost no
00:30memory of light left.
00:33For billions of years, stars brought life to the universe.
00:39The fact that you exist, at all, is because of stars.
00:46Now they're dying out in a star apocalypse.
00:50The effect could be tremendous. It can permeate throughout the universe.
00:56What's causing the die-off? And what happens to life when the lights go out?
01:03Eventually, the whole entire universe starts to get a little bit...weird.
01:20For over 4.5 billion years, the sun has bathed our home planet with light.
01:31Its bright, stable glow helps life flourish.
01:36But hidden in the night sky, other planetary systems haven't been so lucky.
01:43Hanging right above your head every night, we see up there these dead corpses of stars.
01:50Four hundred light years from Earth lies the SDSS J1228 system, a disk of debris orbits
01:58the faintly glowing leftovers of a dead star.
02:03J1228 is a dead star. It is the core of a star that had aged, blown off its outer layers,
02:10revealed the core, which is about the size of the Earth, but has about half the mass
02:14of the star in it. And we call these white dwarfs.
02:25May 2018. Astronomers investigated J1228 using the world's largest optical telescope, the
02:34Grand Telescopio Canarius. They discovered what appears to be a ball of iron orbiting
02:41the white dwarf. The lump of metal, less than 400 miles across, could be the exposed core
02:50of a destroyed planet. It's a clue to this system's past.
02:57It's always a little poignant when you see evidence of a planet around a dead star. You
03:01think back to when that star was shining and could there have been life in that solar system.
03:06The J1228 system is a cosmic graveyard. It might look different to our own solar system,
03:14but this is our future.
03:18This discovery of a dead planet orbiting a dead star is like looking into a crystal ball.
03:25And is it the future of our own solar system? Yep.
03:29For a glimpse into your future, all you need to do is look up.
03:38Just like J1228, our sun will die, killing off Earth in the process.
03:46This terrifying fate will play out across the galaxy in a star apocalypse.
03:53Our sun is a fairly common type of star in the Milky Way, and so other stars in the Milky
03:59Way will undergo the same sort of fate as the sun and will end up as white dwarfs. And
04:03so any other planets out there orbiting sun-like stars will undergo a similar fate.
04:11Once the stars like our sun have died out, what's going to happen? Could life still survive
04:15around white dwarfs?
04:19July 2018. Heliophysicists tried something a little different. They didn't look at a
04:27star. They listened.
04:30In a sense, stars are noisy. There is gas moving around inside of them. There's turbulence.
04:36There's huge parcels of gas moving up and down. It's not really an orchestra playing
04:41coherently this beautiful music. It's more of a cacophony. It's more like just a bunch
04:45of people shouting for attention. But all of this comes together to make the surface
04:49vibrate, and this is telling us what's going on deep, deep inside the star, which we cannot
04:54see directly otherwise.
04:58Just as the seismic waves from earthquakes tell us about our planet's interior, this
05:03cosmic performance grants us special access into our sun.
05:09It provides a window into the physics of the interior of stars. What are they doing inside?
05:16To understand the fate of sun-like stars, we must look inside them. Buried within are
05:22clues to how they live and why they die.
05:26The core, the very center, that's where the action is. That's where the star is fusing
05:31light elements into heavier elements. That works like a hydrogen bomb. It's the same
05:36thing. If you compress hydrogen enough, it gets very hot, and the pressure gets very
05:42high, and it fuses into helium and generates energy, heat. And that's what's happening
05:47in the core of every star.
05:50Because of their enormous mass, stars have huge amounts of gravity. This gravity pushes
05:56inwards, trying to collapse the star. But fusion energy from the core stops that happening.
06:04It's really this sort of very balanced dance between gravity pushing in, fusion energy
06:11pushing out.
06:12You can think of a star as losing energy continuously to the outside world, and gravity is saying,
06:19yes, I'm going to take over, but no. The nuclear reactions inside a star replenish
06:26the energy that's lost and keep the star hot and pressurized inside so that the pressure-gravity
06:34balance can be maintained.
06:38This balance keeps sun-like stars alive for up to 10 billion years, until the star's gas
06:45tank runs dry.
06:49It's going to run out of fuel, and when that happens, it's going to die. But what is that
06:54going to look like? How is this going to happen?
06:59One hundred million years ago, things in the J1228 system started to get ugly. First, the
07:07star grew large, really large.
07:12Once the center starts fusing heavier elements, the outside will swell into what will eventually
07:17be a red giant star.
07:20J1228 transformed into a red giant. Its outer layers blew off, extending out over 40 million
07:30miles.
07:32When stars like our sun die, it's not a quiet affair. It's very violent and ugly and messy.
07:38They turn into red giants, and they turn themselves inside out and vomit all over the
07:43solar system.
07:47When J1228 swelled into a red giant, nearby planets were stuck in a kill zone. The dying
07:56star engulfed them, or fried them with temperatures of over 1200 degrees Fahrenheit. Atmospheres
08:04disappeared. Oceans boiled away. But one planet survived J1228's death throes.
08:13Here's a case where a planet survived, in some sense, the death of its own star and
08:20is still hanging around, still hanging on, hoping for something new.
08:26The red giant's expanding outer layers separated from the star's core.
08:33With no active fusion, the core collapsed into a white dwarf.
08:37The white dwarf's dense gravity then went to work on the one surviving planet.
08:45A planet that might have been orbiting the normal star can gradually spiral in toward
08:51the white dwarf, and then eventually the gravity of the white dwarf pulls on the near side
08:57of the planet more than on the far side, and that tears it apart.
09:02What we're seeing here is a dead star dining on its own solar system. That's what is in
09:10the future for the sun.
09:16J1228 feasted on the remains of its rocky worlds, leaving behind a disk of debris and
09:23the planetary core. It's a glimpse of Earth's future.
09:31What happened here around this white dwarf is going to happen to Earth. It's going to
09:35be stripped of its atmosphere, its crust, and its mantle, and the only thing that will
09:40remain will be the core.
09:44Fried and ripped apart by a dying star is not a good way to go. Fortunately for life
09:50on Earth, our own sun isn't dying just yet.
09:55The sun is middle-aged. It's four and a half billion years old, and it's going to go on
09:59for another five or six billion years. We've got a little bit of time before our sun pukes
10:05all over the solar system.
10:09Our home planet may be safe for now, but systems like J1228 show us that sun-like stars are
10:16destined to die, killing off any life orbiting them.
10:23Sun-like stars aren't the only stars dying across the cosmos. There are others out there,
10:29and they are all doomed.
10:33There's a wonderful rainbow of stars out there of all different shapes, all different sizes,
10:38and all different colors.
10:41We're talking down to, you know, fractions of the mass of the sun, up to hundreds of
10:45times the mass of the sun.
10:47When it comes to the star apocalypse, size matters. The bigger and brighter the star,
10:54the faster it dies.
11:05Our universe is a vast expanse of death and destruction. All the stars are destined to
11:13die, but not all at once.
11:19There's not going to be one particular point where all the lights turn off at the same
11:23time. It's more like a power outage, where different grids go off at different times
11:29until, like, there's the one last light bulb that'll just go off.
11:35This is because stars come in different sizes.
11:39The way a star dies has everything to do with the amount of mass it started life with. It
11:44carries that all the way through its lifetime.
11:47The sun is a medium-sized star, living a stable existence for billions of years.
11:55Giant stars are different. They live fast and die young.
12:00A star like the sun, which is a medium-sized star, it lives about 10 billion years. The
12:08really massive stars, they live maybe 10 million years.
12:14Massive stars can be tens or even hundreds of times more massive than the sun. When it
12:19comes to lifespan, that's a problem.
12:24A massive star has more fuel to burn in a nuclear sense. So you might naively think
12:30that it lasts longer, but it's the exact opposite.
12:34Massive stars can only access hydrogen fuel in their core. The rest is trapped in the
12:41outer layers and can't be used as fuel.
12:45If there's hydrogen in the core, you're good. If there's hydrogen outside of the core, it
12:50can't be used. If it's not in your fuel tank, it's not doing you any good.
12:56Massive stars also have more gravity than smaller stars. So they have to burn their
13:03hydrogen fuel faster to prevent the star collapsing.
13:09They burn their candle on both ends. Because of their incredible mass, their fusion reactions
13:15in the core happen at an incredible rate.
13:18Giant stars are kind of fast and furious. They are bright, they live their life, and
13:24they die very quickly.
13:27When a giant star's fuel runs out, the core collapses catastrophically under the overwhelming
13:34force of gravity.
13:36And then, boom, supernova.
13:46The death of a giant star triggers one of the biggest bangs in the universe. The blast
13:56would instantly vaporize nearby planets.
14:03These star deaths are also critical for life.
14:08When massive stars die, they release heavy elements they've been making through the course
14:12of their lives. And sometimes they even make new ones. And it's these heavier elements
14:17that are essential for life.
14:21We owe our existence to stars that formed billions of years ago.
14:31In May 2018, we spotted evidence of ancient stars creating the stuff of life.
14:38We picked up an infrared light signal from a distant galaxy named MACS 1149-JD1.
14:49The signal was ionized oxygen.
14:54It's been traveling for 13.3 billion years, so the oxygen formed when the universe was
15:01very young.
15:05Just 500 million years after the Big Bang, this oxygen formed in the hearts of massive
15:12stars.
15:15The presence of oxygen tells us that there needed to be massive stars in the early universe
15:19in order to synthesize hydrogen and helium into heavier elements like oxygen, and then
15:24explode to eject that oxygen back into the interstellar and intergalactic medium.
15:33Extreme pressure in the cores of the stars produces oxygen and other elements like carbon
15:42and nitrogen.
15:44Supernova blasts spread these elements across the universe, helping to create new generations
15:50of stars, and most importantly, us.
15:57If there is one single fact that you should care about in all of science, and this is
16:01my favorite fact, is that you and I are a consequence of star death.
16:10Before you can have life, you need to have the kind of elements out of which life forms.
16:15You need carbon, you need nitrogen, you need oxygen.
16:18You need the elements that are the backbone to the biology that makes us possible.
16:24Where did those elements come from?
16:25Well, they came from stars.
16:27They came from stars that formed in the early universe before even the sun existed.
16:32The huge size of massive stars quickly signs their death warrants.
16:38Their explosive ends helps create new stars, and even life.
16:44The fact that you exist at all is because of stars.
16:50But probing galaxies across the universe, we've discovered something else.
16:56The star apocalypse isn't just killing stars, it's stopping them from ever being born.
17:04Star formation is dying, and in fact it's dying rather quickly.
17:08The universe right before our eyes is becoming a darker place.
17:12It's running out of fuel, and eventually no more stars will be made at all.
17:28Life on Earth follows a series of regular patterns.
17:32Day after day, the sun rises and sets.
17:39And stars light up the darkness of the night sky.
17:44The reason I got into astronomy to begin with was because I grew up in a rural part of the
17:48country and the sky was beautiful and dark.
17:50You go outside at night and you look up and you can see thousands of stars.
17:55But it won't be that way forever.
17:582016, a network of telescopes across the world measured the energy output of over 200,000
18:09galaxies.
18:10They discovered that in the last two billion years, the universe has lost half its brightness.
18:19The night sky is getting darker as stars flicker out of existence.
18:26Ten billion years ago, the universe kind of hit its peak and lots of stars were shining.
18:33It was an incredibly bright place.
18:35But in the last couple billion years, it's really overall become a less bright place.
18:42The darkening universe isn't just a sign that stars are dying.
18:47It seems there's a problem with star birth as well.
18:52When we look into the universe's past, what we find is that long ago, stars were forming
18:57at a much higher rate.
18:58Right now, what we see is that really stars are dying off faster than they're being born.
19:05A Milky Way-type galaxy today produces about seven stars per year.
19:09However, 11 billion years ago, a galaxy like our own would have produced ten times more
19:15stars.
19:17In the early universe, old stars died and new ones formed in their place from the material
19:24left over.
19:25It was a cycle that kept the cosmos bright.
19:29Not anymore.
19:32It kind of sucks for us.
19:34We like a bright universe.
19:35We like all this energy and life that's vibrating through the universe.
19:40But that's just not always going to be the case.
19:43The universe is already winding down.
19:49One of the biggest mysteries in galaxy evolution is figuring out how galaxies stop forming
19:54their stars.
19:55We really don't know the answer yet, and it's really important for us to figure out why.
20:01Because in the end, stars really equal life.
20:06To find out what is shutting off the stars, we study galaxy clusters.
20:16These giant regions of space contain hundreds of galaxies bound together by gravity.
20:25Slowly the clusters pull new galaxies into them, causing something strange to happen.
20:32What we see happening when a galaxy falls into a cluster is that its star formation
20:37is quenched.
20:38It's shut off.
20:40The cause of this quenching effect has baffled scientists for decades.
20:47Then in October 2018, an international team of astronomers investigated this long-standing
20:54mystery.
20:57They tracked variations in quenching across 14 galaxy clusters and found a possible explanation.
21:06The ability a galaxy has to make new stars is related to the larger environment it finds
21:11itself in.
21:12In clusters of galaxies where many galaxies are orbiting around each other, we see interactions
21:17that strip gas and dust away from galaxies, the stuff that makes up stars literally just
21:24thrown off into space.
21:27Stars form from dense parcels of cold gas, something galaxies are filled with.
21:34But when a galaxy is dragged into a cluster, everything changes.
21:40Clusters of galaxies contain a lot of hot gas, whereas you need cold gas inside of a
21:45galaxy in order to form stars.
21:47And when a galaxy is moving through this hot gas, then the cold gas inside is stripped
21:53away.
21:55If this new study is right, and galaxy clusters are stripping away star-forming gas, new starlight
22:03will become rare.
22:06Looking over the history of the universe and how much gas was out there and how much is
22:11still left, I think it's fair to say that most of the stars that will ever be made already
22:17have been made.
22:18They've already been born.
22:22Thanks to the shortage of star-forming gas, stars won't just be dying in the universe.
22:28They'll go extinct.
22:29And the first to go will be the largest.
22:34As the universe runs out of gas, fewer of these stars are being made.
22:38Eventually, sometime in the future, all the high-mass and even medium-mass stars like
22:43the sun, they'll be gone.
22:45What does that mean for life?
22:51Some of the brightest stars will disappear forever.
22:55Can life survive the monsters that dead stars leave behind?
23:02The long-term fate of the universe is not a pretty sight.
23:07Some very interesting creatures can start to appear.
23:18In the star apocalypse, the first stars to fade away will be the brightest.
23:24The giant stars, followed by the mid-sized suns.
23:30The universe will become unrecognizable.
23:34The far future will be a very dim universe, especially for creatures like us.
23:39If there's no more gas, no more new stars, it gets dark.
23:45Scared of the dark?
23:47You will be.
23:49Because 100 billion years from now, in the shadows of this new universe, monsters will
23:56come out to play.
23:58Now we find ourselves in the era of stars and starlight.
24:02What comes after you could think of as the era of the dead corpses of old stars.
24:08We already see the corpses of dead stars scattered throughout the cosmos.
24:14Black holes, pulsars, white dwarfs.
24:20What happens when more stars die out and the dead take over?
24:25Can life survive?
24:27It's actually possible that life in the universe will survive, but we're going to have to get
24:31more creative.
24:34January 2019.
24:38The Gaia satellite studied 15,000 white dwarfs within 300 light years of Earth.
24:46These are the corpses of sun-like stars.
24:50White dwarfs are the remnants, the cores, of stars like the sun after they die.
24:55There's no more fusion going on inside of a white dwarf, so it's just kind of sitting
24:58there cooling off.
25:00But it turns out there's a slight reprieve.
25:06White dwarf corpses usually cool off and dim over tens of billions of years.
25:12Gaia's data showed something different, something we've never seen before.
25:18Some of the older dead stars aren't dimming at all.
25:23We used to think that white dwarfs could really only dim over time.
25:27After all, there's no source of fusion, no source of energy in their interiors.
25:32But new studies with the Gaia satellite have shown that there must be some other energy
25:37source keeping those older white dwarfs shining bright.
25:43Something is giving these white dwarf corpses a spark, bringing them back from the dead
25:49as zombies.
25:52The leading contender is that the insides of white dwarfs actually crystallize.
26:00Up to six billion years after dying, the hot carbon and oxygen matter inside the white
26:06dwarf cools and crystallizes, becoming solid, giving the dead star a lifeline.
26:14This actually releases energy.
26:17As the star cools, it winds up releasing a little bit more energy than it otherwise would.
26:23This unusual heat source could warm up a nearby frozen planet, giving life a second
26:29chance.
26:30There will be some extra energy available from these objects, so this is the time that
26:35we have to cuddle up close to the zombies.
26:40Crystallization can rejuvenate old white dwarfs, and the process could even provide a spectacular
26:47setting for an orbiting planet.
26:51We have a special name for cooled down crystallized carbon and oxygen.
26:55We call them diamonds.
26:57The long-term fate of our universe will be sprinkled with all these glittering diamonds.
27:05A zombie that comes to life and shines like a diamond might be pretty to look at, but
27:10it's still no guarantee that life could survive here.
27:16You can kind of think of these white dwarfs as maybe making a little more energy for the
27:22universe, but even that's going to eventually run out.
27:25The whole thing becomes a gigantic crystal, and again, it's just going to start cooling
27:30and fading away.
27:33The zombie fizzles out into a dark cinder, giving off almost no light at all.
27:41There's another monster lurking in the cosmos.
27:48When a star that's much more massive than the sun dies, it explodes violently.
27:52And during that explosion, the core collapses and becomes an incredibly dense, small object,
27:58one of the most wonderful, real monsters in the universe.
28:05This is a pulsar, PSR B0329 plus 54, 3,000 light years away from us.
28:14The pulsar has the mass of the sun, but is just 12 miles across.
28:19Its rapid spin generates beams of radiation from its poles, bringing the zombie to life.
28:29Now we've discovered an alien world orbiting this zombie star.
28:36In 2017, a new planet was discovered around a pulsar, about twice the mass of the earth,
28:43and that's really incredible.
28:46This pulsar planet sounds intriguing, but the prospects for life aren't good.
28:53Orbiting a pulsar would be a brutal environment for life.
28:58It's highly unlikely that there's life, because the radiation from the system would be overwhelming
29:03and likely blow away the atmosphere.
29:10Perhaps it's time to think outside the box, by looking at one of the most mysterious objects
29:16in the universe, a brown dwarf.
29:21A brown dwarf is basically a failed star.
29:24A brown dwarf is a star that wasn't quite big enough or massive enough to cause hydrogen
29:28fusion at its center.
29:31Brown dwarfs are much smaller than the sun, but they do generate some heat, and they have
29:39enough gravity to hold planets in orbit.
29:43But compared to a fully fledged star, they're small fry.
29:49You could imagine a scenario where a brown dwarf forms with, you know, a family of planets,
29:55but these planets won't get a lot of heat and light from that brown dwarf.
30:02Planets orbiting a brown dwarf would freeze very quickly.
30:07But these almost stars do have a trick up their sleeve.
30:14Brown dwarfs are incredibly common, and they often form binary pairs with each other.
30:20What if you had two brown dwarfs that were right at that limit, just not quite enough
30:25material to actually start fusion?
30:28Over time, if they were in a binary star system, they could spiral together, maybe even collide
30:32and combine into a single star.
30:35And then these two failed stars would join together and finally begin to shine.
30:39Two failed stars collide, creating a new Frankenstar.
30:45You could potentially form an actual legitimate star.
30:50A star can start shining and warm up those planets.
30:55Maybe there's a chance for life after all.
30:59As for sustaining life in the universe, none of these options is what you'd call a safe bet.
31:09These are momentary reprieves from the inevitable.
31:12No matter what you do, eventually you're going to run out of these gimmies.
31:17You're going to run out of the get-out-of-jail-free cards.
31:20Inevitably, everything is going to cool and fade away.
31:27This might be game over for stars, and even life.
31:33But there's still a glimmer of hope, hidden in the cosmos.
31:38A star that's not dying.
31:40It appears blessed with eternal life, and its color is red.
31:47Red dwarfs.
31:48We are literally surrounded by them, but they are largely invisible to us.
31:58Illuminating every corner of our night sky is the light of stars.
32:04But what we see with the naked eye doesn't tell the whole story.
32:11The stars that you're seeing are mainly stars like the sun, or even more massive and even
32:15hotter than the sun.
32:16They're bright.
32:17You can see them from a distance.
32:19But amazingly, the most common form of star, by far, are the red dwarf stars.
32:25They're up there right now in the sky, but they're just too small and too faint to see.
32:31Red dwarfs are up to ten times smaller than the sun, and they burn less brightly.
32:37Right now, hidden in the night sky, over three-quarters of the stars in our galaxy are red dwarfs.
32:47And while the larger stars are dying out, we've never seen a red dwarf die, making them
32:53the best bet for life to survive the star apocalypse.
32:58When the most massive stars eventually go out and are not replaced, what will be left
33:05are much, much dimmer stars like red dwarf stars.
33:10We've seen star death across the universe, so why not red dwarfs?
33:15It turns out their size gives them a crucial advantage over larger stars.
33:21The more massive a star is, the hotter it burns.
33:24A red dwarf star burns at a lower temperature, so it doesn't burn through its fuel quite
33:28as quickly as a mid-mass star does.
33:31These are like the economy cars of the universe.
33:34They're just sipping on their nuclear fuel, and they can coast along.
33:39Not only that, despite being smaller, they have access to more fuel.
33:46Our mid-sized sun is split into three layers, a core, a radiation zone, and a convective
33:54layer.
33:55The radiation zone prevents hydrogen in the top layer ever becoming available to the core
34:01to burn, so the sun can only access about 10% of its total hydrogen fuel.
34:09Once the hydrogen in our sun's core runs out, its days are numbered.
34:15In some ways, these mid-sized stars end up starving themselves.
34:19The smaller red dwarfs are different.
34:22They can access all the hydrogen they want.
34:26In low-mass stars, outside of the core, this outer layer is fully convective.
34:32What that means is stuff near the core rises to the surface and then drops back down all
34:37the way to the core.
34:39And that means if you have hydrogen somewhere outside of the core, eventually it's going
34:43to make its way down there, and it can be used for fuel.
34:46The red dwarf has access to everything at the all-you-can-eat buffet.
34:50It can grab stuff from the distant regions at the surface of the star and bring it all
34:55the way down the gullet to the heart of the star.
35:00This all-you-can-eat hydrogen buffet extends the lifespan of red dwarfs to incredible lengths.
35:08The universe is over 13 billion years old, but any red dwarf that age is a toddler.
35:16A red dwarf, even if it was born at the very beginning of the universe when red dwarfs
35:20could first form, even today, it's just a tiny fraction of its lifespan.
35:25They can last for trillions of years, thousands of times the current age of the universe.
35:31Thirteen billion years old, that seems like a long time, but a small red dwarf, it's barely
35:38out of diapers.
35:42Red dwarf stars will not die out for 10 trillion years or more, and we're discovering they
35:49have another trump card that's good news for life.
35:57February 2017, NASA announced the discovery of a system in the Aquarius constellation
36:04called TRAPPIST-1, where seven Earth-sized planets orbit a red dwarf star.
36:12It turns out that red dwarfs apparently are really good at making planets, including planets
36:19that are roughly the size of the Earth.
36:22That's really cool because these stars last a long time.
36:27If they have planets orbiting them with life, they could outlast our solar system by trillions
36:32of years.
36:35Sounds promising, but red dwarfs have an ugly side.
36:41In October 2018, astronomers turned the Hubble Space Telescope to a series of young red dwarf
36:48stars in the Tucana Horologium Association.
36:53They witnessed these infants throwing daily stellar tantrums.
36:59Even though they're the smallest stars, they actually have some of the strongest flares
37:03and storms on them.
37:05Red dwarfs can emit flares 10,000 times more powerful than the sun.
37:11These flares would cook any nearby planets.
37:15When a red dwarf star forms, they're rotating very rapidly, and this creates a lot of magnetic
37:21activity, which creates flares and mass ejections.
37:26For life to exist, it would have to wait for infant red dwarfs to grow up.
37:33As a red dwarf gets older, there's drag between the magnetic fields in space as it
37:39rotates, and that has the effect of slowing down its rate of rotation, and so this means
37:43the activity settles down.
37:45So maybe later in the life of a red dwarf star, they can support planets with life.
37:54Red dwarf stars will dominate the future universe, and may give life a chance to survive.
38:02These small red stars are extremely long-lived, but no star is immortal.
38:09Even though they're really going through their nuclear fuel very slowly, there's just not
38:15enough fuel to last forever.
38:18These little stars will die out eventually.
38:22Unlike their larger stellar siblings, they'll go quietly.
38:27What actually just gets hotter, and the color of a star depends on its temperature.
38:32So as the red dwarf gets hotter, it turns bluer.
38:35So sometime in the very distant future, some of these red dwarfs are actually going to
38:40become blue dwarfs.
38:42The universe isn't old enough for blue dwarfs to exist yet.
38:47But trillions of years from now, a dim blue glow will complete the star apocalypse.
38:55There will be a last star.
38:58One last red dwarf may be now turning blue as it warms up, but it too will eventually
39:04cool off, fade away.
39:08And there will be no more stars in the universe.
39:10It is inevitable.
39:14In this dark, starless universe, prospects for life seem impossible.
39:20But will something else take the place of stars?
39:27The universe at this time is nothing like the universe of today.
39:30There's no light, and it's really cold and very lonely.
39:37When all of the stars die and the light goes away, anything that relies on the heat and
39:41the processes from these stars will start to die.
39:47Once all the lights go out, the only things that will remain will be the leftovers.
39:53With stars as we know them long gone, could something else spark into existence in this
39:59cosmic wasteland?
40:01You think that's it, no more star formation.
40:04But the universe still has a few tricks up its sleeve.
40:08Over the history of the universe, generations of stars have lived and died.
40:13They released heavy metal elements into the universe, building materials for a new kind
40:19of star.
40:20And stars born from these new materials can do things their ancestors couldn't.
40:28As you enrich the universe, as more and more metals get produced over time, you can lower
40:35the temperature needed for fusion reactions in a star.
40:40With lower temperatures needed for fusion, stars have become smaller and smaller.
40:47Currently, the smallest possible star is a little under 10% the sun's mass, but eventually
40:52it may be possible to form stars that have around 4% the sun's mass.
40:59Hundreds of trillions of years in the future, a new star may dominate the universe, built
41:05from scraps left over from generations of dead stars.
41:10A star so small that it burns cold instead of hot.
41:18One of the weirdest types of stars that scientists hypothesize might exist in the far future
41:23is the frozen star.
41:26You can start forming stars that are very, very small and very cold, where nuclear fusion
41:33is happening in the core, but the surfaces are cold.
41:42These small cold objects will be thousands of times dimmer than the faintest star we
41:47see today.
41:49So cold, the temperatures on the surface could reach just 32 degrees Fahrenheit, and ice
41:56clouds may form in the star's atmosphere.
42:01They are so much cooler than stars now, they could actually have ice, water ice, on their
42:08surface, even though they are technically stars.
42:12It's literal water ice covering the surface of a star.
42:16The same ice that you can use for ice skating, or ice racing, or curling.
42:22You could do all of this on the surface of a star in the far future.
42:28It's hard to predict if life could arise on planets orbiting frozen stars.
42:36We won't know until one appears.
42:41And that won't be for a long, long time.
42:45The universe is far too young for even the first one of these things to even be a glimmer
42:50of an idea.
42:52So if you want to wait, you know, a quadrillion years, eh, we can find out.
42:57Stars help create us, building and spreading the ingredients for life to develop.
43:03But the coming star apocalypse may mean the end of life, just not for a while.
43:12Small red stars will continue to illuminate the darkness, safe havens for life to survive
43:19and even flourish.
43:21As for us on Earth, we should be most thankful for one star.
43:27Because without it, we simply wouldn't exist.
43:31I really want you to never experience a sunny day again and not think about this.
43:36The sun someday will burn out, and so will all of the other stars.
43:40We are in this wonderful era of light and warmth coming out of the sky, and everything
43:46is going to go dark, absolutely everything, everywhere in the universe.
43:51So for the time being, you know, enjoy the light, step outside, enjoy the sun, and think
43:57about how lucky we are to live in this time.