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The fate of our galaxy
hangs in the balance.
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00:00:08,660 --> 00:00:14,900
The Milky Way is dying,
and we don't know why.
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00:00:14,900 --> 00:00:19,800
Our galaxy, like all galaxies,
has a limited life-span.
4
00:00:19,800 --> 00:00:23,640
After that, it's lights out.
5
00:00:23,640 --> 00:00:26,510
The race is on
to find a smoking gun.
6
00:00:28,850 --> 00:00:32,610
It's safe to say right now there
are many ways to kill a galaxy.
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00:00:35,820 --> 00:00:39,750
It's a cosmic
crime scene investigation.
8
00:00:39,760 --> 00:00:42,920
Is it murder most foul?
9
00:00:42,930 --> 00:00:45,030
Or is it
death by natural causes?
10
00:00:47,330 --> 00:00:49,800
The suspects are lined up.
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00:00:49,800 --> 00:00:52,270
The interrogation is underway.
12
00:00:56,270 --> 00:00:58,570
It's another example of this big
universe of ours
13
00:00:58,580 --> 00:01:01,110
throwing puzzles at us
that now we have to solve.
14
00:01:04,050 --> 00:01:07,280
What is killing the Milky Way?
15
00:01:07,280 --> 00:01:10,290
16
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captions paid for by
discovery communications
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Earth.
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Our home.
19
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Just one of 100 billion planets
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00:01:34,640 --> 00:01:38,910
orbiting 400 billion stars
21
00:01:38,920 --> 00:01:42,580
that make up an immense
galactic spiral...
22
00:01:44,590 --> 00:01:47,120
the Milky Way.
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00:01:49,890 --> 00:01:52,490
Galaxies are where stars form,
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and, of course,
planets form around stars,
25
00:01:54,830 --> 00:01:56,660
so the story of the Earth,
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00:01:56,670 --> 00:01:58,900
of yourself, of the solar system
27
00:01:58,900 --> 00:02:02,340
has everything to do
with the story of the galaxies.
28
00:02:05,640 --> 00:02:10,880
The story of the Milky Way
begins 13.6 billion years ago,
29
00:02:10,880 --> 00:02:13,480
just after the big bang.
30
00:02:13,480 --> 00:02:17,550
It's a time when there are
no planets and no stars...
31
00:02:17,550 --> 00:02:22,760
Just a vast, lumpy soup
of superheated hydrogen gas.
32
00:02:25,500 --> 00:02:28,660
Over millions of years,
the temperature drops,
33
00:02:28,670 --> 00:02:33,070
and gravity compresses
the lumps down, until eventually
34
00:02:33,070 --> 00:02:37,870
the hydrogen molecules fuse
and ignite a star.
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00:02:42,350 --> 00:02:47,850
In time, billions of stars
burst into life.
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00:02:47,850 --> 00:02:51,120
And the Milky Way
begins to take shape.
37
00:02:56,190 --> 00:02:58,930
You can think of a galaxy
as sort of like a human being.
38
00:02:58,930 --> 00:03:01,360
When you're young and in your
adolescent stage,
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00:03:01,360 --> 00:03:02,830
you're vibrant and active.
40
00:03:02,830 --> 00:03:05,600
That's a young galaxy
forming stars in a crazy way,
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00:03:05,600 --> 00:03:07,240
and it's not
even fully formed yet.
42
00:03:07,240 --> 00:03:09,900
At a certain point,
galaxy reaches middle age,
43
00:03:09,910 --> 00:03:12,510
and a middle-aged galaxy really
is what it's going to be...
44
00:03:12,510 --> 00:03:13,810
It has its shape...
45
00:03:13,810 --> 00:03:17,080
But in the long run, a galaxy
will stop forming stars,
46
00:03:17,080 --> 00:03:20,720
and eventually, just like we
all die, our galaxy will die.
47
00:03:24,020 --> 00:03:27,820
So, at what stage of life
is the Milky Way?
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00:03:27,820 --> 00:03:30,430
Is it a healthy,
active youngster,
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00:03:30,430 --> 00:03:32,590
or is it heading
for its deathbed?
50
00:03:35,800 --> 00:03:39,530
Scientists can determine
each galaxy's stage of life
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00:03:39,540 --> 00:03:42,400
by its color.
52
00:03:42,410 --> 00:03:47,240
So, we see different colors
of galaxies in the universe.
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00:03:47,240 --> 00:03:49,780
We see galaxies
that are tinted blue
54
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and galaxies
that are tinted red.
55
00:03:54,520 --> 00:03:59,720
When we see a blue galaxy, that
tends to be a younger galaxy
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00:03:59,720 --> 00:04:04,030
full of bright,
hot, newly formed stars.
57
00:04:06,500 --> 00:04:08,660
When we see a redder galaxy,
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00:04:08,670 --> 00:04:11,830
that tends
to be a dimmer, older galaxy
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00:04:11,840 --> 00:04:15,670
that isn't forming new stars
in the present moment.
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00:04:15,670 --> 00:04:18,870
All its stars are aged
and older and redder,
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00:04:18,880 --> 00:04:22,510
and so the entire galaxy
casts a different hue.
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00:04:27,880 --> 00:04:32,820
So, what color is our galaxy?
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00:04:32,820 --> 00:04:38,790
It's a simple question, but the
answer is hard to come by,
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even though we've been looking
at the Milky Way
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for thousands of years.
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The term "Milky Way" is ancient.
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It goes back to a time
when in the dark sky,
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00:04:49,070 --> 00:04:51,310
people noticed
there was this light band
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00:04:51,310 --> 00:04:53,640
that actually went
from horizon to horizon,
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00:04:53,640 --> 00:04:54,880
and that band turned out
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to be made of thousands
and thousands of stars
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actually too far away
to see individually.
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00:04:59,820 --> 00:05:01,620
But it took us
a long time to realize
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00:05:01,620 --> 00:05:06,550
what the shape and the scale
of the Milky Way galaxy is.
75
00:05:06,560 --> 00:05:09,420
The amazing thing to think about
is that we actually don't know
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00:05:09,430 --> 00:05:11,730
our home galaxy
very well at all.
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00:05:11,730 --> 00:05:15,430
We actually live in the middle
of this disk of gas and dust,
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00:05:15,430 --> 00:05:19,430
and that obscures our view of
the larger Milky Way.
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00:05:19,440 --> 00:05:22,000
Using visible light,
we can't even see to the center,
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let alone the other side
of the Milky Way galaxy.
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00:05:30,180 --> 00:05:32,780
The solution
is to use a form of light
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that passes through the gas
and dust...
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00:05:36,750 --> 00:05:38,320
Infrared.
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00:05:41,830 --> 00:05:45,690
This is the Sloan
digital sky survey telescope
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00:05:45,700 --> 00:05:50,130
at the Apache point observatory
in new Mexico.
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It's mapping the galaxy
using infrared
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00:05:53,440 --> 00:05:57,210
and giving scientists
unprecedented insights.
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The first sensitive infrared
observations really weren't done
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00:06:03,010 --> 00:06:04,980
till the last 15 years,
90
00:06:04,980 --> 00:06:07,280
and each of these new windows
on the universe
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00:06:07,280 --> 00:06:09,080
teach us different things.
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00:06:14,160 --> 00:06:15,720
In the last 15 years,
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00:06:15,730 --> 00:06:21,060
Sloan has surveyed
more than 250 million stars,
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00:06:21,060 --> 00:06:26,330
analyzing their light to work
out the color of the Milky Way.
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00:06:26,340 --> 00:06:30,540
And what scientists
saw shocked them.
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00:06:30,540 --> 00:06:32,770
Until very recently,
we thought the Milky Way
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00:06:32,780 --> 00:06:34,780
was a young, healthy galaxy,
98
00:06:34,780 --> 00:06:36,240
but now there's evidence
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00:06:36,250 --> 00:06:39,680
that we may be entering
the pathway to death.
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00:06:41,990 --> 00:06:46,520
The Sloan telescope reveals that
star production in our galaxy
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is falling through the floor.
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The Milky Way is dying.
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00:06:52,500 --> 00:06:56,160
And when it stops
forming new stars,
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its time will be up.
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00:07:00,200 --> 00:07:02,770
Paradoxically,
our galaxy still has
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00:07:02,770 --> 00:07:08,710
star-forming gas in the tank,
so it should be healthy,
107
00:07:08,710 --> 00:07:11,210
but something is killing it off.
108
00:07:15,350 --> 00:07:18,050
So, the Milky Way galaxy
is this wonderful disk
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00:07:18,050 --> 00:07:21,920
filled with rich hydrogen gas,
lots of dense dust clouds.
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00:07:21,930 --> 00:07:24,530
It has everything you need there
for star formation,
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00:07:24,530 --> 00:07:27,860
but it seems to be slowing down
and even turning off,
112
00:07:27,860 --> 00:07:31,530
and right now, we don't really
understand what the culprit is.
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With a galaxy killer at large,
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scientists embark upon the
biggest murder investigation
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in the history of the universe.
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Everything in science,
when you're exploring a problem,
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is a bit like a crime scene.
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You've got the evidence
laid out in front of you...
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00:07:57,190 --> 00:08:00,360
and we have to
figure out who done it.
120
00:08:16,610 --> 00:08:20,610
Our home in the universe
is dying...
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00:08:20,610 --> 00:08:26,980
Not the Earth,
but our galaxy, the Milky Way.
122
00:08:26,980 --> 00:08:30,520
It's been producing stars
for billions of years,
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00:08:30,520 --> 00:08:32,990
but soon, it will stop.
124
00:08:35,060 --> 00:08:37,730
Our own sun formed
about 4 1/2 billion years ago
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00:08:37,730 --> 00:08:39,130
in the Milky Way galaxy,
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00:08:39,130 --> 00:08:42,400
and we are not
the oldest star by far.
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And yet, tragically,
we actually seem to be
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00:08:44,600 --> 00:08:48,340
one of the last generations
of new stars in the Milky Way.
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00:08:48,340 --> 00:08:51,770
Current projections suggest
that in about 4 billion years,
130
00:08:51,780 --> 00:08:53,780
star formation may have
ceased all together,
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00:08:53,780 --> 00:08:55,640
which is almost just
a blink of an eye
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00:08:55,650 --> 00:08:57,550
in the life cycle
of the universe.
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00:09:01,920 --> 00:09:05,890
To find out why, scientists
launch an investigation.
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00:09:05,890 --> 00:09:08,190
The most crucial question?
135
00:09:08,190 --> 00:09:11,060
How is the milky way dying?
136
00:09:12,900 --> 00:09:16,130
To kill a galaxy, you have
to get rid of the cold gas,
137
00:09:16,130 --> 00:09:18,330
because that's
what stars form from.
138
00:09:18,340 --> 00:09:19,800
There are many ways
you can do this.
139
00:09:19,800 --> 00:09:21,900
You can blast it out
from the inside.
140
00:09:21,910 --> 00:09:24,110
You can draw it out
from the outside.
141
00:09:24,110 --> 00:09:26,510
You can heat it up
so it's no longer cold.
142
00:09:26,510 --> 00:09:28,010
You can use it all up,
143
00:09:28,010 --> 00:09:30,150
and there's even more ways
you can stop it.
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00:09:30,150 --> 00:09:31,710
What we have to do is figure out
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00:09:31,720 --> 00:09:34,320
which way is happening
in our galaxy.
146
00:09:36,890 --> 00:09:42,420
Perhaps the culprit is inside
the Milky Way itself.
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00:09:42,430 --> 00:09:45,830
A clue comes from
another galaxy entirely.
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This is w2246-0526.
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00:09:54,670 --> 00:09:58,670
Scientists call it a hot,
dust-obscured galaxy,
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00:09:58,680 --> 00:10:01,040
or "hot dog" for short.
151
00:10:03,810 --> 00:10:06,920
This galaxy is
12 1/2 billion light-years away.
152
00:10:06,920 --> 00:10:09,780
It's the most luminous galaxy
we know of in the universe.
153
00:10:09,790 --> 00:10:12,920
It has the light
of 300 trillion stars.
154
00:10:17,190 --> 00:10:20,900
The source of the intense light
is not its stars,
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00:10:20,900 --> 00:10:25,900
but a mysterious object
at the galaxy's center.
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00:10:25,900 --> 00:10:29,400
It's a million times smaller
than the galaxy itself.
157
00:10:32,140 --> 00:10:35,980
There's only one thing
that small and that powerful...
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00:10:38,680 --> 00:10:40,920
a supermassive black hole.
159
00:10:44,750 --> 00:10:47,860
So, supermassive black holes,
as the name suggests,
160
00:10:47,860 --> 00:10:49,390
are indeed supermassive.
161
00:10:49,390 --> 00:10:53,800
These are billions of times
more massive than our sun.
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00:10:53,800 --> 00:10:55,860
These are gigantic objects.
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00:10:58,770 --> 00:11:02,940
The gravity in the supermassive
black hole is off the charts.
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00:11:06,480 --> 00:11:08,540
It sucks in incredible amounts
165
00:11:08,550 --> 00:11:11,880
of the hot dog's
vital star-forming gas.
166
00:11:15,920 --> 00:11:18,450
And as the gas swirls
to form a disk,
167
00:11:18,460 --> 00:11:22,390
the intense friction superheats
it to millions of degrees
168
00:11:22,390 --> 00:11:27,160
and, in some galaxies,
triggers huge jets.
169
00:11:27,160 --> 00:11:29,560
When a lot of material falls
onto that black hole,
170
00:11:29,570 --> 00:11:31,970
it creates incredibly
energetic jets
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00:11:31,970 --> 00:11:34,870
that can be tens of thousands
of light-years across.
172
00:11:34,870 --> 00:11:36,940
All of a sudden,
you have this blowtorch
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00:11:36,940 --> 00:11:39,610
in the middle of the galaxy.
174
00:11:39,610 --> 00:11:41,840
Black hole jets
are bad for galaxies
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because they can shut down
star formation.
176
00:11:43,980 --> 00:11:46,580
They can heat gas up,
blow gas out of galaxies,
177
00:11:46,580 --> 00:11:48,150
and they could really kill them.
178
00:11:50,490 --> 00:11:54,060
A supermassive black hole
is cooking the hot dog.
179
00:11:57,490 --> 00:11:59,260
What's going on in our galaxy?
180
00:12:08,710 --> 00:12:12,870
In 2016, scientists at Harvard
discovered damning evidence
181
00:12:12,880 --> 00:12:16,310
that may link the Milky Way's
supermassive black hole
182
00:12:16,310 --> 00:12:18,080
to the galaxy's demise.
183
00:12:21,080 --> 00:12:22,550
Just like the hot dog,
184
00:12:22,550 --> 00:12:26,650
the Milky Way is surrounded by
a vast cloud of blown-out gas,
185
00:12:28,790 --> 00:12:34,060
and the scientists traced
the gas back to its source...
186
00:12:34,060 --> 00:12:39,000
Sagittarius a-star,
our supermassive black hole.
187
00:12:42,010 --> 00:12:44,110
Well, it turns out
our supermassive black hole
188
00:12:44,110 --> 00:12:47,680
had a bit of a hiccup about
6 million years ago.
189
00:12:47,680 --> 00:12:50,510
There's evidence that
some matter must have fallen
190
00:12:50,510 --> 00:12:53,010
into that black hole,
and if it fell in too quickly,
191
00:12:53,020 --> 00:12:56,080
it would have gotten superheated
by its own friction,
192
00:12:56,090 --> 00:13:00,890
and this would have acted,
in a sense, like an explosion.
193
00:13:00,890 --> 00:13:03,220
And that event was huge.
194
00:13:03,230 --> 00:13:06,660
Our galaxy expelled
an incredible amount of gas...
195
00:13:06,660 --> 00:13:10,100
130 billion times
the mass of the Sun.
196
00:13:10,100 --> 00:13:11,970
Large amount of gas.
197
00:13:14,900 --> 00:13:16,900
This event must have been
very catastrophic
198
00:13:16,910 --> 00:13:19,640
for the inner parts
of the galaxy.
199
00:13:19,640 --> 00:13:22,380
Luckily, Earth is in
the outer parts of the galaxy,
200
00:13:22,380 --> 00:13:24,780
where we were able
to survive this event.
201
00:13:28,620 --> 00:13:31,590
Is this the smoking gun?
202
00:13:31,590 --> 00:13:36,720
Is our own supermassive black
hole killing the Milky Way?
203
00:13:36,730 --> 00:13:38,990
The evidence seems to mount up.
204
00:13:41,560 --> 00:13:45,530
But Sagittarius a-star
has an alibi.
205
00:13:45,540 --> 00:13:48,740
It exploded too late.
206
00:13:48,740 --> 00:13:51,610
Sagittarius a-star
got very active,
207
00:13:51,610 --> 00:13:54,040
very explosive
about 6 million years ago,
208
00:13:54,040 --> 00:13:56,510
but that's so recent, it
shouldn't have really affected
209
00:13:56,510 --> 00:13:57,950
the star formation rates.
210
00:13:57,950 --> 00:13:59,310
Something else is going on.
211
00:13:59,320 --> 00:14:02,180
There must be another culprit
besides the black hole.
212
00:14:04,390 --> 00:14:08,420
Studies suggest our supermassive
black hole must have been active
213
00:14:08,430 --> 00:14:10,890
hundreds of millions
of years ago
214
00:14:10,890 --> 00:14:15,360
to stop all star formation
in our galaxy.
215
00:14:15,370 --> 00:14:19,230
Sagittarius a-star
wasn't active at that time,
216
00:14:19,240 --> 00:14:22,140
so it's no longer a suspect.
217
00:14:24,210 --> 00:14:27,580
The hunt is on for
a different galaxy killer,
218
00:14:27,580 --> 00:14:31,850
and scientists are widening
the investigation.
219
00:14:31,850 --> 00:14:34,420
Maybe the killer
isn't inside our galaxy.
220
00:14:34,420 --> 00:14:37,620
It could be that we suffered
a hit-and-run.
221
00:15:05,010 --> 00:15:07,610
Our universe is a crime scene.
222
00:15:09,910 --> 00:15:15,050
Star production in the Milky Way
is breaking down.
223
00:15:15,050 --> 00:15:17,150
Our galaxy is dying,
224
00:15:17,150 --> 00:15:20,790
and astronomers are examining
the body for clues.
225
00:15:23,030 --> 00:15:28,600
The Milky Way's disk is made up
of three sections...
226
00:15:28,600 --> 00:15:33,300
A nucleus, home to the galaxy's
supermassive black hole...
227
00:15:36,210 --> 00:15:41,510
a dense, central bulge
10,000 light-years across,
228
00:15:41,510 --> 00:15:43,180
and the spiral arms...
229
00:15:43,180 --> 00:15:47,020
Full of gas, dust,
and billions of stars.
230
00:15:49,090 --> 00:15:53,790
The spiral arms should be flat,
but they're rippling.
231
00:15:53,790 --> 00:15:57,060
Is this a clue
for the cosmic detectives?
232
00:15:58,860 --> 00:16:00,900
Today, we look at the edge
of the Milky Way,
233
00:16:00,900 --> 00:16:02,970
and we see mysterious
ripples in its gas,
234
00:16:02,970 --> 00:16:05,170
and we wonder,
what's the origin?
235
00:16:05,170 --> 00:16:07,270
Something must have
caused it to happen.
236
00:16:07,270 --> 00:16:09,740
Something like that
just doesn't happen on its own.
237
00:16:09,740 --> 00:16:11,640
The real question is, why?
238
00:16:16,250 --> 00:16:20,820
Whatever caused the ripples
didn't hang around.
239
00:16:20,820 --> 00:16:23,850
Is this evidence of
a galactic hit-and-run?
240
00:16:26,920 --> 00:16:29,090
January 2016.
241
00:16:32,060 --> 00:16:36,100
Astronomers studying data
from the vista telescope
242
00:16:36,100 --> 00:16:38,100
discover something incredible...
243
00:16:42,210 --> 00:16:44,740
three nearby stars.
244
00:16:44,740 --> 00:16:47,480
On their own, nothing special,
245
00:16:47,480 --> 00:16:51,080
except they've recently
left our galaxy,
246
00:16:51,080 --> 00:16:55,650
and they're traveling
at 350,000 miles an hour.
247
00:16:59,420 --> 00:17:01,860
So, we've discovered these stars
that are careening
248
00:17:01,860 --> 00:17:04,630
out of the galaxy
at super-high velocities.
249
00:17:04,630 --> 00:17:06,900
Could these three stars
somehow be responsible
250
00:17:06,900 --> 00:17:08,800
for warping
the Milky Way's disk?
251
00:17:08,800 --> 00:17:10,270
Well, absolutely not.
252
00:17:10,270 --> 00:17:13,500
The Milky Way is so much more
massive than just three stars.
253
00:17:13,500 --> 00:17:16,910
Three stars alone
can't warp a galaxy,
254
00:17:16,910 --> 00:17:20,280
but those three stars can be
indicative of more stars.
255
00:17:20,280 --> 00:17:22,780
They can be indicative
of the presence of, say,
256
00:17:22,780 --> 00:17:25,980
a dwarf galaxy,
and that can warp the galaxy.
257
00:17:27,950 --> 00:17:31,390
Dwarf galaxies are abundant.
258
00:17:31,390 --> 00:17:34,690
But a tiny fraction
of the size of a major galaxy,
259
00:17:34,690 --> 00:17:36,260
like the Milky Way.
260
00:17:38,330 --> 00:17:41,700
So they're difficult to detect.
261
00:17:41,700 --> 00:17:45,900
But these three bright stars
show there's a dwarf galaxy
262
00:17:45,900 --> 00:17:50,210
hiding beyond the edge
of the Milky Way.
263
00:17:50,210 --> 00:17:53,340
And scientists can study
the trio of stars
264
00:17:53,340 --> 00:17:55,080
to rewind the clock
265
00:17:55,080 --> 00:17:59,280
and track back the past
movements of the dwarf galaxy.
266
00:18:02,150 --> 00:18:04,950
Simulations suggest
that millions of years ago,
267
00:18:04,960 --> 00:18:06,220
this dwarf galaxy
268
00:18:06,220 --> 00:18:08,290
punched through the plane
of the Milky Way.
269
00:18:13,230 --> 00:18:18,000
As the fast-moving dwarf galaxy
hurtles towards the Milky Way,
270
00:18:18,000 --> 00:18:23,740
millions of stars seem set
on a collision course.
271
00:18:23,740 --> 00:18:26,070
Catastrophe looks inevitable.
272
00:18:27,980 --> 00:18:30,980
But appearances
can be deceptive.
273
00:18:32,850 --> 00:18:35,320
When galaxies collide, the first
thing you might imagine
274
00:18:35,320 --> 00:18:37,950
is that the stars collide, but
actually, that doesn't happen.
275
00:18:37,960 --> 00:18:39,760
Galaxies are mostly empty space.
276
00:18:39,760 --> 00:18:41,960
If you took the Sun,
which is really big...
277
00:18:41,960 --> 00:18:43,690
It's a million miles across...
278
00:18:43,690 --> 00:18:47,660
And shrunk it down to the size
of a piece of pollen,
279
00:18:47,660 --> 00:18:51,030
the galaxy itself would be twice
the size of the pacific ocean,
280
00:18:51,040 --> 00:18:54,500
and the nearest star to the Sun
would be a mile away.
281
00:18:54,510 --> 00:18:57,670
Those tiny pieces of pollen are
never going to hit each other.
282
00:19:01,610 --> 00:19:05,750
The distances involved
are staggering.
283
00:19:05,750 --> 00:19:07,420
And at the moment of impact,
284
00:19:07,420 --> 00:19:09,990
most of the stars
from the two galaxies
285
00:19:09,990 --> 00:19:12,620
miss each other entirely.
286
00:19:12,620 --> 00:19:16,660
But that doesn't mean
the Milky Way is safe.
287
00:19:18,830 --> 00:19:21,600
Even though the stars
just pass each other,
288
00:19:21,600 --> 00:19:25,430
they do gravitationally
interact as they come close,
289
00:19:25,440 --> 00:19:28,800
and this gravitational
interaction sets them
290
00:19:28,810 --> 00:19:30,410
on a course that is different
291
00:19:30,410 --> 00:19:32,840
than if they were
to live by themselves.
292
00:19:35,680 --> 00:19:37,780
In much the same way
that taking a stone
293
00:19:37,780 --> 00:19:39,450
and dropping it
into a still pond
294
00:19:39,450 --> 00:19:41,580
creates ripples in the water,
295
00:19:41,590 --> 00:19:44,250
a galaxy like this slamming
into the Milky Way
296
00:19:44,250 --> 00:19:46,660
can create ripple effects
throughout the disk.
297
00:19:49,960 --> 00:19:51,630
The ripples in the Milky Way
298
00:19:51,630 --> 00:19:55,030
stretch across tens
of thousands of light-years.
299
00:19:57,200 --> 00:20:02,540
Still, this hit-and-run isn't
enough to kill the Milky Way.
300
00:20:02,540 --> 00:20:06,210
It only causes a flesh wound.
301
00:20:06,210 --> 00:20:11,710
But what if this dwarf galaxy
isn't acting alone?
302
00:20:11,720 --> 00:20:13,680
What if it has accomplices?
303
00:20:16,220 --> 00:20:18,950
There are a lot of
dwarf galaxies out there,
304
00:20:18,960 --> 00:20:22,220
and it turns out collisions
between these dwarf galaxies
305
00:20:22,230 --> 00:20:25,090
and big galaxies, like
the Milky Way, are common.
306
00:20:25,100 --> 00:20:26,360
They happen all the time.
307
00:20:26,360 --> 00:20:28,400
Right now, there are
several dwarf galaxies
308
00:20:28,400 --> 00:20:30,430
that the Milky Way
is swallowing up.
309
00:20:30,430 --> 00:20:32,630
In fact, a really fun thing is
that we're actually closer
310
00:20:32,640 --> 00:20:34,800
to the core of one
of these galaxies...
311
00:20:34,810 --> 00:20:36,740
The Canis Majoris
dwarf galaxy...
312
00:20:36,740 --> 00:20:38,570
Than we are to the core
of the Milky Way.
313
00:20:38,580 --> 00:20:41,510
So some of the stars that you
see around you in the night sky
314
00:20:41,510 --> 00:20:44,250
are actually stars
from a different galaxy.
315
00:20:44,250 --> 00:20:47,350
So, what happens when all these
dwarf galaxies come together
316
00:20:47,350 --> 00:20:50,220
and start pulling and tugging
on a larger galaxy?
317
00:20:56,030 --> 00:20:59,860
Cosmologists believe there could
be hundreds of dwarf galaxies
318
00:20:59,860 --> 00:21:02,400
surrounding the Milky Way.
319
00:21:05,240 --> 00:21:08,740
A collision with just one of
these dwarf galaxies
320
00:21:08,740 --> 00:21:12,470
may have rippled the
Milky Way's spiral arms,
321
00:21:14,480 --> 00:21:17,680
but a gang of dwarf galaxies
322
00:21:17,680 --> 00:21:23,190
could have a far bigger
and far more deadly effect.
323
00:21:23,190 --> 00:21:25,490
Dwarf galaxies and the way
they interact with big galaxies,
324
00:21:25,490 --> 00:21:26,720
like the Milky Way,
325
00:21:26,720 --> 00:21:29,690
can inflect tremendous
change in our universe.
326
00:21:29,690 --> 00:21:32,330
When they slam into a galaxy,
they can change its structure.
327
00:21:32,330 --> 00:21:35,160
The Milky Way would not look
anything like it looks today
328
00:21:35,170 --> 00:21:38,200
without those dwarf galaxies.
329
00:21:38,200 --> 00:21:40,940
Repeated dwarf-galaxy collisions
330
00:21:40,940 --> 00:21:44,640
could have radically altered the
shape of the Milky Way itself.
331
00:21:47,240 --> 00:21:51,010
Their gravitational disruptions
could have created a distinctive
332
00:21:51,010 --> 00:21:55,080
and possibly fatal feature
in the middle of our galaxy...
333
00:21:55,090 --> 00:21:56,590
The galactic bar.
334
00:21:59,990 --> 00:22:02,260
The center of the Milky Way
is elongated.
335
00:22:02,260 --> 00:22:04,160
Instead of it being
shaped like a sphere,
336
00:22:04,160 --> 00:22:07,330
it's more shaped like a bar,
and the bar is made by stars
337
00:22:07,330 --> 00:22:11,470
actually orbiting in this
sort of elongated way.
338
00:22:11,470 --> 00:22:14,340
And this bar can be bad
for the health of the galaxy
339
00:22:14,340 --> 00:22:16,910
because what they do
is help to funnel gas
340
00:22:16,910 --> 00:22:18,740
into the core of the galaxy.
341
00:22:18,740 --> 00:22:23,140
The loss of this gas could be a
way of stopping star formation.
342
00:22:26,620 --> 00:22:29,820
The bar-shaped bulge
at the center of the Milky Way
343
00:22:29,820 --> 00:22:32,790
sweeps our galaxy's
star-building gas
344
00:22:32,790 --> 00:22:35,020
into the galactic nucleus.
345
00:22:35,030 --> 00:22:36,630
Here, it gets gobbled up
346
00:22:36,630 --> 00:22:39,460
by our galaxy's
supermassive black hole.
347
00:22:43,530 --> 00:22:47,700
Without the star-building
material, no new stars can form,
348
00:22:47,700 --> 00:22:51,140
and the galaxy dies.
349
00:22:51,140 --> 00:22:53,170
So, is it case closed?
350
00:22:53,180 --> 00:22:55,740
Are dwarf galaxies
killing the Milky Way?
351
00:22:55,750 --> 00:22:59,780
Is the murder weapon
a galactic bar?
352
00:22:59,780 --> 00:23:02,650
So, it's possible that
the formation of these bars
353
00:23:02,650 --> 00:23:06,350
helps turn off star formation
in the very core of the galaxy,
354
00:23:06,360 --> 00:23:09,260
but that's just the central
regions of the galaxy.
355
00:23:09,260 --> 00:23:11,130
That doesn't explain
what's going farther out
356
00:23:11,130 --> 00:23:12,960
in the spiral arms.
357
00:23:12,960 --> 00:23:16,460
So, if star formation really is
shutting down in the Milky Way,
358
00:23:16,470 --> 00:23:18,770
it's not really
the fault of the bar.
359
00:23:20,940 --> 00:23:25,310
Dwarf galaxies cause the
Milky Way grievous bodily harm
360
00:23:25,310 --> 00:23:28,380
by creating the galactic bar.
361
00:23:28,380 --> 00:23:33,820
But they're off the hook
for attempted galactic murder.
362
00:23:33,820 --> 00:23:36,550
The investigation continues,
363
00:23:36,550 --> 00:23:41,760
and it could be about
to take a dramatic twist.
364
00:23:41,760 --> 00:23:44,030
It might not be that
the galaxy's being murdered.
365
00:23:44,030 --> 00:23:46,330
It could just be
eating itself to death.
366
00:24:11,850 --> 00:24:15,850
The Milky Way
is being killed off.
367
00:24:15,850 --> 00:24:18,620
And the perpetrator
remains at large.
368
00:24:20,790 --> 00:24:22,760
Scientists
investigating the crime
369
00:24:22,760 --> 00:24:24,490
are running out of suspects.
370
00:24:26,530 --> 00:24:28,660
But the hunt
for clues continues,
371
00:24:30,640 --> 00:24:35,870
so astronomers are examining
the dying body of the Milky Way.
372
00:24:35,870 --> 00:24:39,040
Our galaxy is
a hazy disk of stars
373
00:24:39,040 --> 00:24:42,610
surrounded by a halo
of superheated gas.
374
00:24:44,480 --> 00:24:48,450
It's over 100,000
light-years across.
375
00:24:48,450 --> 00:24:50,620
But it hasn't always
been so big.
376
00:24:52,890 --> 00:24:55,090
When you think about things
so vast,
377
00:24:55,090 --> 00:24:57,130
so gigantic and ancient
as galaxies,
378
00:24:57,130 --> 00:24:58,590
you're kind of tempted to think
379
00:24:58,600 --> 00:25:00,160
that they're
very stable objects,
380
00:25:00,170 --> 00:25:02,130
that they don't change
much over time,
381
00:25:02,130 --> 00:25:03,930
but we now know
that our own galaxy
382
00:25:03,940 --> 00:25:05,870
is the product of many
smaller galaxies
383
00:25:05,870 --> 00:25:07,670
that came together over time,
384
00:25:07,670 --> 00:25:10,370
and there are other galaxies
still colliding with us.
385
00:25:13,340 --> 00:25:15,610
We see galaxies
eating each other all the time.
386
00:25:15,610 --> 00:25:18,310
They collide,
and if one galaxy is very big
387
00:25:18,320 --> 00:25:20,180
and one galaxy is very small,
388
00:25:20,190 --> 00:25:23,450
the little galaxy falls into
the big one, gets torn apart,
389
00:25:23,450 --> 00:25:27,560
and becomes a part
of that bigger galaxy.
390
00:25:27,560 --> 00:25:29,890
The Milky Way might be dying,
391
00:25:29,890 --> 00:25:33,360
but it's still a monster
foraging through the universe,
392
00:25:33,360 --> 00:25:35,730
swallowing smaller
galaxies whole.
393
00:25:39,300 --> 00:25:41,370
It consumes their stars.
394
00:25:41,370 --> 00:25:45,440
But it also has a taste
for their star-building gas.
395
00:25:47,310 --> 00:25:50,650
And it doesn't have
to collide with other galaxies
396
00:25:50,650 --> 00:25:53,980
to feed off of them.
397
00:25:53,990 --> 00:25:56,790
Now, the lifeblood of a galaxy
is hydrogen gas.
398
00:25:56,790 --> 00:25:59,060
That's what actually
creates new stars.
399
00:25:59,060 --> 00:26:02,020
So as a dwarf galaxy
passes by the Milky Way,
400
00:26:02,030 --> 00:26:05,230
the tremendously massive
halo of the Milky Way,
401
00:26:05,230 --> 00:26:08,600
all of that gas, can draw off
material from the dwarf galaxy,
402
00:26:08,600 --> 00:26:10,670
adding it to the Milky Way.
403
00:26:10,670 --> 00:26:12,900
So in this way,
the Milky Way drains away
404
00:26:12,900 --> 00:26:15,940
the lifeblood of other galaxies.
405
00:26:15,940 --> 00:26:18,510
In some sense, you could say
it's a vampire
406
00:26:18,510 --> 00:26:21,310
because a vampire sucks the life
out of other things
407
00:26:21,310 --> 00:26:22,750
so it can remain young.
408
00:26:26,680 --> 00:26:33,990
In its 13 billion-year life,
our vampire galaxy has feasted.
409
00:26:33,990 --> 00:26:37,260
Consuming the lifeblood
of its galactic victims,
410
00:26:37,260 --> 00:26:40,100
the Milky Way has grown fat.
411
00:26:42,530 --> 00:26:45,670
But could this monstrous
feeding frenzy be a factor
412
00:26:45,670 --> 00:26:47,500
in the Milky Way's demise?
413
00:26:50,410 --> 00:26:52,310
Once again, crucial evidence
414
00:26:52,310 --> 00:26:56,910
comes from the
Sloan digital sky survey.
415
00:26:56,920 --> 00:26:59,980
Their telescope maps
the stars in our galaxy,
416
00:26:59,980 --> 00:27:03,790
but it also maps
the galaxies in our universe.
417
00:27:06,260 --> 00:27:11,760
Looking at distant galaxies
is like looking back in time.
418
00:27:11,760 --> 00:27:14,160
Because the farther away
they are,
419
00:27:14,170 --> 00:27:16,930
the longer their light
takes to reach us.
420
00:27:19,800 --> 00:27:24,740
We see the most distant
galaxies not as they are now,
421
00:27:24,740 --> 00:27:29,410
but as they were...
Billions of years ago.
422
00:27:29,410 --> 00:27:30,850
So, when you look
at these galaxies,
423
00:27:30,850 --> 00:27:33,120
you're seeing them as they were
when they were very young,
424
00:27:33,120 --> 00:27:36,120
and you're seeing these galaxies
as they are more recently,
425
00:27:36,120 --> 00:27:38,490
so you can actually look
at the evolution...
426
00:27:38,490 --> 00:27:42,390
How galaxies change over time
as the universe ages.
427
00:27:44,530 --> 00:27:46,100
While studying the data,
428
00:27:46,100 --> 00:27:49,100
scientists make
a dramatic discovery.
429
00:27:49,100 --> 00:27:53,670
They find spiral galaxies,
just like the Milky Way,
430
00:27:53,670 --> 00:27:57,410
dying all over the universe.
431
00:27:57,410 --> 00:28:00,240
And what connects them
is their mass.
432
00:28:02,280 --> 00:28:03,850
There seems to be
an upper weight limit
433
00:28:03,850 --> 00:28:06,680
for the sizes of
spiral galaxies.
434
00:28:06,680 --> 00:28:09,150
Up to about a trillion times
the mass of the Sun,
435
00:28:09,150 --> 00:28:12,050
we see spiral galaxies
that continue to form stars,
436
00:28:12,060 --> 00:28:14,020
but once they pass
this threshold,
437
00:28:14,030 --> 00:28:16,430
galaxies tend to die
and run out of stars.
438
00:28:18,600 --> 00:28:20,700
While devouring
the star-building gas
439
00:28:20,700 --> 00:28:22,900
of smaller galaxies,
440
00:28:22,900 --> 00:28:26,000
the Milky Way
may have grown obese,
441
00:28:26,000 --> 00:28:30,010
and now it could be choking
to death on its own dinner.
442
00:28:30,010 --> 00:28:32,270
But how?
443
00:28:32,280 --> 00:28:34,980
Once a spiral galaxy
is sufficiently big,
444
00:28:34,980 --> 00:28:37,580
it's going to have an
incredible gravitational force,
445
00:28:37,580 --> 00:28:39,980
so any gas that it pulls
to itself
446
00:28:39,980 --> 00:28:43,550
is going to come in
at an incredibly high speed.
447
00:28:43,550 --> 00:28:45,760
That gas is going to be
superheated.
448
00:28:49,660 --> 00:28:52,290
The superheated gas
moves so quickly
449
00:28:52,300 --> 00:28:56,400
that it's prevented
from falling into the Milky Way.
450
00:28:56,400 --> 00:29:01,570
The gas is too energetic for our
galaxy's gravity to pull it in.
451
00:29:01,570 --> 00:29:05,780
Instead, it stays in the halo
around the Milky Way,
452
00:29:05,780 --> 00:29:10,510
and our galaxy's food
supply is choked off.
453
00:29:10,520 --> 00:29:14,950
Eventually, our galaxy
will starve.
454
00:29:14,950 --> 00:29:16,850
This will only happen
if the Milky Way
455
00:29:16,850 --> 00:29:20,290
is over the star-building
weight limit.
456
00:29:20,290 --> 00:29:24,830
But how exactly
do you weigh a galaxy?
457
00:29:24,830 --> 00:29:27,300
One basic way
we can weigh a galaxy
458
00:29:27,300 --> 00:29:30,000
is measure how fast
the stars are moving within it.
459
00:29:30,000 --> 00:29:32,940
So the faster the stars orbit
around the center of the galaxy,
460
00:29:32,940 --> 00:29:35,810
the more massive the galaxy is.
461
00:29:35,810 --> 00:29:40,110
This method of weighing
the Milky Way relies on gravity.
462
00:29:40,110 --> 00:29:43,380
Fast-moving stars need
more gravity to hold them
463
00:29:43,380 --> 00:29:44,710
in their orbits,
464
00:29:44,720 --> 00:29:49,590
and more gravity means
more galactic mass.
465
00:29:49,590 --> 00:29:53,690
When scientists use this
information to run the math,
466
00:29:53,690 --> 00:29:57,230
the horrible truth is revealed.
467
00:29:57,230 --> 00:29:59,930
We've passed
kind of a critical level.
468
00:29:59,930 --> 00:30:03,500
The Milky Way is far too massive
for its own health,
469
00:30:03,500 --> 00:30:06,340
and we've entered
the beginning of the end.
470
00:30:06,340 --> 00:30:09,540
We're running out of gas,
and I mean this literally.
471
00:30:09,540 --> 00:30:11,410
Gas clouds form stars,
472
00:30:11,410 --> 00:30:14,840
and as they form stars,
they're used up,
473
00:30:14,850 --> 00:30:17,080
and so our gas tank
is getting closer and closer
474
00:30:17,080 --> 00:30:19,580
to empty every day.
475
00:30:23,350 --> 00:30:26,390
The investigation into
the killing of the Milky Way
476
00:30:26,390 --> 00:30:27,760
is closed.
477
00:30:29,960 --> 00:30:31,330
The verdict?
478
00:30:31,330 --> 00:30:35,130
The greedy Milky Way
is killing itself.
479
00:30:37,770 --> 00:30:41,770
Over millions of years,
star formation grinds to a halt,
480
00:30:41,770 --> 00:30:43,670
and the galaxy dies.
481
00:30:46,310 --> 00:30:51,450
But could
the galaxy be resurrected?
482
00:30:51,450 --> 00:30:53,320
We seem to be telling
a very sad story.
483
00:30:53,320 --> 00:30:55,520
We're talking about the demise
of the Milky Way galaxy...
484
00:30:55,520 --> 00:30:57,290
The end of star formation...
485
00:30:57,290 --> 00:30:59,120
But maybe it's just
a little bit too soon
486
00:30:59,120 --> 00:31:00,890
to write the death
announcement yet.
487
00:31:00,890 --> 00:31:03,390
Hope could be
just over the horizon.
488
00:31:06,230 --> 00:31:09,470
In space and astrophysics,
really anything is possible.
489
00:31:30,980 --> 00:31:35,750
The shocking case of our
dying galaxy has been solved.
490
00:31:35,750 --> 00:31:38,090
There was no killer.
491
00:31:38,090 --> 00:31:41,760
Turns out, the Milky Way
is eating itself to death.
492
00:31:46,000 --> 00:31:49,470
But is this really the end?
493
00:31:49,470 --> 00:31:52,440
Could salvation
be heading our way?
494
00:31:54,340 --> 00:31:56,210
Even if star formation is
turning off
495
00:31:56,210 --> 00:31:57,570
in the Milky Way now,
496
00:31:57,580 --> 00:31:59,380
we know that it's on
a collision course
497
00:31:59,380 --> 00:32:01,340
with the Andromeda galaxy.
498
00:32:01,350 --> 00:32:02,510
They're moving toward each other
499
00:32:02,510 --> 00:32:06,820
at hundreds of thousands
of miles per hour.
500
00:32:06,820 --> 00:32:08,320
A collision sounds
like something
501
00:32:08,320 --> 00:32:09,850
that's always destructive,
502
00:32:09,860 --> 00:32:11,860
but that's not
necessarily the case.
503
00:32:14,790 --> 00:32:15,990
The Milky Way's collision
504
00:32:15,990 --> 00:32:19,930
with our giant
galactic neighbor Andromeda
505
00:32:19,930 --> 00:32:23,100
won't happen
for another 4 billion years.
506
00:32:25,440 --> 00:32:29,070
By then, star formation
in both of these galaxies
507
00:32:29,070 --> 00:32:30,770
will have stopped completely.
508
00:32:33,550 --> 00:32:37,550
But a giant meet-up
could change all that.
509
00:32:40,820 --> 00:32:42,290
As an isolated galaxy,
510
00:32:42,290 --> 00:32:45,690
the Milky Way is already
in its wind-down phase.
511
00:32:45,690 --> 00:32:49,360
It's not producing as many
new stars as it used to.
512
00:32:49,360 --> 00:32:53,530
But there is one way to generate
a new round of star formation,
513
00:32:53,530 --> 00:32:57,230
and that's through
a galactic merger event.
514
00:32:57,240 --> 00:32:59,200
When Andromeda
gets close enough,
515
00:32:59,200 --> 00:33:01,300
the mutual gravity
between the two galaxies
516
00:33:01,310 --> 00:33:05,610
will start to stretch them out,
pulling them out like Taffy.
517
00:33:05,610 --> 00:33:08,550
Stars will be pulled out into
these long, looping streamers,
518
00:33:08,550 --> 00:33:09,680
and then the galaxies
519
00:33:09,680 --> 00:33:12,850
will physically pass
through each other.
520
00:33:12,850 --> 00:33:16,620
Eventually, the two galaxies
will draw back together again
521
00:33:16,620 --> 00:33:19,260
and merge into
one gigantic galaxy,
522
00:33:19,260 --> 00:33:21,190
and at that point,
all of these gas clouds
523
00:33:21,190 --> 00:33:24,360
will flash into star formation.
524
00:33:29,070 --> 00:33:33,600
As the galaxies merge,
they'll be reborn.
525
00:33:33,610 --> 00:33:36,570
Two dying spiral-shaped galaxies
526
00:33:36,580 --> 00:33:40,840
become a single living
elliptical galaxy
527
00:33:40,850 --> 00:33:44,850
called Milkomeda.
528
00:33:44,850 --> 00:33:47,450
Imagine you're living in
the far future of the galaxy
529
00:33:47,450 --> 00:33:49,050
and you see the night sky
530
00:33:49,050 --> 00:33:51,620
while the Milky Way
and Andromeda are colliding.
531
00:33:51,620 --> 00:33:53,920
It will look like
a very different place.
532
00:33:53,930 --> 00:33:56,160
Rather than one band
across the night sky,
533
00:33:56,160 --> 00:33:58,900
you might have two
as the two disks come together.
534
00:33:58,900 --> 00:34:00,560
It will be a miraculous sight,
535
00:34:00,570 --> 00:34:03,400
but a very, very different
place than we have today.
536
00:34:06,710 --> 00:34:11,880
Our sky will light up for the
first time in billions of years.
537
00:34:11,880 --> 00:34:15,310
Star formation will flare
across the galaxy.
538
00:34:17,650 --> 00:34:20,550
But is it too soon to celebrate?
539
00:34:24,290 --> 00:34:26,660
This new round of star formation
540
00:34:26,660 --> 00:34:30,490
during the merger
of our two galaxies...
541
00:34:30,500 --> 00:34:32,860
While it's very cool
for a little bit,
542
00:34:32,870 --> 00:34:37,600
once it's over, that kind of
sends the new galaxy
543
00:34:37,600 --> 00:34:39,000
into a death spiral.
544
00:34:39,000 --> 00:34:41,770
When new stars are born
in this new galaxy,
545
00:34:41,770 --> 00:34:46,080
many of them are going to
be hot, large, blue stars.
546
00:34:46,080 --> 00:34:49,280
Eventually, those young, hot
stars are going to start to die,
547
00:34:49,280 --> 00:34:51,380
and when they do,
they're going to explode
548
00:34:51,380 --> 00:34:53,380
violently as supernovae.
549
00:34:58,860 --> 00:35:00,860
And those supernovae
are going to start
550
00:35:00,860 --> 00:35:03,390
blasting gas out of the galaxy.
551
00:35:07,070 --> 00:35:08,600
All of the gas is gone.
552
00:35:08,600 --> 00:35:10,900
There's no more stuff
to form stars.
553
00:35:10,900 --> 00:35:12,670
And that's what kills a galaxy.
554
00:35:16,440 --> 00:35:18,740
It'll take
hundreds of millions of years
555
00:35:18,740 --> 00:35:23,350
for Milkomeda
to run out of star-building gas.
556
00:35:23,350 --> 00:35:27,650
And then our new elliptical
galaxy will starve.
557
00:35:29,450 --> 00:35:32,060
But the final blow
is still to come.
558
00:35:34,490 --> 00:35:36,730
Another issue to consider
is what happens
559
00:35:36,730 --> 00:35:38,760
to the two supermassive
black holes
560
00:35:38,760 --> 00:35:41,460
at the cores of
the two galaxies.
561
00:35:41,470 --> 00:35:44,000
Well, initially, they're going
to orbit each other,
562
00:35:44,000 --> 00:35:48,570
stirring up a lot of turbulence,
and they're going to combine.
563
00:35:48,570 --> 00:35:51,510
And because there's
a lot of new, hot, fresh gas,
564
00:35:51,510 --> 00:35:54,680
our new galaxy is
going to be a quasar.
565
00:35:54,680 --> 00:35:57,210
And that quasar is going
to turn up the heat,
566
00:35:57,220 --> 00:35:58,850
it's going to turn up
the turbulence,
567
00:35:58,850 --> 00:36:02,090
and this means star formation
is going to be shut off.
568
00:36:04,360 --> 00:36:07,960
The combined power of the
supermassive black holes
569
00:36:07,960 --> 00:36:11,960
help create a quasar
that tears through the galaxy.
570
00:36:15,070 --> 00:36:18,270
It releases ferocious
beams of radiation
571
00:36:18,270 --> 00:36:21,840
that blast through Milkomeda's
star-forming gas.
572
00:36:25,180 --> 00:36:27,080
It's only just been reborn,
573
00:36:27,080 --> 00:36:31,650
but our newly enlarged galaxy
is once again dying.
574
00:36:34,350 --> 00:36:39,520
Vast galaxies, like Milkomeda,
seem doomed from the start.
575
00:36:39,520 --> 00:36:43,630
Their size creates too many
problems for star formation.
576
00:36:46,130 --> 00:36:49,400
Or... so we thought.
577
00:36:49,400 --> 00:36:51,740
The more galaxies we see,
the more we realize
578
00:36:51,740 --> 00:36:53,800
there's a lot out there
we haven't discovered,
579
00:36:53,810 --> 00:36:55,570
and there's a new class
of galaxies
580
00:36:55,570 --> 00:36:57,970
only recently identified.
581
00:36:57,980 --> 00:37:02,580
These galaxies are more than 10
times the mass of the Milky Way.
582
00:37:02,580 --> 00:37:06,680
And, intriguingly,
they're still forming stars.
583
00:37:06,690 --> 00:37:08,420
Apparently
we've missed something.
584
00:37:32,240 --> 00:37:37,480
4 billion years from now,
the Milky Way is no more.
585
00:37:37,480 --> 00:37:40,610
After colliding with Andromeda,
it's reborn
586
00:37:40,610 --> 00:37:44,520
as a giant elliptical galaxy
called Milkomeda.
587
00:37:46,450 --> 00:37:50,890
Scientists thought galaxies
this big were doomed.
588
00:37:50,890 --> 00:37:53,960
But is hope on the horizon?
589
00:37:56,430 --> 00:38:00,000
The Sloan digital sky survey
has spent a decade
590
00:38:00,000 --> 00:38:04,570
studying
over a million galaxies.
591
00:38:04,570 --> 00:38:08,970
It's discovered a rare
but enormous kind of galaxy...
592
00:38:08,980 --> 00:38:10,740
A super spiral.
593
00:38:13,650 --> 00:38:16,580
These super-spiral galaxies
are spiral galaxies
594
00:38:16,580 --> 00:38:18,150
that are incredibly super,
595
00:38:18,150 --> 00:38:21,920
and by "super," I mean
they have four times the size,
596
00:38:21,920 --> 00:38:23,620
10 times the mass,
597
00:38:23,620 --> 00:38:25,290
and they're weird
because they exceed
598
00:38:25,290 --> 00:38:28,290
the supposed weight limit
for spiral galaxies.
599
00:38:28,300 --> 00:38:31,300
So they shouldn't have
new stars, but they do.
600
00:38:31,300 --> 00:38:33,630
They're very healthy galaxies.
601
00:38:39,240 --> 00:38:43,140
Scientists have found
just 53 super spirals.
602
00:38:46,250 --> 00:38:50,350
Super-spiral galaxies show
that in rare situations,
603
00:38:50,350 --> 00:38:54,920
massive galaxies continue
to produce new stars.
604
00:38:58,790 --> 00:39:02,330
So, is this a lifeline
for Milkomeda?
605
00:39:05,170 --> 00:39:07,200
When we think about
two galaxies colliding,
606
00:39:07,200 --> 00:39:08,830
a lot of our computer models
suggest
607
00:39:08,840 --> 00:39:10,440
that they really
mess each other up.
608
00:39:10,440 --> 00:39:12,140
Things get very chaotic.
609
00:39:12,140 --> 00:39:15,970
But over time, could they settle
back down into a spiral shape?
610
00:39:15,980 --> 00:39:18,840
And, in fact, that may be
what happens with super spirals.
611
00:39:18,850 --> 00:39:21,410
One of the clues is that many
super-spiral galaxies
612
00:39:21,410 --> 00:39:22,950
have double cores.
613
00:39:22,950 --> 00:39:25,420
Instead of there just being
one supermassive black hole,
614
00:39:25,420 --> 00:39:29,250
there are actually
two orbiting each other.
615
00:39:29,260 --> 00:39:31,860
The fact that we see spiral
galaxies with two cores
616
00:39:31,860 --> 00:39:33,890
makes it possible
that you could have a collision
617
00:39:33,890 --> 00:39:36,430
and still survive
as a spiral galaxy.
618
00:39:36,430 --> 00:39:39,400
So maybe there's hope that even
the Milky Way will be a spiral
619
00:39:39,400 --> 00:39:41,430
once it collides with Andromeda.
620
00:39:45,640 --> 00:39:49,440
Picture the scene...
6 billion years in the future.
621
00:39:51,640 --> 00:39:54,250
Milkomeda drifts
through the universe...
622
00:39:56,220 --> 00:40:00,250
not as an elliptical galaxy,
but as a super spiral.
623
00:40:02,820 --> 00:40:07,490
This shape means the galaxy
is far more stable.
624
00:40:07,490 --> 00:40:09,530
The damaging heat and turbulence
625
00:40:09,530 --> 00:40:13,030
generated by Milkomeda's
supermassive black holes
626
00:40:13,030 --> 00:40:19,070
can't disrupt star-building gas
way out in the spiral arms.
627
00:40:19,070 --> 00:40:24,910
Far from dying off,
our galaxy lives on...
628
00:40:24,910 --> 00:40:28,610
Larger than ever before.
629
00:40:28,620 --> 00:40:31,880
But that isn't
the end of the story.
630
00:40:31,890 --> 00:40:34,990
Tens of billions
of years from now,
631
00:40:34,990 --> 00:40:38,960
could the galaxy
continue to grow?
632
00:40:38,960 --> 00:40:41,190
Our local group of galaxies...
633
00:40:41,190 --> 00:40:43,730
Milky Way, Andromeda,
Triangulum...
634
00:40:43,730 --> 00:40:47,470
And then a collection
of dwarf satellite galaxies...
635
00:40:47,470 --> 00:40:50,540
Is gravitationally
bound together,
636
00:40:50,540 --> 00:40:53,140
and eventually,
we're all glued together
637
00:40:53,140 --> 00:40:55,340
into a single massive object.
638
00:40:57,080 --> 00:40:58,540
What does this mean?
639
00:40:58,550 --> 00:41:00,980
This means we might be part of
one of the largest structures
640
00:41:00,980 --> 00:41:03,180
in the universe.
641
00:41:03,180 --> 00:41:05,450
During
its billion years of life,
642
00:41:05,450 --> 00:41:10,050
the Milky Way changes
beyond recognition.
643
00:41:10,060 --> 00:41:13,390
It suffers countless collisions,
644
00:41:13,390 --> 00:41:17,560
feasts on many smaller galaxies,
645
00:41:17,560 --> 00:41:20,630
and gives birth
to innumerable stars.
646
00:41:22,840 --> 00:41:24,900
We talk about
the life cycle of galaxies...
647
00:41:24,900 --> 00:41:28,110
How they're born, how they live
healthy lives making new stars,
648
00:41:28,110 --> 00:41:30,210
and eventually
how they die away.
649
00:41:30,210 --> 00:41:32,380
It's really not
as depressing as that.
650
00:41:32,380 --> 00:41:34,480
Everything in
the universe changes.
651
00:41:36,550 --> 00:41:41,850
Galaxies like ours are in
a constant state of flux.
652
00:41:41,860 --> 00:41:44,660
So when it comes to
the Milky Way,
653
00:41:44,660 --> 00:41:47,290
death really isn't the end.
654
00:41:49,200 --> 00:41:50,760
What we see in our universe
655
00:41:50,760 --> 00:41:54,530
is that there's always
a process of birth and rebirth,
656
00:41:54,530 --> 00:41:56,100
so the future of the Milky Way
657
00:41:56,100 --> 00:41:58,400
is that it's going to
keep on doing what it does.
658
00:42:01,040 --> 00:42:02,740
Galaxies are ever-changing.
659
00:42:02,740 --> 00:42:04,180
10 billion years ago,
the Milky Way
660
00:42:04,180 --> 00:42:06,010
was nothing like
what it is today,
661
00:42:06,010 --> 00:42:08,010
and certainly,
10 billion years in the future,
662
00:42:08,010 --> 00:42:09,850
it'll be a very different place.
663
00:42:12,420 --> 00:42:14,790
Look, I live in this galaxy.
664
00:42:14,790 --> 00:42:17,390
I hope that it can find a way
to rejuvenate itself
665
00:42:17,390 --> 00:42:19,520
through collisions
or some other process
666
00:42:19,530 --> 00:42:21,260
because that gives me some hope
667
00:42:21,260 --> 00:42:24,130
that it'll go on
for a long, long time.
668
00:42:24,431 --> 00:42:27,931
669
00:42:27,981 --> 00:42:32,531
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