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00:00Our universe is at war.
00:06The universe is a very violent and deadly place.
00:11Entire galaxies fight to the death.
00:14Talk about Clash of the Titans.
00:16It doesn't get more titanic than this.
00:19It's a slaughter.
00:20It's a massacre.
00:21Only the strongest survive.
00:23If a galaxy wants to stay alive, it has to feed on other galaxies.
00:29Our own galaxy also fights for survival.
00:33We are facing the ultimate destruction of the Milky Way galaxy.
00:39These battles are how galaxies live, grow, and die.
00:45These collisions got us to where we are today, and they're going to determine the future
00:49of the universe.
00:59In 2018, astronomers used the Gaia Space Telescope to map our Milky Way galaxy.
01:12They tracked the movements of a billion stars, and they found that some behave very strangely.
01:21When astronomers were mapping stars in our galaxy, they found a whole bunch that were
01:25on similar but very strange orbits.
01:29Most stars in the Milky Way are orbiting in a sort of regular pattern, but these stars
01:34at the center, they're in these highly elongated orbits.
01:37Coming in from very far, swinging around the center of our galaxy, and then going back
01:43out again.
01:44It's a little bit like a comet does.
01:46This group of stars plunges wildly through the center of our galaxy.
01:52When you track their direction and speed on a chart, you get a shape that looks a bit
01:57like a sausage.
02:01This doesn't sound very science-y, but this sausage is really what the stars look like
02:05if you look at the shapes of their orbits in a certain configuration.
02:11What sent so many stars on such a strange path?
02:15It must have been a huge event.
02:17We think these stars are the result of a past cosmic collision.
02:25They are casualties from an enormous battle between the Milky Way and a foreign galactic
02:31army.
02:35They don't move like stars in the Milky Way because they're not from the Milky Way.
02:39These stars are actually alien stars.
02:42They're invaders from outer, outer space.
02:46The attackers left their mark on the Milky Way.
02:50We find similar battle scars on galaxies across the universe.
02:56Our models of galaxy formation are still pretty uncertain.
02:58We still don't really understand how galaxies got to where they are, how we go from the
03:03Big Bang to the Milky Way.
03:06Wars between galaxies have profound consequences for the winners, the losers, and for us.
03:15What we're learning is that these galactic battles have had a huge impact on what the
03:20universe looks like today.
03:22Our understanding of galaxies has changed entirely in the last few decades.
03:26We understand now that every big galaxy like the Milky Way started from many smaller things
03:30colliding, changing each other as they went.
03:34Nearly 10 billion years ago, the sausage stars were part of a foreign galaxy.
03:41It was on a collision course with our home, the Milky Way.
03:46We call this invading army the Sausage Galaxy, or Gaia Enceladus.
03:54The galaxy that we fought probably had about 50 billion stars, so we're talking about something
03:59that is a significant fraction of the size of the Milky Way.
04:05Gaia Enceladus was a tough opponent.
04:10The Milky Way was 20 times its mass, and that makes a huge difference.
04:17When galaxies interact with each other, size definitely matters.
04:21The bigger galaxies are going to dominate over the smaller ones, ripping them apart
04:26and essentially consuming them.
04:29Galaxy interactions are all about bullies.
04:32The bigger you are, the badder you are.
04:35When two galaxies collide, it's like two massive armies marching towards each other.
04:43These galaxies aren't fighting with knives or spears or guns or even nuclear bombs.
04:50They're fighting with something much more powerful, gravity itself.
04:56Each galaxy contains billions of stars and planets, and a supermassive black hole millions
05:03of times the mass of the sun.
05:06That's a lot of gravitational firepower.
05:11As these galaxies approach each other, you can get tidal effects, the same way that the
05:16moon can raise tides on one side of the Earth and the opposite side.
05:22One galaxy can stretch another galaxy along a certain direction.
05:32As Gaia Enceladus advanced towards us, our galaxy's superior gravity grabbed hold of
05:39the smaller galaxy.
05:42As it approached, the gravity from the Milky Way would have stretched it out.
05:47Gaia Enceladus was distorted, but not defeated.
05:52The battle was just beginning.
05:54It would have passed through our galaxy, maybe orbiting a couple of times, before being torn
05:59apart by our gravity.
06:04The Milky Way's gravitational power ripped Gaia Enceladus apart and captured billions
06:11of its stars.
06:15Eventually most of those stars would have then settled down into the disk of the Milky
06:19Way and become a part of it.
06:22Little galaxy try to take all the Milky Way, you're going to get what's coming to you.
06:27Despite winning the battle, the Milky Way suffered serious damage.
06:33The collision with the Sausage Galaxy left a scar on the Milky Way, and when we look
06:37near the center of our galaxy, we see a bulge that's left over from that collision.
06:44The Milky Way isn't the only galaxy scarred by war.
06:50Across the universe, rival armies made up of billions of stars slug it out, leaving
06:57behind distorted and damaged casualties of war.
07:04There's a million different subcategories of them.
07:06There's tadpole galaxies that have long tails, longer than our own galaxy.
07:11There are things like Art Mador 2026, where you see this eerie glowing face, two big eyes
07:16looking right at you from across the universe.
07:20There are galaxies that look like they might have collided with one another and blown holes
07:23through each other.
07:28These battle scars give us important clues about one of the biggest mysteries in astronomy,
07:34how galaxies develop and grow.
07:38But there's a problem.
07:40We can't watch these battles in real time.
07:45The scale of galaxies is huge.
07:47They're hundreds of thousands of light years across.
07:49It's going to take them millions or billions of years to come together.
07:54So it's like looking at one frame from a really energetic fight scene in a movie.
08:00By piecing these snapshots together, astronomers can build up a detailed picture of past conflicts
08:07and discover how these battles transformed galaxies over billions of years.
08:14We have pictures of isolated galaxies.
08:16We have pictures of interacting galaxies.
08:18And we have pictures of aftermath galaxies.
08:22And that's helped us discover something alarming.
08:26The Milky Way faces yet another attack from an enemy armed with an enormous secret weapon.
08:35Will our solar system survive the onslaught?
08:46Across the universe, galaxies are at war.
08:52Their main weapon, gravity.
08:55It tears the combatants into weird and wonderful shapes.
09:00Our galaxy didn't escape the mayhem.
09:04It's peppered with battle scars.
09:08The overall shape of the Milky Way is a flat disk of stars and gas, except recently we
09:14have found out that at the edges, it's actually warped a little bit like the brim of a fedora.
09:19The stars actually dip down below the plane on one side and dip above it on the other.
09:26We think the attacker was one of our satellites, a galaxy that orbits the Milky Way like the
09:32moon orbits the Earth.
09:34It's called the Sagittarius Dwarf Galaxy.
09:38From looking at how the stars move in the Milky Way, we suspect that the Sagittarius
09:43Dwarf Galaxy has actually crashed through the Milky Way a few times on its course of
09:48its orbit around the galaxy.
09:50It came in about six billion years ago, hit the disk hard about two billion years ago,
09:55and crashed again about a billion years ago.
09:57And our gravity has pulled it out into a gigantic looping stream of stars that is moving in
10:03and out of our Milky Way.
10:07The war is not over.
10:09The insurgent galaxy will return.
10:14When galaxies interact, often they're caught in this huge cosmic dance where they revolve
10:19around each other a few times or they even crash through each other and then come back
10:23around.
10:24The Sagittarius Dwarf Galaxy looks like it's crashing in with ever increasing frequency.
10:31A new skirmish could take place in the next hundred million years.
10:36So should we be worried about these attacks?
10:40Because the Sagittarius Dwarf Galaxy is so small compared to the Milky Way, it will do
10:45some damage at the beginning.
10:47But because we're so massive, we can absorb the impact.
10:50I mean, this galaxy, it's looking for a fight, but it's also 10,000 times smaller than us.
10:56So this is going to be no sweat at all.
11:02So far, the Milky Way has been victorious, but the danger isn't over.
11:10We are surrounded by enemies.
11:13Our local neighborhood of galaxies has three major galaxies, but up to 50 smaller ones.
11:21All these galaxies are potential troublemakers.
11:24Each one of these could be armies that rise up against us.
11:30The two most famous galaxies that orbit the Milky Way are the Large and Small Magellanic
11:35Clouds.
11:37These are two independent dwarf galaxies that you can see in the night sky from the Southern
11:41Hemisphere.
11:46We thought the Large Magellanic Cloud orbited our galaxy at a safe distance of 160,000 light
11:53years.
11:55We thought it would stay that way, and we thought it was harmless.
12:00Now a new discovery shows we were wrong on all counts.
12:06The new factor that changed our view of the Magellanic Cloud is we found out it has a
12:11lot more dark matter than we thought.
12:16Dark matter, the most mysterious stuff in the universe.
12:22Dark matter is literally what it sounds like.
12:25It's matter that we cannot see, but it has gravity and can affect objects that we can
12:30see.
12:33Adding in this extra dark matter makes the Large Magellanic Cloud at least twice as massive
12:40as predicted.
12:42So its gravity is double what we thought.
12:47It's secretly been gathering allies, has been gathering dark matter on its side, and now
12:54it's a much bigger threat than we thought before.
12:58So it's not just going to orbit us, it's going to collide with the Milky Way.
13:04Moving at nearly a million miles an hour, the Large Magellanic Cloud will not swing
13:09past us.
13:11It will attack.
13:13The Large Magellanic Cloud is one-tenth the mass of the Milky Way.
13:20That's enough to make a pretty big punch.
13:23In about two and a half billion years, it will smash into our galaxy.
13:32It's going to plow through the disk of the Milky Way.
13:35It's going to blow a cavity.
13:37It might even damage our spiral arms.
13:42Earth sits in one of those spiral arms.
13:47Could our planet become collateral damage?
13:52If the Large Magellanic Cloud passes through the plane of our galaxy near our location,
13:57that could have dire consequences.
14:01The gravitational clash between the invader and the Milky Way could hurl stars and planets
14:07out of our galaxy.
14:10Earth could be one of them.
14:13Our planet's very close to its own star.
14:16So the odds are that you'll just get ripped out along with your star.
14:21So we'd be moving along with the sun, even as the sun gets jettisoned from our galaxy.
14:27And it'll move off out into intergalactic space.
14:30And that's not terrible.
14:31I mean, it's not going to get destroyed, but it's a little lonely.
14:38Our view of the night sky would radically change.
14:43We'd be able to see much more of the Milky Way, especially if we got kicked up above
14:49the plane of the galaxy.
14:50We'd be able to see the whole shebang.
14:53Just look at any image of a spiral galaxy.
14:57They're gorgeous.
14:58Now, imagine seeing your night sky filled with the face on spiral galaxy.
15:04That would be like waking up to my face every morning.
15:07Spectacular.
15:08If we were unlucky, our home planet could have a close encounter with an invading star.
15:21The odds are very low that another star will pass close by the sun.
15:27But those odds aren't zero.
15:29It could happen that another star passes close enough to affect the planets.
15:33And if that were to happen, it could upset the delicate balance in the solar system.
15:40We don't know where the Earth could end up.
15:44It might find its way into the sun.
15:46You just don't know.
15:47Or there might just be a rain of comets into our inner solar system.
15:51Our own planet might be flung out, in which case this would be a death knell for all life
15:57on Earth.
15:58I'm not someone who's like a doom and gloom person, but like that would be insane.
16:04You don't know what's going to happen, but most of the options are bad.
16:10All these nightmare scenarios will extinguish life.
16:18Earth might survive, but our cosmic zip code will take a severe beating.
16:25The Milky Way galaxy is bigger than the Large Magellanic Cloud, so we are going to win,
16:31but it's going to hurt us for a long time.
16:35The Large Magellanic Cloud will leave our galaxy battered, bruised, but ultimately undefeated.
16:43But there's a far bigger threat looming over the Milky Way.
16:47It's going to face an opponent that it can't defeat.
16:52Will this mega-collision be the Milky Way's last stand?
16:59In the not-too-distant future, galactically speaking, a much, much larger battle is due
17:06for the Milky Way.
17:09A battle with a local superpower, the Andromeda Galaxy.
17:15We thought this huge galaxy might wound us in the future.
17:21Now, recent evidence reveals it's going to make a full-scale assault.
17:28We've known for a long time that Andromeda is heading more or less toward us, but we
17:33didn't know exactly in what direction.
17:35But in recent years, we've been able to pinpoint this a lot better, and yeah, it's heading
17:42right for us.
17:46Data from the Hubble Space Telescope shows the two galaxies will collide in about four
17:51billion years, and it will be a monumental battle.
17:59This collision that is coming, and it is coming, is not going to be anything like the Milky
18:04Way has experienced before in its 10 or 12 billion year history.
18:08This is a galaxy of comparable size.
18:11This is two heavyweight prize fighters coming at it.
18:15Warriors with the same gravitational firepower.
18:19Simulations suggest a clash of the titans.
18:23Each of them with half a trillion stars in them.
18:26That sounds like a pretty spectacular collision.
18:30Fights between equally matched galaxies are rare and messy.
18:35When the battle kicks off, there will be no good news for either side.
18:43When the Andromeda galaxy and the Milky Way galaxy start to get close, they're going to
18:48start affecting each other profoundly.
18:50Tendrils of stars are going to be thrown out.
18:53Gas is going to be thrown out.
18:56It won't be a single impact.
19:00Gravity will send the two opponents into a spiraling dance of death.
19:07The first pass is actually not a direct hit.
19:09They're going to swing past each other, in fact.
19:13At this point, their gravitational interaction is going to slow them down and they're going
19:16to come back toward each other.
19:18The galaxies will collide and fly apart again, inflicting more and more damage with each
19:25clash.
19:27If you were to go outside and look up, you could see the disk of our galaxy getting ripped
19:32apart by tidal interactions with Andromeda.
19:38The two beautiful spiral galaxies will tear each other apart, leaving one vast elliptical
19:48galaxy.
19:51The fate of the Andromeda-Milky Way battle is that they will merge.
19:57This is going to be one gigantic galaxy.
20:04And that presents a problem.
20:05What are we going to call this new galaxy?
20:07Of course, my nerd colleagues have come up with names like Milk-Omeda, Androma-Way, whatever.
20:14Those are corny.
20:15We should just call it Hakeem.
20:23With a trillion stars, it will be one of the biggest galaxies in the universe.
20:31In the Hakeem galaxy, things are going to be completely new.
20:35First off, it's going to be a really good-looking galaxy.
20:38Let's get that straight from the get-go.
20:40Second, it's going to be powerful.
20:43And I'm talking powerful.
20:45This may be the most remarkable galaxy in the history of the universe.
20:51Milk-Omeda, or Hakeem if you prefer, will become the undisputed boss of our cosmic neighborhood.
21:00Its calm appearance concealing a history of violence.
21:07It's the result of a complete war zone of mergers over the course of billions of years.
21:12Many galaxies crashing together, fully reconfiguring each time.
21:16And slowly you grow this smooth, placid, big blob of stars.
21:23After billions of years of warfare, our galaxy will finally be peaceful.
21:31But before its honorable discharge, Milk-Omeda may produce one final devastating act of war.
21:41Imagine World War II, and then all of a sudden one of the sides comes up with the Death Star.
21:47That's what we're talking about here.
21:51A weapon of cosmic destruction.
22:06When giant galaxies clash, the battles are spectacular and destructive.
22:15The victors steal huge numbers of stars and vast amounts of gas.
22:21As fuel for the ultimate superweapon.
22:26The special weapon that these monster galaxies have is a giant death ray.
22:32A jet of material racing across thousands of light years.
22:39These huge outbursts of energy blast out of the center of the colliding galaxies.
22:45They produce more energy in one second than the sun will in its entire 10 billion year
22:52lifetime.
22:54We call them jets.
22:58These incredibly powerful jets aren't just brief features.
23:02They can be sustained for millions of years, and they can maintain their structure for
23:07thousands of light years.
23:11It's like turning on a garden hose in Chicago and using it to water a garden in London.
23:18Exactly what triggered these jets was a mystery.
23:21Then in June 2018, astronomers in Hawaii captured something stunning.
23:27A jet forming during a galactic collision.
23:34The team found something really incredible.
23:36They found two galaxies that were in a cosmic collision and actually found an active jet
23:42in one of these galaxies.
23:43It was the first time anything like this has been discovered.
23:46When galaxies collide, the clash drives huge clouds of gas and dust towards their centers.
23:56The supermassive black holes start to feed.
24:01The gas that was in those galaxies starts to funnel toward the black hole and fall upon it.
24:08Not all this gas ends up inside the supermassive black hole.
24:13Powerful magnetic fields carry some of this matter to the poles and blast it out in tight,
24:20narrow jets.
24:21A superweapon is born.
24:25This discovery helps us understand how giant elliptical galaxies form.
24:31Knowing that mergers of spiral galaxies can cause these jets helps us put together a complete
24:36picture of how these huge elliptical galaxies might be formed.
24:43The discovery doesn't answer all our questions.
24:47There's another mystery.
24:48How did the supergiant galaxies that dwarf the Milky Way get so big?
24:54Our Milky Way galaxy is big-ish.
24:58It's slightly bigger than average.
25:00But IC 1101, for example, is more than 50 times larger than our home galaxy and has
25:06more than a trillion, with a T, trillion stars in it.
25:11The biggest galaxies make the Milky Way look like an ant.
25:16These galactic giants pose a problem.
25:20There hasn't been enough time since the birth of the universe for them to become so large,
25:27even by conquering smaller galaxies.
25:30When we look into the distant universe, we see something very strange that we don't quite
25:34understand.
25:35We see enormous galaxies that existed just a billion years after the Big Bang.
25:40And even though these cosmic collisions help explain how galaxies get bigger, they don't
25:45quite explain everything about how galaxies grow over time.
25:49So we still have a big mystery on our hands here.
25:55So in 2019, an international team investigated a very large galaxy over 300 million light
26:02years away.
26:04We call it NGC 6240.
26:08NGC 6240 was being studied because it had two supermassive black holes in it.
26:15Now the galaxy itself looked like it had been disturbed, like something had happened.
26:19They thought that potentially it had had a recent merger.
26:28They were expecting to see two supermassive black holes in the galaxy's heart.
26:34As the researchers peered through the layers of gas and dust, they discovered something
26:39surprising.
26:41What we found was staggering.
26:44We found not two, but three supermassive black holes lurking in the center.
26:55It's the first time we've found a galaxy with three supermassive black holes, evidence of
27:02a three galaxy pileup.
27:05This galaxy is an active battlefield of not two, but three armies colliding at once.
27:14And because there are three armies involved, there are three galaxies involved with three
27:19times as much mass, three times as many stars, three times as much material, and three times
27:25as much violence.
27:27This three-way battle may explain how the largest galaxies got so big so fast.
27:35It could be that galaxy mergers are more frequent than what we thought previously,
27:40and therefore galaxies become more massive faster than previously expected.
27:47In the past, galaxies may have battled and collided more often than today.
27:53Back then, galaxies were more densely packed together.
27:58Our universe is expanding as it ages, which means in the past all the galaxies in the
28:04universe were closer together, and that means they had greater chance for their gravitational
28:09interactions to pull them together and smash them together.
28:16When galaxies fight, the big get bigger.
28:20More mass means more gravity, the vital ingredient for victory.
28:26But galactic conflict doesn't always result in growth.
28:31A strange new astronomical object had scientists confused.
28:39They just looked like stars from the ground.
28:41However, with the advent of Hubble and beautiful space-based telescopes, it was possible to
28:47look at these stars again and actually discover that they were galaxies.
28:58They're kind of crazy.
28:59They're a huge number of stars, but crammed into an incredibly tiny space on an astrophysical
29:06scale, something 500 times smaller than our Milky Way galaxy.
29:12We call them ultra-compact dwarf galaxies, or UCDs.
29:18You might imagine the difference between the Milky Way galaxy and a UCD as the difference
29:25between a cloud and a rock, where the rock is just the same kind of material but compressed
29:32to just incredibly high densities compared to some fluffy, gaseous thing.
29:39What are these strange galaxies?
29:42They seem to break all the rules.
29:45To find out, astronomers zoomed in to a particularly dense ultra-compact dwarf galaxy called M60
29:54UCD-1.
29:55M60 UCD-1 is 300 light years across.
30:00It's tiny.
30:01It's a pinpoint compared to our enormous galaxy.
30:04Our galaxy has 200 or more billion stars in it, and M60 UCD-1 only has 140 million.
30:11But they're packed into this incredibly tight volume.
30:15The night sky inside the galaxy would look very different from our own.
30:21On Earth, when you look at the night sky, you see a few thousand stars, but if you were
30:25in M60 UCD-1, you wouldn't just see a few thousand stars, you would see hundreds of
30:31thousands of stars in the night sky.
30:34That would be amazing.
30:42As the astronomers look deeper into the heart of this tiny galaxy, things got even weirder.
30:50They found a supermassive black hole, much bigger than expected.
30:56It actually has a black hole that's bigger, five times bigger than the black hole at the
31:01center of our Milky Way galaxy.
31:04When we see supermassive black holes inside of galaxies, they tend to scale with the size
31:09of the galaxy itself.
31:11A bigger galaxy has a bigger supermassive black hole.
31:15Why does such a tiny little object have such an oversized central black hole?
31:20The only possible explanation?
31:22This tiny galaxy was once much larger.
31:27These galaxies might have begun their lives as, in fact, much bigger galaxies, and that
31:32what we see today was really just the very central, densest part of a much larger galaxy.
31:40Based on the size of its supermassive black hole, M60 UCD-1 may once have contained many
31:47billions of stars.
31:50Something captured them.
31:52And we don't have to look far to find the aggressor, a nearby supergalaxy with lots
31:58of gravitational firepower, M60.
32:01M60 is a monster.
32:04It has a trillion stars in it.
32:06It's bigger than the Milky Way, and we're pretty big.
32:09The battle was not a full-on frontal assault.
32:12M60 raided its smaller opponent, capturing its troops.
32:18This is more of a stealthy, guerrilla hit and run, where we're going to move in, pick
32:23off some of your troops, and then get out before you even notice.
32:28All that's left from one of these drive-by galaxy interactions is this supermassive black
32:34hole with a fraction of its original stars.
32:38The conflict devastated M60 UCD-1.
32:43Over 98% of its stellar army were captured and became prisoners of war.
32:48It used to be a big galaxy, but it suffered one too many defeats, and now it's a fallen
32:56empire.
32:58We can frame this battle between M60 and M60 UCD-1 as just a battle, but in fact, it's
33:05a slaughter.
33:06It's a massacre.
33:08These small galaxies get all their troops removed, but the HQ, the supermassive black
33:15hole, remains, but it doesn't have any troops left.
33:18Eventually, M60 will conquer its battered opponent, destroying what's left of the compact
33:26galaxy.
33:27It'll get ripped apart even further, and more and more stars will be consumed by the bigger
33:32galaxy.
33:33So, chances are, this little dwarf is eventually going to be pulled apart and become a part
33:38of M60.
33:42But while this little galaxy is down, it is most certainly not yet out.
33:48It's likely to stage a counterattack.
33:51Its final mission, a kamikaze charge that'll leave M60 reeling.
34:01It's absolutely chaotic.
34:05Stars are sent off flying in every direction, and they're moving at really high velocities.
34:13And when stars are stripped out of galaxies, they become what we call rogue stars.
34:20These ejected, runaway stars will leave the galaxy at two million miles an hour.
34:28They'll hurtle through the emptiness of space, never to be seen again.
34:36But even this dramatic assault can't change the inevitable outcome.
34:41M60 UCD-1 is doomed.
34:51In the great game of galactic warfare, losing can be catastrophic.
34:55For weak and small galaxies, resistance is futile.
35:01Pillaged for resources by their more powerful opponents, they slowly become burnt-out wrecks.
35:10But some peaceful galaxies face an equally terrible fate.
35:16They starve to death.
35:25Galactic wars are vicious.
35:27They destroy many galaxies.
35:30But violent conflicts can also give galaxies new life.
35:35Case in point, galaxy NGC 4485.
35:40NGC 4485 has a nickname of the Two-Faced Galaxy, like the Batman villain, because it has two
35:47different halves of the galaxy doing completely different things.
35:50Half of the galaxy is sort of old and calm and relatively quiescent, whereas half of
35:55it appears to be undergoing a sort of fireworks display of new star formation.
36:02Why are new stars only born in one half of this galaxy?
36:08We found a clue on the edge of a photo taken by the Hubble Space Telescope.
36:15It was evidence of an attack by another galaxy.
36:19We think that another galaxy passed through it just off-center in a way that strongly
36:24perturbed the gas on one half of the galaxy.
36:30The Two-Faced Galaxy's skirmish gave it a gravitational jolt, forcing clouds of gas
36:36together.
36:38When we think of galaxies, we think of stars, and of course galaxies are made of stars.
36:42But of course gas is the stuff that stars are made of.
36:48When two galaxies collide, the gravitational duel can trigger a huge burst of star formation.
36:59You need something to give a galaxy a push, and that's exactly what a galaxy collision
37:03does.
37:04And when gas clouds collide, they compress, and when they compress, you get knots in them
37:09that can compress more and form stars.
37:15So you can think of these collisions as very violent events, but ultimately it can breathe
37:19new life into a galaxy.
37:22But the spoils of war don't last long.
37:26In the short term, the victor galaxy can come out glorious with so many new stars.
37:33But this celebration is short-lived, because that round of star formation quickly uses
37:38up the material available.
37:41If a galaxy wants to stay alive, it has to feed on other galaxies.
37:50But what happens when opponents are closely matched in gravitational firepower?
37:57When two equal-mass galaxies meet, when two armies of equal size meet, they can exhaust
38:04all their fuel.
38:06They can kill all their troops, so at the end of it, there's nobody left to fight.
38:10The galaxies themselves can be stripped of gas and dust, all the things you need to actually
38:15make star formation happen.
38:22Once the gas is used up, the galaxy can't create any new stars.
38:26It can only get more gas by looting other galaxies.
38:32We can sometimes see a gas stream coming from one galaxy and joining to the other, and then
38:38the gas from that galaxy is now free and it starts to make stars again.
38:44You're bringing in new ingredients to form stars, new gas, new dust, to become part of
38:49this larger galaxy.
38:50All sorts of weird and wonderful things can happen in galaxy mergers.
38:58A star is born when a huge cloud of gas collapses under gravity, triggering nuclear fusion.
39:06A star ignites and begins to shine.
39:10It's a glorious sight and often a sign that the war is finally ending.
39:16When two galaxies come together, boom, all of a sudden, stars light up all over the place.
39:24In this moment as you catch this system getting excited, the star formation rate's ramping
39:28up and it's sort of a cosmic fireworks display, as if they're celebrating the end of this
39:33hostile encounter with the other galaxy, this war.
39:38But it's also a sign that the victorious galaxy is using up its gas supply.
39:46So galaxies constantly need to raid new targets, and that raises an important question.
39:53What happens if there's a galaxy just alone in space?
39:56Nothing else is colliding with it, sort of a pacifist galaxy.
40:00The poster child for these peace-loving galaxies is NGC 1277.
40:08NGC 1277 is a very peculiar galaxy.
40:11It's pretty big and its stars are extremely old.
40:16It basically hasn't formed new stars in the last 10 billion years, so it's kind of the
40:21veteran's home of galaxies.
40:24NGC 1277 lives in a rough part of the cosmos called the Perseus Cluster.
40:31Thousands of other galaxies surround NGC 1277, and they are all ready for a fight.
40:38So you might ask, why hasn't it had encounters with other galaxies that might rejuvenate it?
40:44The answer, once again, is gravity.
40:48NGC 1277 sits inside this massive galaxy cluster that has a ton of mass.
40:54And if you look at its position, it's fairly near the center of the cluster.
41:01The combined gravity of thousands of galaxies pulls on NGC 1277, accelerating it to 2 million
41:09miles an hour.
41:13And so it has spent the last few billion years traveling faster and faster until now it's
41:18almost at its fastest pace.
41:23It's very hard for gravity to just catch it or catch one of its neighbors and bring them
41:27together to merge with each other.
41:30NGC 1277 has no chance of grabbing new gas to make new stars.
41:38It's dying.
41:39All it has left are old, red stars.
41:43When it comes to galaxies, red is dead.
41:47No new stars means no big stars, no blue stars, just small, dim, red dwarfs.
41:58Galaxies that don't fight just fade away.
42:01At that point, the history of the universe becomes really kind of boring.
42:06All the stars will simply start to die out.
42:08Eventually there will be the last star formed in the Milky Way, with no new galaxy bringing
42:12fresh material.
42:14Without galaxy collisions, the universe dies.
42:24Galactic battles mix things up and replenish gas supplies, and our own galaxy has reaped
42:32the benefits.
42:34Our Milky Way galaxy fought a massive battle, but that battle may have been necessary to
42:41build solar systems like the one we live in right now.
42:48Clashes with the Sagittarius dwarf galaxy occurred at the same time the sun formed.
42:56It's possible that we owe our very existence to the collision with the Sagittarius dwarf
43:01galaxy.
43:03Maybe the gas that ultimately gave rise to the birth of our solar system once came from
43:08another galaxy entirely.
43:12So galactic wars are both creative and destructive.
43:18Galaxies are built from collisions.
43:21Galaxies survive from collisions, and galaxies can also die from collisions.
43:28Far from being destructive events, colliding galaxies may be the reason that you and I
43:33are here.
43:37Galactic warfare has revolutionized our understanding of how galaxies live and die.
43:45Ultimately it's these galaxy mergers that are one of the great engines of all structure
43:49growth in the universe.
43:52These collisions got us to where we are today, and they're going to determine the future
43:56of all the universe.