How the Universe Works - S06E04 - Death of the Milky Way
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00:00The fate of our galaxy hangs in the balance.
00:10The Milky Way is dying, and we don't know why.
00:16Our galaxy, like all galaxies, has a limited lifespan.
00:21After that, it lights out.
00:24The hunt is on to find the cause.
00:29It's safe to say right now there are many ways to kill a galaxy.
00:35It's a cosmic-wide investigation.
00:40Is it murder most foul, or is it death by natural causes?
00:48And every possible reason is under suspicion and will be scrutinized.
00:57It's another example of this big universe of ours throwing puzzles at us that now we
01:01have to solve.
01:05What is killing the Milky Way?
01:28Earth, just one of 100 billion planets, orbiting 400 billion stars that make up an immense
01:40galactic spiral, our home, the Milky Way.
01:51Galaxies are where stars form, and of course planets form around stars.
01:56So the story of the Earth, of yourself, of the solar system, has everything to do with
02:01the story of the galaxies.
02:04Our galaxy's story began 13.6 billion years ago, just after the Big Bang.
02:12It was a time when there were no planets and no stars, just a vast, lumpy soup of superheated
02:22hydrogen gas.
02:26Over millions of years, the temperature dropped and gravity compressed the lumps down, until
02:33eventually the hydrogen molecules fused and ignited a star.
02:43In time, billions of stars burst into life, and the Milky Way started to take shape.
02:57You can think of a galaxy as sort of like a human being.
03:00When you're young and in your adolescent stage, you're vibrant and active.
03:04That's a young galaxy forming stars in a crazy way, and it's not even fully formed yet.
03:08At a certain point, a galaxy reaches middle age, and a middle-aged galaxy really is what
03:12it's going to be.
03:13It has its shape.
03:15But in the long run, a galaxy will stop forming stars, and eventually, just like we all die,
03:20our galaxy will die.
03:25So at what stage of life is the Milky Way?
03:28Is it a healthy, active, young galaxy, or is it heading for its deathbed?
03:37Astronomers can determine a galaxy's stage of life by its color.
03:43So we see different colors of galaxies in the universe.
03:48We see galaxies that are tinted blue and galaxies that are tinted red.
03:55When we see a blue galaxy, that tends to be a younger galaxy, full of bright, hot, newly
04:03formed stars.
04:07When we see a redder galaxy, that tends to be a dimmer, older galaxy that isn't forming
04:14new stars in the present moment.
04:16All its stars are aged and older and redder, and so the entire galaxy casts a different
04:22hue.
04:29So what color is our galaxy?
04:33It's a simple question, but the answer is hard to come by, even though we've been looking
04:40at the Milky Way for thousands of years.
04:45The term Milky Way is ancient.
04:47It goes back to a time when in the dark sky, people noticed there was this light band that
04:52actually went from horizon to horizon.
04:54And that band turned out to be made of thousands and thousands of stars, actually too far away
04:59to see individually.
05:00But it took us a long time to realize what the shape and the scale of the Milky Way galaxy
05:05is.
05:07The amazing thing to think about is that we actually don't know our home galaxy very well
05:11at all.
05:12We actually live in the middle of this disk of gas and dust, and that obscures our view
05:17of the larger Milky Way.
05:20Using visible light, we can't even see to the center, let alone the other side of the
05:24Milky Way galaxy.
05:31The solution is to use a form of light that passes through the gas and dust, infrared.
05:42This is the Sloan Digital Sky Survey Telescope at the Apache Point Observatory in New Mexico.
05:50It's mapping the galaxy, using infrared, and giving scientists unprecedented insights.
06:00The first sensitive infrared observations really weren't done until the last 15 years.
06:06And each of these new windows on the universe teach us different things.
06:15In the last 15 years, Sloan has surveyed more than 250 million stars, analyzing their
06:22light to work out the color of the Milky Way.
06:27And what scientists saw shocked them.
06:31Until very recently, we thought the Milky Way was a young, healthy galaxy.
06:35But now there's evidence that we may be entering the pathway to death.
06:43The Sloan Telescope revealed that star production in our galaxy is rapidly dropping.
06:49The Milky Way is dying.
06:52And when it stops forming new stars, its time will be up.
06:59Paradoxically, our galaxy still has enough star-forming gas, so it should be healthy.
07:08But something is killing it off.
07:16So the Milky Way galaxy is this wonderful disk filled with rich hydrogen gas, lots of
07:21dense dust clouds.
07:22It has everything you need there for star formation, but it seems to be slowing down
07:27and even turning off.
07:29And right now we don't really understand what the culprit is.
07:34The Milky Way has been producing stars for billions of years.
07:39But just how old is our own star, the Sun?
07:44Is it a relatively new star in our galaxy?
07:49Our own Sun formed about four and a half billion years ago in the Milky Way galaxy, and we
07:53are not the oldest star by far.
07:57And yet, tragically, we actually seem to be one of the last generations of new stars in
08:01the Milky Way.
08:03Our projections suggest that in about four billion years, star formation may have ceased
08:07altogether, which is almost just the blink of an eye in the life cycle of the universe.
08:17To find out why, one crucial question needs to be answered.
08:22How is the Milky Way dying?
08:25To kill a galaxy, you have to get rid of the cold gas, because that's what stars form from.
08:33There are many ways you can do this.
08:34You can blast it out from the inside.
08:36You can draw it out from the outside.
08:38You can heat it up so it's no longer cold.
08:41You can use it all up.
08:42And there's even more ways you can stop it.
08:44What we have to do is figure out which way is happening in our galaxy.
08:52Perhaps the culprit is inside the Milky Way itself.
08:57A clue comes from another galaxy entirely.
09:03This is W2246-0526.
09:08Scientists call it a hot dust-obscured galaxy, or hot dog for short.
09:18This galaxy is 12.5 billion light-years away.
09:21It's the most luminous galaxy we know of in the universe and has a light of 300 trillion
09:26stars.
09:32The source of the intense light is not its stars, but a mysterious object at the galaxy's
09:38centre.
09:41It's one million times smaller than the galaxy itself.
09:46And there's only one thing that small and that powerful.
09:52A supermassive black hole.
09:59So supermassive black holes, as the name suggests, are indeed supermassive.
10:04These are billions of times more massive than our sun.
10:08These are gigantic objects.
10:14The gravity of the supermassive black hole is huge.
10:22It sucks in incredible amounts of the hot dog's vital star-forming gas.
10:28And as the gas swirls to form a disk, the intense friction superheats it to millions
10:34of degrees and, in some galaxies, triggers huge jets.
10:41When a lot of material falls onto that black hole, it creates incredibly energetic jets
10:46that can be tens of thousands of light-years across.
10:49All of a sudden you have this blowtorch in the middle of the galaxy.
10:54Black hole jets are bad for galaxies because they can shut down star formation.
10:58They can heat gas up, blow gas out of galaxies, and it can really kill them.
11:05A supermassive black hole is cooking the hot dog.
11:12So is something similar happening at the centre of the Milky Way?
11:18In 2016, scientists at Harvard discovered damning evidence that may link the Milky Way's
11:28supermassive black hole to our galaxy's demise.
11:37Similar to the hot dog, the Milky Way is surrounded by a vast cloud of blown-out gas.
11:45And the scientists trace the gas back to its source, Sagittarius A star, our supermassive
11:52black hole.
11:54Well, it turns out our supermassive black hole had a bit of a hiccup about six million
12:01years ago.
12:02There's evidence that some matter must have fallen into that black hole, and if it fell
12:06in too quickly, it would have gotten superheated by its own friction.
12:10And this would have acted, in a sense, like an explosion.
12:15And that event was huge.
12:17Our galaxy expelled an incredible amount of gas, 130 billion times the mass of the sun.
12:25Large amount of gas.
12:29This event must have been very catastrophic for the inner parts of the galaxy.
12:34Luckily, Earth is in the outer parts of the galaxy where we were able to survive this event.
12:43Has the cause been found?
12:45Is our own supermassive black hole killing the Milky Way?
12:49Yet there's a problem with this theory.
12:56Sagittarius A star has an alibi.
12:59It exploded too late.
13:04Sagittarius A star got very active, very explosive about six million years ago.
13:08But that's so recent, it shouldn't have really affected the star formation rates.
13:12Something else is going on.
13:13There must be another culprit besides the black hole.
13:20Studies suggest our supermassive black hole had to have been active hundreds of millions
13:24of years ago to stop all star formation in our galaxy.
13:31Sagittarius A star wasn't active at that time and thus is no longer considered the cause
13:36for killing our galaxy.
13:40So scientists have to widen their search for a different killer.
13:46Maybe the killer isn't inside our galaxy.
13:48It could be that we suffered a hit and run.
14:01Sagittarius A star production in the Milky Way is breaking
14:20down.
14:21A sign that our galaxy is dying.
14:26And astronomers are on the hunt for clues as to why.
14:35The Milky Way's disk is made up of three sections.
14:39A nucleus, home to the galaxy's supermassive black hole.
14:44A dense central bulge that's 10,000 light years across.
14:53And a spiral arms full of gas, dust, and billions of stars.
15:01The spiral arms should be flat, but they're rippling.
15:06Is this a clue as to why our galaxy is dying?
15:12Today we look at the edge of the Milky Way and we see mysterious ripples in its gas and
15:16we wonder what's the origin.
15:18Something must have caused it to happen.
15:20Something like that just doesn't happen on its own.
15:22The real question is why?
15:29Whatever caused the ripples didn't stay.
15:33Is this evidence of a galactic hit and run?
15:41In January 2016, astronomers studying data from the Vista telescope discovered something
15:48incredible.
15:55Three nearby stars that on their own are nothing special, except they recently left our galaxy
16:02and are traveling at over 560,000 kilometers per hour.
16:12So we've discovered these stars that are careening out of the galaxy at super high velocities.
16:17Could these three stars somehow be responsible for warping the Milky Way's disk?
16:21Well, absolutely not.
16:23The Milky Way is so much more massive than just three stars.
16:26Three stars alone can't warp a galaxy, but those three stars can be indicative of more
16:32stars.
16:33They can be indicative of the presence of, say, a dwarf galaxy, and that can warp a galaxy.
16:40Oftentimes we're really distracted by the big stuff, like the biggest galaxies.
16:44But in many cases, it's the smallest things that matter, and these dwarf galaxies really
16:49are the case of these really small things might be the most important things after all.
16:55Fifteen years ago, the smallest galaxy we knew of had about 100,000 stars.
17:00Today we know of tens of galaxies with just 100 stars or 1,000 stars.
17:05The reason why galaxies are so small, how small they get, are cutting-edge questions.
17:11And in fact, the most populous galaxies in the universe are probably these galaxies with
17:15just a few hundred stars.
17:19There could be hundreds of millions of these dwarf galaxies in the universe, but because
17:24they contain so few stars, they're very hard to find.
17:30And spotting dwarf galaxies that are close to the Milky Way is even harder, because their
17:34stars are hidden among the starscape of our galaxy.
17:39However, there's one type of star that can give a dwarf galaxy's location away.
17:47When we look at the night sky, and we see stars in a particular region of the sky, they
17:51can be all different sort of distances.
17:53It's very difficult to tell, but there are certain stars for which we can measure their
17:57distance accurately.
17:59One type is a Cepheid variable star.
18:02Cepheid variable stars are bright, so we can see them to a great distance.
18:06They change their brightness in a very characteristic way, so we know they're Cepheid variables.
18:11And based on how fast they change their brightness, we know how bright they really are, so we
18:15can tell how far away they are.
18:17So if we find a group of Cepheid variables in the halo of our galaxy, then what we know
18:22is that that's a cluster that shouldn't be there unless there's a dwarf galaxy there.
18:30The group of stars found by VISTA were very distinct Cepheid variable stars.
18:39Dwarf galaxies are abundant, but a tiny fraction of the size of a major galaxy like the Milky
18:44Way, and thus they're difficult to detect.
18:51But these three bright stars show there's a dwarf galaxy hiding beyond the edge of the
18:56Milky Way.
18:59Furthermore, scientists can study the trio of stars to rewind the clock and track back
19:07the past movements of this dwarf galaxy.
19:12Simulation suggests that millions of years ago, this dwarf galaxy punched through the
19:16plane of the Milky Way.
19:23As the fast-moving dwarf galaxy hurtled toward the Milky Way, millions of stars appeared
19:29set to collide.
19:33Catastrophe looked inevitable, but appearances can be deceptive.
19:42When galaxies collide, the first thing you might imagine is that the stars collide, but
19:46actually that doesn't happen.
19:48Galaxies are mostly empty space.
19:49If you took the sun, which is really big, it's a million miles across, and shrunk it
19:54down to the size of a piece of pollen.
19:57The galaxy itself would be twice the size of the Pacific Ocean, and the nearest star
20:01to the sun would be a mile away.
20:04Those tiny pieces of pollen are never going to hit each other.
20:11The distances involved are staggering, because of which, at the moment of impact, most of
20:18the stars from the two galaxies missed each other entirely.
20:23But that didn't mean the Milky Way escaped unharmed.
20:29Even though the stars just pass each other, they do gravitationally interact as they come
20:34close.
20:35And this gravitational interaction sets them on a course that is different than if they
20:40were to live by themselves.
20:46In much the same way that taking a stone and dropping it into a still pond creates
20:49ripples in the water, a galaxy like this, slamming into the Milky Way, can create ripple
20:54effects throughout the disk.
21:00The ripples in the Milky Way stretch across tens of thousands of light years.
21:07Yet this collision wasn't enough to kill our galaxy.
21:11It only caused a minor wound.
21:15But what if this dwarf galaxy isn't the only one?
21:19What if there are others?
21:26There are a lot of dwarf galaxies out there, and it turns out collisions between these
21:31dwarf galaxies and big galaxies like the Milky Way are common.
21:35They happen all the time.
21:36Right now, there are several dwarf galaxies that the Milky Way is swallowing up.
21:40In fact, a really fun thing is that we're actually closer to the core of one of these
21:44galaxies, the Canis Majoris dwarf galaxy, than we are to the core of the Milky Way.
21:48So some of the stars that you see around you in the night sky are actually stars from a
21:52different galaxy.
21:54So what happens when all these dwarf galaxies come together and start pulling and tugging
21:58on a larger galaxy?
22:05Cosmologists believe there could be hundreds of dwarf galaxies surrounding the Milky Way.
22:16A collision with just one of these dwarf galaxies rippled the Milky Way's spiral arms.
22:22But a group of dwarf galaxies could have a far bigger and far more deadly effect.
22:33Dwarf galaxies and the way they interact with big galaxies like the Milky Way can inflict
22:37tremendous change in our universe.
22:39When they slam into a galaxy, they can change its structure.
22:42The Milky Way would not look anything like it looks today without those dwarf galaxies.
22:49Repeated dwarf galaxy collisions could have radically altered the shape of the Milky Way
22:53itself.
22:56Their gravitational disruptions could have created a distinctive and possibly fatal feature
23:01in the middle of our galaxy.
23:05The galactic bar.
23:10The center of the Milky Way is elongated.
23:12Instead of it being shaped like a sphere, it's more shaped like a bar.
23:15And the bar is made by stars actually orbiting in this sort of elongated way.
23:21And this bar can be bad for the health of the galaxy, because what they do is help to
23:26funnel gas into the core of the galaxy.
23:28The loss of this gas could be a way of stopping star formation.
23:35The bar-shaped bulge at the center of the Milky Way sweeps our galaxy's star-building
23:41gas into the galactic nucleus.
23:44Here, it's consumed by our galaxy's supermassive black hole.
23:52Without the star-building material, no new stars can form, and the galaxy dies.
23:59So is this the reason our galaxy is dying?
24:03Are dwarf galaxies killing the Milky Way with a galactic bar?
24:09So it's possible that the formation of these bars helps turn off star formation in the
24:14very core of the galaxy.
24:16But that's just the central regions of the galaxy.
24:19That doesn't explain what's going farther out in the spiral arms.
24:23So if star formation really is shutting down in the Milky Way, it's not really the fault
24:27of the bar.
24:32Dwarf galaxies cause the Milky Way serious damage by creating the galactic bar.
24:37But they're not responsible for killing the Milky Way.
24:44The search continues, and it could take a dramatic turn.
24:51It might not be that the galaxy's being murdered.
24:53It could just be eating itself to death.
25:15Our galaxy, the Milky Way, is being slowly eradicated by something as yet unknown.
25:33Scientists have identified some possible causes, but nothing as yet has been confirmed.
25:39So the hunt for clues within the body of the Milky Way continues.
25:47Our galaxy is a hazy disk of stars, surrounded by a halo of superheated gas.
25:55It's over 100,000 light years across.
25:59But it hasn't always been so big.
26:04When you think about things so vast, so gigantic and ancient as galaxies, you're kind of tempted
26:09to think that they're very stable objects, that they don't change much over time.
26:13But we now know that our own galaxy is the product of many smaller galaxies that came
26:17together over time, and there are other galaxies still colliding with us.
26:24We see galaxies eating each other all the time.
26:26They collide.
26:27And if one galaxy is very big and one galaxy is very small, the little galaxy falls into
26:32the big one, gets torn apart and becomes a part of that bigger galaxy.
26:39The Milky Way might be dying, but it's still a monster foraging through the universe, swallowing
26:46smaller galaxies whole.
26:52It consumes their stars, but it also has a taste for their star-building gas.
26:58And it doesn't have to collide with other galaxies to feed off them.
27:05Now the lifeblood of a galaxy is hydrogen gas.
27:08That's what actually creates new stars.
27:10So as a dwarf galaxy passes by the Milky Way, the tremendously massive halo of the Milky
27:15Way, all of that gas can draw off material from the dwarf galaxy, adding it to the Milky
27:20Way.
27:21So in this way, the Milky Way drains away the lifeblood of other galaxies.
27:27In some sense, you could say it's a vampire, because a vampire sucks the life out of other
27:32things so it can remain young.
27:39During its 13 billion years of existence, our vampire galaxy has feasted, consuming
27:46the lifeblood of its galactic victims.
27:49The Milky Way has grown fat.
27:54But could this feeding frenzy be a factor in the Milky Way's demise?
28:01Once again, crucial evidence comes from the Sloan Digital Sky Survey.
28:08Their telescope maps the stars in our galaxy, but it also maps the galaxies in our universe.
28:19Looking at distant galaxies is like looking back in time, because the further away they
28:24are, the longer their light takes to reach us.
28:30We see the most distant galaxies not as they are now, but as they were billions of years
28:37ago.
28:40So when you look at these galaxies, you're seeing them as they were when they were very
28:43young, and you're seeing these galaxies as they are more recently.
28:47So you can actually look at the evolution, how galaxies change over time as the universe
28:52ages.
28:56While studying the data, scientists made a dramatic discovery.
29:01They found spiral galaxies just like the Milky Way dying all over the universe.
29:07And what connected them was their mass.
29:12There seems to be an upper weight limit for the sizes of spiral galaxies.
29:17Up to about a trillion times the mass of the sun, we see spiral galaxies that continue
29:21to form stars.
29:22But once they pass this threshold, galaxies tend to die and run out of stars.
29:30While devouring the star-building gas of smaller galaxies, the Milky Way may have grown obese.
29:36And now it could be choking to death on all it consumed.
29:41But how?
29:42Once a spiral galaxy is sufficiently big, it's going to have an incredible gravitational
29:48force.
29:49Any gas that it pulls to itself is going to come in at an incredibly high speed.
29:54That gas is going to be superheated.
29:59The superheated gas moves so quickly that it's prevented from falling into the Milky
30:04Way.
30:07The gas is too energetic for our galaxy's gravity to pull it in.
30:10Instead, it stays in the halo around the Milky Way.
30:16And our galaxy's food supply is choked off.
30:21Eventually, our galaxy will starve.
30:27This will only happen if the Milky Way is over the star-building weight limit.
30:31But how do you weigh a galaxy?
30:36One basic way we can weigh a galaxy is measure how fast the stars are moving within it.
30:41So the faster the stars orbit around the center of the galaxy, the more massive the galaxy
30:46is.
30:47This method of weighing the Milky Way relies on gravity.
30:51Fast-moving stars need more gravity to hold them in their orbits, and more gravity means
30:57more galactic mass.
31:00When scientists used this information to do the calculations, the horrible truth was revealed.
31:07We've passed kind of a critical level.
31:10The Milky Way is far too massive for its own health.
31:14And we've entered the beginning of the end.
31:17We're running out of gas, and I mean this literally.
31:20Gas clouds form stars, and as they form stars, they're used up.
31:25And so our gas tank is getting closer and closer to empty every day.
31:35The investigation into the slow death of the Milky Way is solved.
31:40The cause?
31:41The gluttonous Milky Way is killing itself.
31:47Over millions of years, star formation will grind to a halt, and the galaxy will die.
31:57But could the galaxy be resurrected?
32:02We seem to be telling a very sad story.
32:04We're talking about the demise of the Milky Way galaxy, the end of star formation.
32:08But maybe it's just a little bit too soon to write the death announcement yet.
32:12Hope could be just over the horizon.
32:17In space, in astrophysics, really anything is possible.
32:38The shocking case of our dying galaxy has been solved.
32:44There was no killer.
32:47It transpires that the Milky Way is eating itself to death.
32:51But is this really the end?
32:56Could salvation be headed our way?
33:02Even if star formation is turning off in the Milky Way now, we know that it's on a collision
33:07course with the Andromeda galaxy.
33:09They're moving toward each other at hundreds of thousands of miles per hour.
33:15A collision sounds like something that's always destructive, but that's not necessarily the
33:19case.
33:23The Milky Way's collision with our giant galactic neighbour, Andromeda, won't happen for another
33:28four billion years.
33:32By then, star formation in both of these galaxies will have stopped completely.
33:39But this giant coming together changes all that.
33:49As an isolated galaxy, the Milky Way is already in its wind-down phase.
33:54It's not producing as many new stars as it used to.
33:57But there is one way to generate a new round of star formation, and that's through a galactic
34:03merger event.
34:05When Andromeda gets close enough, the neutral gravity between the two galaxies will start
34:09to stretch them out, pulling them out like taffy.
34:14Stars will be pulled out into these long looping streamers, and then the galaxies will physically
34:18pass through each other.
34:20Eventually, the two galaxies will draw back together again and merge into one gigantic
34:27galaxy.
34:28And at that point, all of these gas clouds will flash into star formation.
34:37As the galaxies merge, they'll be reborn.
34:42Two dying spiral-shaped galaxies become a single living elliptical galaxy called Milkomeda.
34:53When you're living in the far future of the galaxy, and you see the night sky while the
34:57Milky Way and Andromeda are colliding, it will look like a very different place.
35:02Rather than one band across the night sky, you might have two as the two disks come together.
35:06It will be a miraculous sight, but a very, very different place than we have today.
35:14When gas clouds ignite to create new stars, it doesn't just stop there.
35:18But as the new stars are born, they produce energy.
35:21It's a ripple effect throughout the galaxy.
35:24New gas clouds collide in response to that.
35:26New stars are born.
35:27And it's just a chain reaction throughout the whole system.
35:35This means our sky will light up for the first time in billions of years.
35:41And star formation will flare across the galaxy.
35:46But is it too soon to celebrate?
35:53This new round of star formation during the merger of our two galaxies, while it's very
36:00cool for a little bit, once it's over, that kind of sends the new galaxy into a death spiral.
36:08When new stars are born in this new galaxy, many of them are going to be hot, large blue stars.
36:15Eventually, those young hot stars are going to start to die.
36:18And when they do, they're going to explode violently as supernovae.
36:28And those supernovae are going to start blasting gas out of the galaxy.
36:36All the gas is gone.
36:37There's no more stuff to form stars.
36:40And that's what kills a galaxy.
36:47It will take hundreds of millions of years for Milkomeda to run out of star-building gas.
36:54Our new elliptical galaxy will then starve.
36:58But there is one final blow still to come.
37:04Another issue to consider is what happens to the two supermassive black holes at the
37:08cores of the two galaxies?
37:10Well, initially, they're going to orbit each other, stirring up a lot of turbulence, and
37:15they're going to combine.
37:17And because there's a lot of new, hot, fresh gas, our new galaxy is going to be a quasar.
37:24And that quasar is going to turn up the heat, it's going to turn up the turbulence, and
37:28this means star formation is going to be shut off.
37:34The combined power of the supermassive black holes will help create a quasar that will
37:39tear through the galaxy.
37:45It will release ferocious beams of radiation that will blast through Milkomeda's star-forming
37:50gas.
37:53It will have only just been reborn, but our newly enlarged galaxy will be dying once again.
38:05Vast galaxies such as Milkomeda seem doomed from the start.
38:11Their size creates too many problems for star formation.
38:15Or so we thought.
38:18The more galaxies we see, the more we realize there's a lot out there we haven't discovered.
38:23And there's a new class of galaxies only recently identified.
38:27These galaxies are more than ten times the mass of the Milky Way.
38:31And intriguingly, they're still forming stars.
38:36Apparently we've missed something.
38:43So does this mean there is hope on the horizon?
38:49The Sloan Digital Sky Survey has spent a decade studying over a million galaxies.
38:54And it's discovered a rare but enormous kind of galaxy.
38:59A super spiral.
39:05These super spiral galaxies are spiral galaxies that are incredibly super.
39:09And by super, I mean they have four times the size, ten times the mass, and they're
39:15weird because they exceed the supposed weight limit for spiral galaxies.
39:20So they shouldn't have new stars, but they do.
39:23They're very healthy galaxies.
39:30Scientists have found just 53 super spirals.
39:39Super spiral galaxies demonstrate that in rare situations, massive galaxies continue
39:45to produce new stars.
39:47So is this a lifeline for Milkomeda?
39:50When we think about two galaxies colliding, a lot of our computer models suggest that
40:00they really mess each other up.
40:02Things get very chaotic.
40:03But over time, could they settle back down into a spiral shape?
40:07And in fact, that may be what happens with super spirals.
40:10One of the clues is that many super spiral galaxies have double cores.
40:14Instead of there just being one super massive black hole, there are actually two orbiting
40:18each other.
40:19The fact that we see spiral galaxies with two cores makes it possible that you could
40:24have a collision and still survive as a spiral galaxy.
40:28So maybe there's hope that even the Milky Way will be a spiral once it collides with
40:32Andromeda.
40:35Picture the scene.
40:39Six billion years from now, Milkomeda drifts through the universe.
40:45Not as an elliptical galaxy, but as a super spiral.
40:53This shape means the galaxy is far more stable.
40:58The damaging heat and turbulence generated by Milkomeda's super massive black holes can't
41:04disrupt star building gas way out in the spiral arms.
41:10Far from dying off, our galaxy will live on, larger than ever before.
41:18We've started to make computer models about how the Milky Way and Andromeda will collide.
41:22So it's possible that these two spiral galaxies won't just become a big mess.
41:27We may settle down to becoming a super spiral ourself.
41:36The Milky Way could one day be a super spiral, but that isn't the end of the story.
41:45Tens of billions of years from now, could the galaxy continue to grow?
41:53Our local group of galaxies, Milky Way, Andromeda, Triangulum, and then a collection of dwarf
42:00satellite galaxies, is gravitationally bound together.
42:05And eventually we're all glued together into a single massive object.
42:11What does this mean?
42:13This means we might be part of one of the largest structures in the universe.
42:18During its billions of years of life, the Milky Way will go through many changes.
42:25It suffers countless collisions, feasts on many smaller galaxies, and gives birth to
42:34innumerable stars.
42:37We talk about the life cycle of galaxies, how they're born, how they live healthy lives
42:41making new stars, and eventually how they die away.
42:44It's really not as depressing as that.
42:47Everything in the universe changes.
42:51Galaxies like ours are in a constant state of flux.
42:55So when it comes to the Milky Way, death really isn't the end.
43:04What we see in our universe is that there's always a process of birth and rebirth.
43:09So the future of the Milky Way is that it's going to keep on doing what it does.
43:15Things are ever-changing.
43:17Ten billion years ago, the Milky Way was nothing like what it is today, and certainly ten billion
43:21years in the future, it'll be a very different place.
43:25Look, I live in this galaxy.
43:29I hope that it can find a way to rejuvenate itself through collisions or some other process,
43:34because that gives me some hope that it'll go on for a long, long time.