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0:00:03 > 0:00:05- Earth - our planet.
0:00:07 > 0:00:09- The sun is our closest star.
0:00:13 > 0:00:16- And along with - 200 billion other stars...
0:00:16 > 0:00:19- ..we live in a galaxy - called the Milky Way.
0:00:22 > 0:00:24- But where did all the stars - come from?
0:00:26 > 0:00:28- How was our galaxy created?
0:00:33 > 0:00:35- Scientists finally know the answer.
0:00:38 > 0:00:43- Our solar system orbits a black hole - at the centre of our galaxy.
0:00:47 > 0:00:51- It's responsible - for the death and birth of stars.
0:00:54 > 0:00:57- In this programme, - we follow the story...
0:00:58 > 0:01:00- ..of how the Milky Way was created.
0:01:01 > 0:01:03- And we reveal - how it will one day die.
0:01:13 > 0:01:15- Subtitles
0:01:23 > 0:01:28- The cosmos is full of galaxies - - an array of shapes and sizes.
0:01:32 > 0:01:36- There are billions of galaxies - in the universe.
0:01:36 > 0:01:39- And one of them is our home - - the Milky Way.
0:01:46 > 0:01:51- When you go out on a dark night, you - see what's called the Milky Way...
0:01:52 > 0:01:57- ..a band of stars - travelling across the night sky.
0:02:02 > 0:02:06- It looks like a disc of stars - with spiral arms...
0:02:07 > 0:02:10- ..and a small, thicker bulge - in the middle.
0:02:14 > 0:02:18- I sometimes ask my students - to identify where they live.
0:02:19 > 0:02:24- Well, you would say for example, - New York City, US of A...
0:02:24 > 0:02:27- ..planet earth, - third planet from the sun...
0:02:29 > 0:02:33- ..and then you would say - the Milky Way galaxy.
0:02:37 > 0:02:39- The Milky Way galaxy is our home.
0:02:41 > 0:02:46- Looking up at the night sky, we can - see some of the 200 billion stars...
0:02:47 > 0:02:49- ..that make up the Milky Way.
0:02:53 > 0:02:57- From afar, these stars - create a huge spiral galaxy.
0:03:01 > 0:03:05- Our sun is just a dot - within one of its enormous arms.
0:03:07 > 0:03:10- It's around - 100,000 light years across.
0:03:11 > 0:03:15- Light takes - approximately 100,000 years...
0:03:16 > 0:03:20- ..to travel from one side - of the disc to the other.
0:03:23 > 0:03:28- Until recently, our massive galaxy - kept a big secret.
0:03:33 > 0:03:38- For years, scientists have wondered - how the Milky Way was formed.
0:03:41 > 0:03:44- The Milky Way's past - and the story...
0:03:44 > 0:03:47- ..leads us to where we are - and who we are.
0:03:50 > 0:03:53- To reveal - how our galaxy was formed...
0:03:53 > 0:03:57- ..we must go back - to the beginning of everything.
0:04:02 > 0:04:05- Back to the Big Bang...
0:04:14 > 0:04:16- ..some 13.6 billion years ago.
0:04:17 > 0:04:21- There are no stars, no planets - and no galaxies.
0:04:23 > 0:04:28- How do we go from the early universe - that's almost featureless...
0:04:28 > 0:04:31- ..to this complex - and interesting universe?
0:04:35 > 0:04:39- The early cosmos - is a thick, uniform soup of gas.
0:04:42 > 0:04:46- But then, gravity starts to work.
0:04:48 > 0:04:53- After reaching a certain point, - the gas becomes self-gravitating.
0:04:54 > 0:04:58- As it starts to smooth out, - there's a little spin on it...
0:04:58 > 0:05:00- ..and it rotates, like a ball.
0:05:02 > 0:05:06- At a certain point, it becomes - dense enough for stars to form.
0:05:11 > 0:05:13- Millions of stars burst into life.
0:05:19 > 0:05:24- The stars move closer together, - over time, and they start to rotate.
0:05:30 > 0:05:31- A galaxy has formed.
0:05:34 > 0:05:37- But what's - holding these stars together?
0:05:41 > 0:05:45- The answer lies - at the centre of the galaxy.
0:05:47 > 0:05:51- We took photographs of the centre - of our galaxy...
0:05:51 > 0:05:54- ..then we waited a year - and took another photo.
0:05:55 > 0:05:59- When you do this, you see how - the stars move around the centre.
0:05:59 > 0:06:04- Once you've worked out - the orbits of these stars...
0:06:07 > 0:06:12- ..you know that the centre contains - something with a vast gravity.
0:06:14 > 0:06:16- There is no light - shining in there...
0:06:17 > 0:06:20- ..so the only thing it could be - is a black hole...
0:06:20 > 0:06:24- ..with a mass tens of thousands - times bigger than the sun.
0:06:26 > 0:06:30- Just as the planets - in our solar system orbit the sun...
0:06:31 > 0:06:37- ..our solar system - orbits a supermassive black hole...
0:06:37 > 0:06:41- ..which lies at the centre - of our galaxy.
0:06:47 > 0:06:52- Within the Milky Way, scientists - find a supermassive black hole.
0:06:54 > 0:06:58- Similar black holes can be found - in other galaxies too.
0:07:03 > 0:07:05- There is a direct relationship...
0:07:06 > 0:07:10- ..between the frequencies - of the rays radiated by galaxies...
0:07:13 > 0:07:16- ..and the mass of the black hole - at its centre.
0:07:17 > 0:07:22- We believe that there are - many black holes in the universe...
0:07:22 > 0:07:26- ..the largest of which can be found - at the centre of galaxies.
0:07:28 > 0:07:32- Somehow, the Milky Way with its - billions of bright stars...
0:07:34 > 0:07:38- ..is intrinsically linked - to one supermassive black hole.
0:07:43 > 0:07:45- It is an object - of fascination and mystery.
0:07:45 > 0:07:48- How did it get there? - How did it grow to be so large?
0:07:49 > 0:07:51- Is it going to continue to grow?
0:07:53 > 0:07:56- To work out - the origins of our galaxy...
0:07:57 > 0:08:01- ..we must first find out - how it got its black hole.
0:08:05 > 0:08:10- In the early universe, - the first stars burst into life.
0:08:14 > 0:08:17- But these stars - are nothing like our sun.
0:08:18 > 0:08:21- Those first stars - were very, very massive.
0:08:22 > 0:08:26- One thing that happens with massive - stars is they explode quickly.
0:08:30 > 0:08:34- Before long, the big stars - come to the end of their life...
0:08:34 > 0:08:36- ..and they explode.
0:08:45 > 0:08:49- They exploded as incredibly powerful - supernovae - exploding stars.
0:08:49 > 0:08:52- Their cores collapsed - to form black holes.
0:08:52 > 0:08:55- These may be the first black holes - to form in the universe.
0:08:59 > 0:09:03- Some scientists believe that - a number of smaller black holes...
0:09:04 > 0:09:05- ..came together...
0:09:06 > 0:09:12- ..to create the supermassive black - hole at the centre of our galaxy.
0:09:16 > 0:09:17- But there's a problem.
0:09:18 > 0:09:24- Scientists have found superbright - lights in the very early universe.
0:09:26 > 0:09:28- These aren't stars.
0:09:31 > 0:09:33- They're called quasars.
0:09:34 > 0:09:39- Quasars come from galaxies - that are emitting a lot of energy.
0:09:39 > 0:09:41- At first, they look like stars...
0:09:41 > 0:09:45- ..but once you calculate - how far away they are...
0:09:45 > 0:09:47- ..you realize they aren't stars.
0:09:48 > 0:09:51- Quasars are scattered - across the universe.
0:09:52 > 0:09:56- It seems they're part - of every galaxy's formation.
0:09:57 > 0:09:59- So a black hole evolves...
0:09:59 > 0:10:03- ..and starts to swallow up - everything around it.
0:10:04 > 0:10:05- This creates a quasar.
0:10:11 > 0:10:15- These quasars are brighter - than anything else in the cosmos.
0:10:18 > 0:10:23- When you think of the energy created - when a supernova explodes...
0:10:23 > 0:10:26- ..or when two black holes - fuse together...
0:10:28 > 0:10:30- ..the energy in quasars is massive.
0:10:35 > 0:10:38- We still don't know how they create - all that energy.
0:10:39 > 0:10:43- We just know that a lot of energy - comes from this black hole.
0:10:44 > 0:10:48- Where these supermassive black holes - came from...
0:10:48 > 0:10:51- ..is still a matter for conjecture.
0:10:54 > 0:10:59- This black hole is far larger than - anything created when a star dies.
0:11:01 > 0:11:05- When you consider the mass of - these supermassive black holes...
0:11:06 > 0:11:10- ..you're talking about millions - of times more mass than our sun.
0:11:10 > 0:11:15- A stellar black hole is only five - to ten times the mass of the sun.
0:11:21 > 0:11:26- There's a supermassive black hole - at the centre of our Milky Way.
0:11:28 > 0:11:31- But where did this black hole - come from?
0:11:33 > 0:11:36- Can everything we see around us...
0:11:36 > 0:11:40- ..have come from the darkest depths - of a black hole?
0:11:46 > 0:11:47- .
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0:12:08 > 0:12:11- Our galaxy - the Milky Way.
0:12:15 > 0:12:19- And at the centre of this cluster - of billions of stars...
0:12:19 > 0:12:22- ..there's a supermassive black hole.
0:12:22 > 0:12:25- The idea of black holes - is fairly old.
0:12:25 > 0:12:29- They were first mentioned - by a French mathematician...
0:12:29 > 0:12:31- ..in the 18th century.
0:12:31 > 0:12:36- Evidence that these things - really exist in our universe...
0:12:36 > 0:12:38- ..has only been found recently.
0:12:43 > 0:12:46- We now believe - they are a crucial part...
0:12:46 > 0:12:50- ..of the development of each galaxy, - including ours.
0:12:52 > 0:12:57- They've come from a 1930s idea that - nobody actually thought was true...
0:12:57 > 0:13:03- ..to be a fundamental part of our - understanding of how galaxies form.
0:13:08 > 0:13:12- Some scientists believe that - the stars form first...
0:13:12 > 0:13:16- ..and then create a black hole - at the end of their life.
0:13:21 > 0:13:24- But the discovery of quasars - challenges this.
0:13:24 > 0:13:28- A quasar is a bright light - that's produced by black holes...
0:13:29 > 0:13:31- ..at the beginning of the universe.
0:13:35 > 0:13:37- But how can a black hole...
0:13:38 > 0:13:41- ..create something as remarkable - as our Milky Way?
0:13:45 > 0:13:50- If we think that galaxies form early - in the history of the universe...
0:13:50 > 0:13:55- ..perhaps in the first billion years - of the start of the universe...
0:13:56 > 0:13:58- ..black holes would have formed.
0:13:59 > 0:14:03- So they are very old - - much older than our sun.
0:14:05 > 0:14:08- Go back to the early days - of the cosmos...
0:14:09 > 0:14:11- ..to a time before any stars appear.
0:14:15 > 0:14:19- Back then, - there were clouds of gas everywhere.
0:14:21 > 0:14:26- Over time, gravity begins to - clump the gas together into a ball.
0:14:28 > 0:14:30- A star is born.
0:14:33 > 0:14:36- But here, too much gas and dust - is piled in.
0:14:36 > 0:14:40- Gravity crushes the gas, - making it denser and denser...
0:14:40 > 0:14:43- ..until it reaches - its breaking point.
0:14:43 > 0:14:46- Finally, the gas collapses - so violently...
0:14:46 > 0:14:49- ..it rips through - the fabric of space.
0:14:54 > 0:14:57- A supermassive black hole is born.
0:15:04 > 0:15:07- A black hole that's way bigger - than any black hole...
0:15:07 > 0:15:11- ..that would form - at the end of a star's life.
0:15:11 > 0:15:16- This could explain how black holes - and quasars are so huge...
0:15:16 > 0:15:18- ..so early on in the universe.
0:15:22 > 0:15:27- Some believe that black holes - come first, before stars.
0:15:35 > 0:15:37- What about galaxies? - How do they form?
0:15:39 > 0:15:42- The chicken and egg question is...
0:15:42 > 0:15:47- ..do black holes cause galaxies - to coalesce around them...
0:15:47 > 0:15:51- ..or do the galaxies build up and - hit some crucial, critical size...
0:15:51 > 0:15:55- ..beyond which black holes - must form at their centre?
0:15:55 > 0:15:57- The only way to learn about that...
0:15:57 > 0:16:01- ..is to look out in the universe - and try and find out.
0:16:03 > 0:16:05- After years of searching...
0:16:06 > 0:16:10- ..scientists believe they've found a - young galaxy forming in the cosmos.
0:16:11 > 0:16:14- Henize 2-10 - is a perfect opportunity...
0:16:14 > 0:16:17- ..to observe - how a young galaxy develops.
0:16:24 > 0:16:28- Henize 2-10 is a very interesting, - tiny dwarf galaxy.
0:16:29 > 0:16:32- Originally, - I was studying this galaxy...
0:16:32 > 0:16:35- ..because it has - all this star formation going on.
0:16:36 > 0:16:40- But when I looked at all the data, - I was sort of shocked and excited.
0:16:40 > 0:16:44- I found a supermassive black hole - at the centre of this galaxy.
0:16:47 > 0:16:50- We can't see these - black holes directly.
0:16:50 > 0:16:54- They only release - a small amount of energy...
0:16:54 > 0:16:58- ..and any energy that is released - escapes in a particular way.
0:16:59 > 0:17:02- The only way we see them...
0:17:02 > 0:17:05- ..is by observing stars - being swallowed up by them.
0:17:06 > 0:17:09- This process has ended - in many galaxies...
0:17:09 > 0:17:14- ..so we don't see any evidence - of black holes in these situations.
0:17:14 > 0:17:18- But we do see energy coming from - black holes in some galaxies...
0:17:19 > 0:17:21- ..where stars - are being swallowed up.
0:17:23 > 0:17:27- Finding a black hole in a galaxy - is nothing new.
0:17:27 > 0:17:31- But the real discovery is the size - of this monster black hole.
0:17:34 > 0:17:38- Our best estimate for the mass - of the black hole in Henize 2-10...
0:17:39 > 0:17:41- ..is a million or two solar masses.
0:17:41 > 0:17:46- This is comparable to the mass of - the black hole in the Milky Way.
0:17:46 > 0:17:49- But the Milky Way - is 100,000 light years across...
0:17:49 > 0:17:53- ..but Henize 2-10 is only - a few thousand light years across.
0:17:54 > 0:17:58- It's amazing to find a black hole - that is so massive...
0:17:58 > 0:18:00- ..in a small dwarf galaxy.
0:18:03 > 0:18:06- Scientists didn't think - such a tiny galaxy...
0:18:07 > 0:18:09- ..could contain - such a large black hole.
0:18:17 > 0:18:19- It's a groundbreaking discovery.
0:18:19 > 0:18:24- In Henize 2-10, the black hole - is more developed than the galaxy.
0:18:25 > 0:18:29- It suggests the black hole - came first, then the galaxy.
0:18:38 > 0:18:42- Henize 2-10 could be a blueprint - for how all galaxies first form...
0:18:44 > 0:18:47- ..including our own galaxy, - the Milky Way.
0:18:48 > 0:18:50- It's fascinating.
0:18:50 > 0:18:53- It could be evidence - that black holes form first...
0:18:53 > 0:18:55- ..then galaxies form around them.
0:18:59 > 0:19:03- Everything we see in our sky - - the stars and the planets...
0:19:04 > 0:19:08- ..may all have started - as a supermassive black hole.
0:19:14 > 0:19:18- But how can something - so dark and frightening...
0:19:18 > 0:19:22- ..create something - as glorious as the Milky Way?
0:19:23 > 0:19:25- Where did the stars come from?
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0:19:53 > 0:19:58- Some scientists now believe - that our galaxy, the Milky Way...
0:19:58 > 0:20:01- ..was created - from a supermassive black hole.
0:20:05 > 0:20:08- But how can something - so dark and frightening...
0:20:09 > 0:20:11- ..create billions of bright stars?
0:20:22 > 0:20:26- Because black holes - aren't just black - far from it.
0:20:32 > 0:20:38- The name comes from the idea - that nothing escapes a black hole.
0:20:43 > 0:20:46- But even though - a black hole is a dark place...
0:20:47 > 0:20:52- ..the gases and matter - around the black holes thrive.
0:20:58 > 0:21:02- Quasars prove that - these massive black holes...
0:21:02 > 0:21:05- ..throw out more light - than whole galaxies.
0:21:07 > 0:21:10- Black holes - don't just swallow matter.
0:21:10 > 0:21:11- They also spit it out.
0:21:13 > 0:21:19- Some matter leaks from around - the edges of these black holes.
0:21:19 > 0:21:21- But more often than not...
0:21:22 > 0:21:25- ..once matter is sucked - into a black hole...
0:21:25 > 0:21:29- ..it stays there - for the lifespan of the galaxy.
0:21:34 > 0:21:38- In the early universe, - the supermassive black hole...
0:21:38 > 0:21:42- ..starts to feast - on the gas and dust around it.
0:21:46 > 0:21:50- But when it eats too much, - it generates so much energy...
0:21:50 > 0:21:54- ..that even the black hole's gravity - can't contain it.
0:21:59 > 0:22:04- Suddenly, highly energized atoms - and light is thrown from the core.
0:22:10 > 0:22:13- How do stars form - around such violence?
0:22:16 > 0:22:20- Scientists have found a black hole - which may hold the key.
0:22:21 > 0:22:24- A naked black hole.
0:22:26 > 0:22:29- A naked black hole sits on its own - in the universe...
0:22:30 > 0:22:33- ..without stellar matter around it.
0:22:33 > 0:22:34- It's very unusual.
0:22:36 > 0:22:40- This naked black hole - has either escaped from a galaxy...
0:22:40 > 0:22:42- ..or eaten its host galaxy.
0:22:43 > 0:22:46- It begins to eat - any gas that surrounds it...
0:22:47 > 0:22:50- ..and, as a result, - it generates emissions.
0:22:51 > 0:22:56- This black hole emits - a huge jet of light from its centre.
0:22:58 > 0:23:03- This jet is smashing into dust - and gas in its neighbouring galaxy.
0:23:04 > 0:23:09- Rather than destroy that galaxy, - it's helping to build it.
0:23:11 > 0:23:16- There's a lot of indirect evidence - about the behaviour of black holes.
0:23:17 > 0:23:19- We see a gravitational effect...
0:23:19 > 0:23:22- ..as they draw matter - from other galaxies.
0:23:22 > 0:23:26- As our sun and moon - create tides on earth...
0:23:26 > 0:23:30- ..we see a tidal effect - between a black hole and galaxies.
0:23:30 > 0:23:35- This colossal jet is the spark - needed to create billions of stars.
0:23:36 > 0:23:38- The black hole - is emitting radiation.
0:23:39 > 0:23:43- When this radiation runs into - all the gas in the galaxy...
0:23:43 > 0:23:46- ..gas clumps together - and new stars get made.
0:23:49 > 0:23:53- Direct evidence - that black holes can create stars.
0:24:00 > 0:24:05- After these stars are created, they - start to orbit the black hole...
0:24:06 > 0:24:08- ..building the galaxy.
0:24:14 > 0:24:17- It's possible that the black hole...
0:24:18 > 0:24:21- ..could have created - many of the stars we see today...
0:24:22 > 0:24:25- ..including our closest star - - the sun.
0:24:29 > 0:24:33- It's amazing that black holes - existed as theoretical constructs...
0:24:33 > 0:24:37- ..that many physicists involved in - developing them didn't believe in.
0:24:38 > 0:24:42- Now we see that even perhaps our - very existence depends upon them.
0:24:42 > 0:24:46- They've gone - from objects in our imagination...
0:24:46 > 0:24:49- ..to objects - on which our life depends.
0:24:56 > 0:24:59- The first stars of the Milky Way...
0:24:59 > 0:25:02- ..sparked into life - 13 billion years ago.
0:25:06 > 0:25:08- The galaxy starts to take shape.
0:25:12 > 0:25:17- And in the early cosmos, - the Milky Way is not alone.
0:25:20 > 0:25:23- Before long, - it does something remarkable.
0:25:23 > 0:25:27- It becomes a cannibal - and begins to eat other galaxies.
0:25:34 > 0:25:38- From time to time, galaxies - can move closer to each other.
0:25:41 > 0:25:44- When they do come together...
0:25:44 > 0:25:48- ..the bigger of the two galaxies - starts to eat the other.
0:25:49 > 0:25:52- This in turn - creates one vast galaxy.
0:25:56 > 0:26:01- We have evidence that the Milky Way - has eaten several other galaxies.
0:26:03 > 0:26:06- The stream of stars - we see in the night sky...
0:26:06 > 0:26:10- ..is the remnants of a galaxy - torn apart by the Milky Way.
0:26:11 > 0:26:16- It happens throughout the universe. - Bigger galaxies eat smaller ones.
0:26:19 > 0:26:22- The infant universe - is like a war zone.
0:26:23 > 0:26:26- Dwarf galaxies battle each other...
0:26:26 > 0:26:29- ..and join forces - to create larger galaxies.
0:26:36 > 0:26:40- To understand how a galaxy is formed - from this chaos...
0:26:41 > 0:26:46- ..each skater represents a - small galaxy in the infant universe.
0:26:48 > 0:26:52- The process of a big galaxy - swallowing a smaller one...
0:26:53 > 0:26:56- ..isn't like Pac-Man, - gobbling it up.
0:26:57 > 0:27:01- It pulls the galaxy apart - by using the tidal force.
0:27:02 > 0:27:05- This breaks down the smaller galaxy.
0:27:12 > 0:27:15- All that remains - are strings of stars.
0:27:18 > 0:27:22- Smaller galaxies start to orbit - around the Milky Way...
0:27:23 > 0:27:26- ..and slowly they're dismantled - by our galaxy.
0:27:26 > 0:27:29- Dwarf galaxies - smash into each other.
0:27:30 > 0:27:32- The larger one always wins.
0:27:33 > 0:27:37- All this stuff slams together. - Stars are thrown all over the place.
0:27:37 > 0:27:40- They strip mass from each other. - They collide.
0:27:41 > 0:27:44- And any small objects in-between - get eaten up.
0:27:46 > 0:27:49- The game lasts - for over a billion years.
0:27:55 > 0:27:59- In the chaos of collisions, - the Milky Way grows bigger.
0:28:05 > 0:28:06- But it's still hungry.
0:28:08 > 0:28:12- Even this second - - it's eating another galaxy.
0:28:14 > 0:28:17- A lot cannibalism goes on - in the universe.
0:28:18 > 0:28:21- The Milky Way is now devouring - a dwarf galaxy...
0:28:21 > 0:28:23- ..which orbits our galaxy.
0:28:24 > 0:28:28- That dwarf galaxy - is called Sagittarius...
0:28:29 > 0:28:32- ..and it's currently - being eaten by the Milky Way.
0:28:40 > 0:28:45- But in this battle with Sagittarius, - the Milky Way doesn't go unscathed.
0:28:47 > 0:28:50- You can see different populations - of stars in the sky.
0:28:51 > 0:28:54- One population - is original to the Milky Way...
0:28:55 > 0:28:58- ..but others - have come from other galaxies.
0:28:59 > 0:29:03- So you can see many stars - which weren't born in the Milky Way.
0:29:04 > 0:29:07- We're not sure how it affects us.
0:29:09 > 0:29:14- It's possible this will cause us - to travel further out in our galaxy.
0:29:15 > 0:29:16- Away from the centre.
0:29:18 > 0:29:23- But it could do the opposite, - and move us closer to the centre.
0:29:24 > 0:29:26- We don't know.
0:29:27 > 0:29:29- Many questions remain unanswered.
0:29:33 > 0:29:37- All that eating - builds and sculpts our galaxy.
0:29:41 > 0:29:45- Some scientists say it repositioned - the stars in the Milky Way...
0:29:47 > 0:29:50- ..including our star - the sun.
0:29:51 > 0:29:55- It's possible that the sun was born - closer to the middle of the galaxy.
0:29:55 > 0:30:00- It migrated to the suburbs over the - course of a couple of billion years.
0:30:00 > 0:30:03- It's possible that, - when Sagittarius hit the disc...
0:30:03 > 0:30:07- ..it created some spiral arms that - then allowed the sun to migrate out.
0:30:11 > 0:30:13- The sun is now a long way away...
0:30:13 > 0:30:18- ..from the supermassive black hole - at the galaxy's centre.
0:30:18 > 0:30:21- That's good news for life on earth.
0:30:25 > 0:30:28- The black hole - eats everything around it...
0:30:28 > 0:30:32- ..so we're lucky to be so far away - from the centre of our galaxy.
0:30:33 > 0:30:38- It'll take a long time to eat - everything, so we're safe here.
0:30:42 > 0:30:47- By eating smaller galaxies, - the Milky Way continues to grow.
0:30:49 > 0:30:51- And throughout the cosmos...
0:30:51 > 0:30:54- ..it's the same story - for every galaxy.
0:30:57 > 0:31:01- But each collision is different, - making each galaxy unique.
0:31:03 > 0:31:06- Galaxies come in - many shapes and sizes.
0:31:06 > 0:31:08- Some are very small and shapeless.
0:31:08 > 0:31:12- Others are huge elliptical galaxies - like giant cotton balls.
0:31:12 > 0:31:14- Some are like our galaxy...
0:31:14 > 0:31:18- ..flat discs with a central bulge - and sweeping spiral arms.
0:31:18 > 0:31:22- We think those big galaxies - got so big by cannibalizing.
0:31:22 > 0:31:24- By eating their brethren.
0:31:29 > 0:31:33- The most common type of galaxy - are spiral galaxies...
0:31:33 > 0:31:36- ..and the Whirlpool Galaxies.
0:31:38 > 0:31:41- Some are discs, - like the Sombrero Galaxy...
0:31:41 > 0:31:44- ..with a circle of gas and dust.
0:31:45 > 0:31:50- Some, like the golden M87, are among - the oldest galaxies in the universe.
0:31:54 > 0:31:59- There are hundreds of billions - of galaxies in the universe.
0:31:59 > 0:32:03- Each one started small - and grew by devouring others.
0:32:06 > 0:32:10- And our galaxy - continues to grow too.
0:32:10 > 0:32:12- Like a huge cannibal...
0:32:14 > 0:32:19- ..our galaxy has devoured other - galaxies over billions of years.
0:32:21 > 0:32:27- In turn, this created the perfect - conditions for us here on earth.
0:32:28 > 0:32:34- But can anything stop the juggernaut - of our cannibal galaxy?
0:32:35 > 0:32:40- Recently, scientists have found - hardly any new stars.
0:32:41 > 0:32:45- Something is preventing our galaxy - from growing.
0:32:45 > 0:32:47- What is it?
0:32:53 > 0:32:54- .
0:33:05 > 0:33:05- Subtitles
0:33:05 > 0:33:07- Subtitles- - Subtitles
0:33:18 > 0:33:23- Our galaxy - the Milky Way - - is one of the largest in the cosmos.
0:33:25 > 0:33:28- It's now home to billions of stars.
0:33:32 > 0:33:36- It contains 1,000,000,000 stars - and that's a lot of stars!
0:33:37 > 0:33:40- It's a spiral galaxy.
0:33:41 > 0:33:47- We believe the spiral galaxies were - formed by eating smaller galaxies.
0:33:49 > 0:33:52- The best theory we have - about how galaxies formed...
0:33:53 > 0:33:56- ..when the cosmos was very young...
0:33:59 > 0:34:02- ..is that dwarf galaxies form first.
0:34:03 > 0:34:08- Then, a cloud of gas and dust - forms the first stars.
0:34:11 > 0:34:14- They started out - as dwarf galaxies...
0:34:14 > 0:34:17- ..and with time, - dwarf galaxies united...
0:34:17 > 0:34:21- ..to create the giant galaxies - we see today.
0:34:22 > 0:34:27- In the past, the Milky Way was - bursting with star formation.
0:34:27 > 0:34:28- So what's changed?
0:34:37 > 0:34:39- In the early universe...
0:34:39 > 0:34:43- ..our black hole - may have sparked stars into life.
0:34:43 > 0:34:48- But now, that same black hole - may be stopping stars from forming.
0:34:52 > 0:34:53- To find out why...
0:34:53 > 0:34:58- ..scientists must examine the - Milky Way's supermassive black hole.
0:34:59 > 0:35:04- And that's exactly what they did, - thanks to NASA's latest telescope.
0:35:07 > 0:35:11- Fiona Harrison - runs the NuSTAR mission.
0:35:14 > 0:35:17- NuSTAR can see - the highest energy X-rays...
0:35:17 > 0:35:20- ..that can penetrate dust and gas.
0:35:20 > 0:35:24- It enables us to have this view - of this black hole.
0:35:26 > 0:35:32- Scientists didn't have to wait long - to see the black hole in action.
0:35:38 > 0:35:41- We looked and about six hours - after we looked...
0:35:41 > 0:35:45- ..we saw the black hole - get 100 times brighter.
0:35:46 > 0:35:48- How long did that last?
0:35:48 > 0:35:52- Only a few hours, - then it faded back into oblivion.
0:35:52 > 0:35:55- But this event - was what we were looking for.
0:35:56 > 0:36:00- We were all amazed. - There were cheers in the room.
0:36:01 > 0:36:04- It was one of - the most exciting moments...
0:36:04 > 0:36:06- ..and so early on - in the mission too.
0:36:08 > 0:36:12- It's direct evidence that - our black hole is still active...
0:36:13 > 0:36:17- ..and still has the muscle - to control the galaxy.
0:36:17 > 0:36:20- And surrounding the black hole...
0:36:20 > 0:36:24- ..scientists discover - remnants of past violence.
0:36:32 > 0:36:37- Huge lobes of old material hang - above and below the Milky Way disc.
0:36:44 > 0:36:48- These are scars - from the black hole's violent past.
0:36:53 > 0:36:56- The black hole's power - is revealed...
0:36:56 > 0:37:00- ..when it lights up a disc of gas - and dust which spins around it.
0:37:06 > 0:37:09- You can see the gases - collecting into a stream...
0:37:09 > 0:37:12- ..flowing towards the black hole.
0:37:12 > 0:37:15- They gather into a disc - and get hotter...
0:37:15 > 0:37:20- ..before disappearing - into the black hole.
0:37:23 > 0:37:27- NuSTAR detects that gas around - the black hole is heating up...
0:37:28 > 0:37:30- ..to 180 million degrees Fahrenheit.
0:37:34 > 0:37:38- This super heated gas - is bad news for star formation.
0:37:41 > 0:37:43- Gas has to get cold - for it to form stars.
0:37:44 > 0:37:47- That's because the gas - has to get very, very dense...
0:37:47 > 0:37:51- ..so it can collapse into something - with nuclear fusion in its core.
0:37:52 > 0:37:55- So in regions around black holes, - because they're so hot...
0:37:56 > 0:37:57- ..they heat up gas around them...
0:37:58 > 0:38:01- ..limiting the ability - for that gas to turn into stars.
0:38:07 > 0:38:09- Star formation shuts down.
0:38:14 > 0:38:16- Before this...
0:38:16 > 0:38:20- ..the black hole was responsible - for gathering gases and dust...
0:38:22 > 0:38:25- ..igniting the cloud - and forming stars.
0:38:29 > 0:38:30- But now that's changed.
0:38:31 > 0:38:35- The heat generated by the black hole - blows away the cloud of gases.
0:38:39 > 0:38:44- The black hole regulates the number - of stars created in a galaxy.
0:38:48 > 0:38:52- There's a relationship - between the size of a black hole...
0:38:54 > 0:38:57- ..and the size of the galaxy itself.
0:39:01 > 0:39:03- This is a recent discovery.
0:39:05 > 0:39:08- The size of the black hole...
0:39:10 > 0:39:15- ..affects the position of our galaxy - in the family of galaxies.
0:39:21 > 0:39:27- We've only known about this - symbiotic relationship for 15 years.
0:39:27 > 0:39:30- It gives us a clearer picture - of galaxy formation.
0:39:35 > 0:39:40- Scientists are still not sure why - our black hole behaves in this way.
0:39:42 > 0:39:47- All we know is that this regulation - might be essential for us.
0:39:55 > 0:39:57- Looking up at our night sky...
0:39:58 > 0:40:02- ..it looks like our galaxy - is unchanging, eternal.
0:40:05 > 0:40:09- But in the universe, - nothing lasts forever.
0:40:12 > 0:40:15- So what does the future hold - for the Milky Way?
0:40:18 > 0:40:23- It's difficult to predict because - our time on earth is so short...
0:40:23 > 0:40:27- ..compared to - the timescales involved.
0:40:29 > 0:40:32- The Milky Way - is setting its own course.
0:40:32 > 0:40:36- If other galaxies come too close, - they'll be swallowed.
0:40:36 > 0:40:41- If the Milky Way veers too close to - a huge galaxy, we'll be swallowed.
0:40:42 > 0:40:45- This process - would take billions of years.
0:40:49 > 0:40:53- Within 5 billion years, - the Andromeda Galaxy will hit us.
0:40:56 > 0:41:00- The Milky Way is used to - bumping into smaller galaxies...
0:41:00 > 0:41:03- ..and swallowing them - in order to grow.
0:41:07 > 0:41:08- But Andromeda is huge.
0:41:13 > 0:41:18- As Andromeda approaches, it will - appear larger in the night sky.
0:41:19 > 0:41:23- When it approaches us, - you'll see many more stars.
0:41:26 > 0:41:32- At present, we can see a few - thousand stars with the naked eye.
0:41:33 > 0:41:38- As Andromeda gets closer, the - numbers would increase dramatically.
0:41:42 > 0:41:46- As the collision nears, - our night sky changes completely.
0:41:50 > 0:41:52- It would be an awesome sight.
0:41:57 > 0:42:01- But it would have a detrimental - effect on both galaxies.
0:42:01 > 0:42:06- Andromeda is bigger than our galaxy, - so we may come off worse.
0:42:10 > 0:42:15- As it nears, Andromeda grows - larger and larger in our sky.
0:42:15 > 0:42:18- Finally, the galaxies - smash into each other.
0:42:18 > 0:42:21- Stars are torn from their orbits.
0:42:24 > 0:42:30- What's interesting about the event - is that the stars don't collide.
0:42:31 > 0:42:35- Stars are extremely small - compared to the galaxy.
0:42:36 > 0:42:41- It's like two crowds of people - running past each other.
0:42:46 > 0:42:52- Where the two galaxies collide, - a huge explosion creates new stars.
0:42:53 > 0:42:55- One half of the night sky...
0:42:56 > 0:42:58- ..has a huge number of stars.
0:42:59 > 0:43:01- The other half is as it is now.
0:43:02 > 0:43:05- There will be thousands of stars - in the sky
0:43:05 > 0:43:07- What an amazing sight!
0:43:11 > 0:43:13- The night sky will be full of stars.
0:43:13 > 0:43:18- Colourful gas clouds - will erupt like fireworks.
0:43:19 > 0:43:21- It will be great!
0:43:26 > 0:43:28- This is the beginning of the end.
0:43:31 > 0:43:36- The Milky Way and Andromeda - will rip each other apart.
0:43:42 > 0:43:46- Some people believe - that elliptical galaxies form...
0:43:47 > 0:43:51- ..when two spiral galaxies - come together.
0:43:52 > 0:43:56- The pattern of stars in the disc - is destroyed...
0:43:56 > 0:44:01- ..by all the gravity that's created - when two galaxies collide.
0:44:01 > 0:44:06- Interestingly, elliptical galaxies - don't contain many new stars.
0:44:07 > 0:44:11- New stars are created all the time - in spiral galaxies...
0:44:11 > 0:44:15- ..but few are created - in elliptical galaxies.
0:44:16 > 0:44:21- Elliptical galaxies have no - disc of gas at their centre.
0:44:25 > 0:44:30- They suffered so much damage when - the two galaxies came together...
0:44:31 > 0:44:33- ..everything was destroyed.
0:44:40 > 0:44:43- When two galaxies collide, - like us and Andromeda...
0:44:44 > 0:44:47- ..thousands of stars - are created within a year.
0:44:48 > 0:44:53- All the gases coalesce - to create a new generation of stars.
0:45:00 > 0:45:03- The Milky Way and Andromeda - have gone.
0:45:03 > 0:45:05- In their place, a new galaxy.
0:45:10 > 0:45:12- But it's not over.
0:45:13 > 0:45:18- Their two supermassive black holes - hurtle towards each other.
0:45:21 > 0:45:26- Those black holes - will most probably join together.
0:45:26 > 0:45:28- They have so much mass...
0:45:28 > 0:45:33- ..their huge gravitational force - will draw them towards each other.
0:45:34 > 0:45:38- The result is an even bigger - supermassive black hole.
0:45:42 > 0:45:43- It will be fantastic.
0:45:44 > 0:45:47- Two fireballs - rotating around each other...
0:45:48 > 0:45:53- ..until the black holes at the - centre of them finally coalesce.
0:45:55 > 0:45:57- The black holes merge...
0:45:58 > 0:46:03- ..forming a huge supermassive black - hole to rule over a new galaxy.
0:46:07 > 0:46:09- But this new galaxy - is already dying.
0:46:10 > 0:46:14- Over billions of years, - the stars slowly die out.
0:46:16 > 0:46:20- There's no fuel left to create - new stars and replace them.
0:46:24 > 0:46:29- The black hole absorbs all the - matter at the centre of the galaxy.
0:46:32 > 0:46:36- We also believe that black holes - can influence...
0:46:36 > 0:46:38- ..the formation of new stars.
0:46:39 > 0:46:43- Black holes are responsible - for the death and birth of stars.
0:46:47 > 0:46:51- Our galaxy, - with its billions of bright stars...
0:46:51 > 0:46:55- ..has been created - from a supermassive black hole.
0:46:56 > 0:46:59- It's ruled over our Milky Way - from the start.
0:47:00 > 0:47:05- And one day, everything will return - to the place it all began.
0:47:06 > 0:47:09- Back into a supermassive black hole.
0:47:37 > 0:47:40- S4C subtitles by Eirlys A Jones
0:47:40 > 0:47:41- .