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SUB BY : DENI AUROR@
https://aurorarental.blogspot.com/
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The fate of our galaxy hangs in the balance.
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The Milky Way is dying, and we don't know why.
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Our galaxy, like all galaxies, has a limited life-span.
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After that, it's lights out.
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The race is on to find a smoking gun.
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It's safe to say right now there are many ways to kill a galaxy.
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It's a cosmic crime scene investigation.
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Is it murder most foul?
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Or is it death by natural causes?
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The suspects are lined up.
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The interrogation is underway.
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It's another example of this big universe of ours
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throwing puzzles at us that now we have to solve.
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What is killing the Milky Way?
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captions paid for by discovery communications
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Earth.
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Our home.
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Just one of 100 billion planets
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orbiting 400 billion stars
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that make up an immense galactic spiral...
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the Milky Way.
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Galaxies are where stars form,
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and, of course, planets form around stars,
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so the story of the Earth,
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of yourself, of the solar system
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has everything to do with the story of the galaxies.
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The story of the Milky Way begins 13.6 billion years ago,
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just after the big bang.
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It's a time when there are no planets and no stars...
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Just a vast, lumpy soup of superheated hydrogen gas.
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Over millions of years, the temperature drops,
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and gravity compresses the lumps down, until eventually
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the hydrogen molecules fuse and ignite a star.
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In time, billions of stars burst into life.
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And the Milky Way begins to take shape.
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You can think of a galaxy as sort of like a human being.
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When you're young and in your adolescent stage,
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you're vibrant and active.
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That's a young galaxy forming stars in a crazy way,
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and it's not even fully formed yet.
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At a certain point, galaxy reaches middle age,
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and a middle-aged galaxy really is what it's going to be...
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It has its shape...
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But in the long run, a galaxy will stop forming stars,
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and eventually, just like we all die, our galaxy will die.
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So, at what stage of life is the Milky Way?
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Is it a healthy, active youngster,
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or is it heading for its deathbed?
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Scientists can determine each galaxy's stage of life
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by its color.
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So, we see different colors of galaxies in the universe.
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We see galaxies that are tinted blue
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and galaxies that are tinted red.
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When we see a blue galaxy, that tends to be a younger galaxy
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full of bright, hot, newly formed stars.
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When we see a redder galaxy,
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that tends to be a dimmer, older galaxy
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that isn't forming new stars in the present moment.
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All its stars are aged and older and redder,
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and so the entire galaxy casts a different hue.
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So, what color is our galaxy?
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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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people noticed there was this light band
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that actually went from horizon to horizon,
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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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But it took us a long time to realize
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what the shape and the scale of the Milky Way galaxy is.
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The amazing thing to think about is that we actually don't know
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our home galaxy very well at all.
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We actually live in the middle of this disk of gas and dust,
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and that obscures our view of the larger Milky Way.
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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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The solution is to use a form of light
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that passes through the gas and dust...
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Infrared.
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This is the Sloan digital sky survey telescope
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at the Apache point observatory in new Mexico.
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It's mapping the galaxy using infrared
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and giving scientists unprecedented insights.
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The first sensitive infrared observations really weren't done
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till the last 15 years,
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and each of these new windows on the universe
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teach us different things.
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In the last 15 years,
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Sloan has surveyed more than 250 million stars,
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analyzing their light to work out the color of the Milky Way.
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And what scientists saw shocked them.
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Until very recently, we thought the Milky Way
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was a young, healthy galaxy,
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but now there's evidence
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that we may be entering the pathway to death.
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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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And when it stops forming new stars,
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its time will be up.
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Paradoxically, our galaxy still has
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star-forming gas in the tank, so it should be healthy,
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but something is killing it off.
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So, the Milky Way galaxy is this wonderful disk
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filled with rich hydrogen gas, lots of dense dust clouds.
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It has everything you need there for star formation,
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but it seems to be slowing down and even turning off,
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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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and we have to figure out who done it.
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Our home in the universe is dying...
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Not the Earth, but our galaxy, the Milky Way.
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It's been producing stars for billions of years,
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but soon, it will stop.
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Our own sun formed about 4 1/2 billion years ago
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in the Milky Way galaxy,
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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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one of the last generations of new stars in the Milky Way.
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Current projections suggest that in about 4 billion years,
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star formation may have ceased all together,
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which is almost just a blink of an eye
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in the life cycle of the universe.
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To find out why, scientists launch an investigation.
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The most crucial question?
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How is the milky way dying?
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To kill a galaxy, you have to get rid of the cold gas,
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because that's what stars form from.
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There are many ways you can do this.
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You can blast it out from the inside.
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You can draw it out from the outside.
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You can heat it up so it's no longer cold.
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You can use it all up,
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and there's even more ways you can stop it.
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What we have to do is figure out
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which way is happening in our galaxy.
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Perhaps the culprit is inside the Milky Way itself.
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A clue comes from another galaxy entirely.
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This is w2246-0526.
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Scientists call it a hot, dust-obscured galaxy,
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or "hot dog" for short.
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This galaxy is 12 1/2 billion light-years away.
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It's the most luminous galaxy we know of in the universe.
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It has the light of 300 trillion stars.
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The source of the intense light is not its stars,
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but a mysterious object at the galaxy's center.
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It's a million times smaller than the galaxy itself.
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There's only one thing that small and that powerful...
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a supermassive black hole.
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So, supermassive black holes, as the name suggests,
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are indeed supermassive.
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These are billions of times more massive than our sun.
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These are gigantic objects.
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The gravity in the supermassive black hole is off the charts.
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It sucks in incredible amounts
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of the hot dog's vital star-forming gas.
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And as the gas swirls to form a disk,
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the intense friction superheats it to millions of degrees
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and, in some galaxies, triggers huge jets.
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When a lot of material falls onto that black hole,
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it creates incredibly energetic jets
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that can be tens of thousands of light-years across.
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All of a sudden, you have this blowtorch
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in the middle of the galaxy.
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Black hole jets are bad for galaxies
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because they can shut down star formation.
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They can heat gas up, blow gas out of galaxies,
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and they could really kill them.
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A supermassive black hole is cooking the hot dog.
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What's going on in our galaxy?
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In 2016, scientists at Harvard discovered damning evidence
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that may link the Milky Way's supermassive black hole
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to the galaxy's demise.
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Just like the hot dog,
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the Milky Way is surrounded by a vast cloud of blown-out gas,
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and the scientists traced the gas back to its source...
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Sagittarius a-star, our supermassive black hole.
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Well, it turns out our supermassive black hole
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had a bit of a hiccup about 6 million years ago.
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There's evidence that some matter must have fallen
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into that black hole, and if it fell in too quickly,
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it would have gotten superheated by its own friction,
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and this would have acted, in a sense, like an explosion.
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And that event was huge.
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Our galaxy expelled an incredible amount of gas...
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130 billion times the mass of the Sun.
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Large amount of gas.
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This event must have been very catastrophic
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for the inner parts of the galaxy.
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Luckily, Earth is in the outer parts of the galaxy,
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where we were able to survive this event.
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Is this the smoking gun?
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Is our own supermassive black hole killing the Milky Way?
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The evidence seems to mount up.
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But Sagittarius a-star has an alibi.
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It exploded too late.
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Sagittarius a-star got very active,
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very explosive about 6 million years ago,
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but that's so recent, it shouldn't have really affected
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the star formation rates.
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Something else is going on.
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There must be another culprit besides the black hole.
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Studies suggest our supermassive black hole must have been active
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hundreds of millions of years ago
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to stop all star formation in our galaxy.
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Sagittarius a-star wasn't active at that time,
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so it's no longer a suspect.
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The hunt is on for a different galaxy killer,
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and scientists are widening the investigation.
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Maybe the killer isn't inside our galaxy.
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It could be that we suffered a hit-and-run.
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Our universe is a crime scene.
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Star production in the Milky Way is breaking down.
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Our galaxy is dying,
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and astronomers are examining the body for clues.
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The Milky Way's disk is made up of three sections...
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A nucleus, home to the galaxy's supermassive black hole...
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a dense, central bulge 10,000 light-years across,
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and the spiral arms...
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Full of gas, dust, and billions of stars.
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The spiral arms should be flat, but they're rippling.
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Is this a clue for the cosmic detectives?
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Today, we look at the edge of the Milky Way,
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and we see mysterious ripples in its gas,
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and we wonder, what's the origin?
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Something must have caused it to happen.
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Something like that just doesn't happen on its own.
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The real question is, why?
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Whatever caused the ripples didn't hang around.
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Is this evidence of a galactic hit-and-run?
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January 2016.
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Astronomers studying data from the vista telescope
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discover something incredible...
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three nearby stars.
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On their own, nothing special,
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except they've recently left our galaxy,
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and they're traveling at 350,000 miles an hour.
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So, we've discovered these stars that are careening
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out of the galaxy at super-high velocities.
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Could these three stars somehow be responsible
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for warping the Milky Way's disk?
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Well, absolutely not.
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The Milky Way is so much more massive than just three stars.
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Three stars alone can't warp a galaxy,
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but those three stars can be indicative of more stars.
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They can be indicative of the presence of, say,
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a dwarf galaxy, and that can warp the galaxy.
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Dwarf galaxies are abundant.
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But a tiny fraction of the size of a major galaxy,
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like the Milky Way.
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00:17:38,330 --> 00:17:41,700
So they're difficult to detect.
260
00:17:41,700 --> 00:17:45,900
But these three bright stars show there's a dwarf galaxy
261
00:17:45,900 --> 00:17:50,210
hiding beyond the edge of the Milky Way.
262
00:17:50,210 --> 00:17:53,340
And scientists can study the trio of stars
263
00:17:53,340 --> 00:17:55,080
to rewind the clock
264
00:17:55,080 --> 00:17:59,280
and track back the past movements of the dwarf galaxy.
265
00:18:02,150 --> 00:18:04,950
Simulations suggest that millions of years ago,
266
00:18:04,960 --> 00:18:06,220
this dwarf galaxy
267
00:18:06,220 --> 00:18:08,290
punched through the plane of the Milky Way.
268
00:18:13,230 --> 00:18:18,000
As the fast-moving dwarf galaxy hurtles towards the Milky Way,
269
00:18:18,000 --> 00:18:23,740
millions of stars seem set on a collision course.
270
00:18:23,740 --> 00:18:26,070
Catastrophe looks inevitable.
271
00:18:27,980 --> 00:18:30,980
But appearances can be deceptive.
272
00:18:32,850 --> 00:18:35,320
When galaxies collide, the first thing you might imagine
273
00:18:35,320 --> 00:18:37,950
is that the stars collide, but actually, that doesn't happen.
274
00:18:37,960 --> 00:18:39,760
Galaxies are mostly empty space.
275
00:18:39,760 --> 00:18:41,960
If you took the Sun, which is really big...
276
00:18:41,960 --> 00:18:43,690
It's a million miles across...
277
00:18:43,690 --> 00:18:47,660
And shrunk it down to the size of a piece of pollen,
278
00:18:47,660 --> 00:18:51,030
the galaxy itself would be twice the size of the pacific ocean,
279
00:18:51,040 --> 00:18:54,500
and the nearest star to the Sun would be a mile away.
280
00:18:54,510 --> 00:18:57,670
Those tiny pieces of pollen are never going to hit each other.
281
00:19:01,610 --> 00:19:05,750
The distances involved are staggering.
282
00:19:05,750 --> 00:19:07,420
And at the moment of impact,
283
00:19:07,420 --> 00:19:09,990
most of the stars from the two galaxies
284
00:19:09,990 --> 00:19:12,620
miss each other entirely.
285
00:19:12,620 --> 00:19:16,660
But that doesn't mean the Milky Way is safe.
286
00:19:18,830 --> 00:19:21,600
Even though the stars just pass each other,
287
00:19:21,600 --> 00:19:25,430
they do gravitationally interact as they come close,
288
00:19:25,440 --> 00:19:28,800
and this gravitational interaction sets them
289
00:19:28,810 --> 00:19:30,410
on a course that is different
290
00:19:30,410 --> 00:19:32,840
than if they were to live by themselves.
291
00:19:35,680 --> 00:19:37,780
In much the same way that taking a stone
292
00:19:37,780 --> 00:19:39,450
and dropping it into a still pond
293
00:19:39,450 --> 00:19:41,580
creates ripples in the water,
294
00:19:41,590 --> 00:19:44,250
a galaxy like this slamming into the Milky Way
295
00:19:44,250 --> 00:19:46,660
can create ripple effects throughout the disk.
296
00:19:49,960 --> 00:19:51,630
The ripples in the Milky Way
297
00:19:51,630 --> 00:19:55,030
stretch across tens of thousands of light-years.
298
00:19:57,200 --> 00:20:02,540
Still, this hit-and-run isn't enough to kill the Milky Way.
299
00:20:02,540 --> 00:20:06,210
It only causes a flesh wound.
300
00:20:06,210 --> 00:20:11,710
But what if this dwarf galaxy isn't acting alone?
301
00:20:11,720 --> 00:20:13,680
What if it has accomplices?
302
00:20:16,220 --> 00:20:18,950
There are a lot of dwarf galaxies out there,
303
00:20:18,960 --> 00:20:22,220
and it turns out collisions between these dwarf galaxies
304
00:20:22,230 --> 00:20:25,090
and big galaxies, like the Milky Way, are common.
305
00:20:25,100 --> 00:20:26,360
They happen all the time.
306
00:20:26,360 --> 00:20:28,400
Right now, there are several dwarf galaxies
307
00:20:28,400 --> 00:20:30,430
that the Milky Way is swallowing up.
308
00:20:30,430 --> 00:20:32,630
In fact, a really fun thing is that we're actually closer
309
00:20:32,640 --> 00:20:34,800
to the core of one of these galaxies...
310
00:20:34,810 --> 00:20:36,740
The Canis Majoris dwarf galaxy...
311
00:20:36,740 --> 00:20:38,570
Than we are to the core of the Milky Way.
312
00:20:38,580 --> 00:20:41,510
So some of the stars that you see around you in the night sky
313
00:20:41,510 --> 00:20:44,250
are actually stars from a different galaxy.
314
00:20:44,250 --> 00:20:47,350
So, what happens when all these dwarf galaxies come together
315
00:20:47,350 --> 00:20:50,220
and start pulling and tugging on a larger galaxy?
316
00:20:56,030 --> 00:20:59,860
Cosmologists believe there could be hundreds of dwarf galaxies
317
00:20:59,860 --> 00:21:02,400
surrounding the Milky Way.
318
00:21:05,240 --> 00:21:08,740
A collision with just one of these dwarf galaxies
319
00:21:08,740 --> 00:21:12,470
may have rippled the Milky Way's spiral arms,
320
00:21:14,480 --> 00:21:17,680
but a gang of dwarf galaxies
321
00:21:17,680 --> 00:21:23,190
could have a far bigger and far more deadly effect.
322
00:21:23,190 --> 00:21:25,490
Dwarf galaxies and the way they interact with big galaxies,
323
00:21:25,490 --> 00:21:26,720
like the Milky Way,
324
00:21:26,720 --> 00:21:29,690
can inflect tremendous change in our universe.
325
00:21:29,690 --> 00:21:32,330
When they slam into a galaxy, they can change its structure.
326
00:21:32,330 --> 00:21:35,160
The Milky Way would not look anything like it looks today
327
00:21:35,170 --> 00:21:38,200
without those dwarf galaxies.
328
00:21:38,200 --> 00:21:40,940
Repeated dwarf-galaxy collisions
329
00:21:40,940 --> 00:21:44,640
could have radically altered the shape of the Milky Way itself.
330
00:21:47,240 --> 00:21:51,010
Their gravitational disruptions could have created a distinctive
331
00:21:51,010 --> 00:21:55,080
and possibly fatal feature in the middle of our galaxy...
332
00:21:55,090 --> 00:21:56,590
The galactic bar.
333
00:21:59,990 --> 00:22:02,260
The center of the Milky Way is elongated.
334
00:22:02,260 --> 00:22:04,160
Instead of it being shaped like a sphere,
335
00:22:04,160 --> 00:22:07,330
it's more shaped like a bar, and the bar is made by stars
336
00:22:07,330 --> 00:22:11,470
actually orbiting in this sort of elongated way.
337
00:22:11,470 --> 00:22:14,340
And this bar can be bad for the health of the galaxy
338
00:22:14,340 --> 00:22:16,910
because what they do is help to funnel gas
339
00:22:16,910 --> 00:22:18,740
into the core of the galaxy.
340
00:22:18,740 --> 00:22:23,140
The loss of this gas could be a way of stopping star formation.
341
00:22:26,620 --> 00:22:29,820
The bar-shaped bulge at the center of the Milky Way
342
00:22:29,820 --> 00:22:32,790
sweeps our galaxy's star-building gas
343
00:22:32,790 --> 00:22:35,020
into the galactic nucleus.
344
00:22:35,030 --> 00:22:36,630
Here, it gets gobbled up
345
00:22:36,630 --> 00:22:39,460
by our galaxy's supermassive black hole.
346
00:22:43,530 --> 00:22:47,700
Without the star-building material, no new stars can form,
347
00:22:47,700 --> 00:22:51,140
and the galaxy dies.
348
00:22:51,140 --> 00:22:53,170
So, is it case closed?
349
00:22:53,180 --> 00:22:55,740
Are dwarf galaxies killing the Milky Way?
350
00:22:55,750 --> 00:22:59,780
Is the murder weapon a galactic bar?
351
00:22:59,780 --> 00:23:02,650
So, it's possible that the formation of these bars
352
00:23:02,650 --> 00:23:06,350
helps turn off star formation in the very core of the galaxy,
353
00:23:06,360 --> 00:23:09,260
but that's just the central regions of the galaxy.
354
00:23:09,260 --> 00:23:11,130
That doesn't explain what's going farther out
355
00:23:11,130 --> 00:23:12,960
in the spiral arms.
356
00:23:12,960 --> 00:23:16,460
So, if star formation really is shutting down in the Milky Way,
357
00:23:16,470 --> 00:23:18,770
it's not really the fault of the bar.
358
00:23:20,940 --> 00:23:25,310
Dwarf galaxies cause the Milky Way grievous bodily harm
359
00:23:25,310 --> 00:23:28,380
by creating the galactic bar.
360
00:23:28,380 --> 00:23:33,820
But they're off the hook for attempted galactic murder.
361
00:23:33,820 --> 00:23:36,550
The investigation continues,
362
00:23:36,550 --> 00:23:41,760
and it could be about to take a dramatic twist.
363
00:23:41,760 --> 00:23:44,030
It might not be that the galaxy's being murdered.
364
00:23:44,030 --> 00:23:46,330
It could just be eating itself to death.
365
00:24:11,850 --> 00:24:15,850
The Milky Way is being killed off.
366
00:24:15,850 --> 00:24:18,620
And the perpetrator remains at large.
367
00:24:20,790 --> 00:24:22,760
Scientists investigating the crime
368
00:24:22,760 --> 00:24:24,490
are running out of suspects.
369
00:24:26,530 --> 00:24:28,660
But the hunt for clues continues,
370
00:24:30,640 --> 00:24:35,870
so astronomers are examining the dying body of the Milky Way.
371
00:24:35,870 --> 00:24:39,040
Our galaxy is a hazy disk of stars
372
00:24:39,040 --> 00:24:42,610
surrounded by a halo of superheated gas.
373
00:24:44,480 --> 00:24:48,450
It's over 100,000 light-years across.
374
00:24:48,450 --> 00:24:50,620
But it hasn't always been so big.
375
00:24:52,890 --> 00:24:55,090
When you think about things so vast,
376
00:24:55,090 --> 00:24:57,130
so gigantic and ancient as galaxies,
377
00:24:57,130 --> 00:24:58,590
you're kind of tempted to think
378
00:24:58,600 --> 00:25:00,160
that they're very stable objects,
379
00:25:00,170 --> 00:25:02,130
that they don't change much over time,
380
00:25:02,130 --> 00:25:03,930
but we now know that our own galaxy
381
00:25:03,940 --> 00:25:05,870
is the product of many smaller galaxies
382
00:25:05,870 --> 00:25:07,670
that came together over time,
383
00:25:07,670 --> 00:25:10,370
and there are other galaxies still colliding with us.
384
00:25:13,340 --> 00:25:15,610
We see galaxies eating each other all the time.
385
00:25:15,610 --> 00:25:18,310
They collide, and if one galaxy is very big
386
00:25:18,320 --> 00:25:20,180
and one galaxy is very small,
387
00:25:20,190 --> 00:25:23,450
the little galaxy falls into the big one, gets torn apart,
388
00:25:23,450 --> 00:25:27,560
and becomes a part of that bigger galaxy.
389
00:25:27,560 --> 00:25:29,890
The Milky Way might be dying,
390
00:25:29,890 --> 00:25:33,360
but it's still a monster foraging through the universe,
391
00:25:33,360 --> 00:25:35,730
swallowing smaller galaxies whole.
392
00:25:39,300 --> 00:25:41,370
It consumes their stars.
393
00:25:41,370 --> 00:25:45,440
But it also has a taste for their star-building gas.
394
00:25:47,310 --> 00:25:50,650
And it doesn't have to collide with other galaxies
395
00:25:50,650 --> 00:25:53,980
to feed off of them.
396
00:25:53,990 --> 00:25:56,790
Now, the lifeblood of a galaxy is hydrogen gas.
397
00:25:56,790 --> 00:25:59,060
That's what actually creates new stars.
398
00:25:59,060 --> 00:26:02,020
So as a dwarf galaxy passes by the Milky Way,
399
00:26:02,030 --> 00:26:05,230
the tremendously massive halo of the Milky Way,
400
00:26:05,230 --> 00:26:08,600
all of that gas, can draw off material from the dwarf galaxy,
401
00:26:08,600 --> 00:26:10,670
adding it to the Milky Way.
402
00:26:10,670 --> 00:26:12,900
So in this way, the Milky Way drains away
403
00:26:12,900 --> 00:26:15,940
the lifeblood of other galaxies.
404
00:26:15,940 --> 00:26:18,510
In some sense, you could say it's a vampire
405
00:26:18,510 --> 00:26:21,310
because a vampire sucks the life out of other things
406
00:26:21,310 --> 00:26:22,750
so it can remain young.
407
00:26:26,680 --> 00:26:33,990
In its 13 billion-year life, our vampire galaxy has feasted.
408
00:26:33,990 --> 00:26:37,260
Consuming the lifeblood of its galactic victims,
409
00:26:37,260 --> 00:26:40,100
the Milky Way has grown fat.
410
00:26:42,530 --> 00:26:45,670
But could this monstrous feeding frenzy be a factor
411
00:26:45,670 --> 00:26:47,500
in the Milky Way's demise?
412
00:26:50,410 --> 00:26:52,310
Once again, crucial evidence
413
00:26:52,310 --> 00:26:56,910
comes from the Sloan digital sky survey.
414
00:26:56,920 --> 00:26:59,980
Their telescope maps the stars in our galaxy,
415
00:26:59,980 --> 00:27:03,790
but it also maps the galaxies in our universe.
416
00:27:06,260 --> 00:27:11,760
Looking at distant galaxies is like looking back in time.
417
00:27:11,760 --> 00:27:14,160
Because the farther away they are,
418
00:27:14,170 --> 00:27:16,930
the longer their light takes to reach us.
419
00:27:19,800 --> 00:27:24,740
We see the most distant galaxies not as they are now,
420
00:27:24,740 --> 00:27:29,410
but as they were... Billions of years ago.
421
00:27:29,410 --> 00:27:30,850
So, when you look at these galaxies,
422
00:27:30,850 --> 00:27:33,120
you're seeing them as they were when they were very young,
423
00:27:33,120 --> 00:27:36,120
and you're seeing these galaxies as they are more recently,
424
00:27:36,120 --> 00:27:38,490
so you can actually look at the evolution...
425
00:27:38,490 --> 00:27:42,390
How galaxies change over time as the universe ages.
426
00:27:44,530 --> 00:27:46,100
While studying the data,
427
00:27:46,100 --> 00:27:49,100
scientists make a dramatic discovery.
428
00:27:49,100 --> 00:27:53,670
They find spiral galaxies, just like the Milky Way,
429
00:27:53,670 --> 00:27:57,410
dying all over the universe.
430
00:27:57,410 --> 00:28:00,240
And what connects them is their mass.
431
00:28:02,280 --> 00:28:03,850
There seems to be an upper weight limit
432
00:28:03,850 --> 00:28:06,680
for the sizes of spiral galaxies.
433
00:28:06,680 --> 00:28:09,150
Up to about a trillion times the mass of the Sun,
434
00:28:09,150 --> 00:28:12,050
we see spiral galaxies that continue to form stars,
435
00:28:12,060 --> 00:28:14,020
but once they pass this threshold,
436
00:28:14,030 --> 00:28:16,430
galaxies tend to die and run out of stars.
437
00:28:18,600 --> 00:28:20,700
While devouring the star-building gas
438
00:28:20,700 --> 00:28:22,900
of smaller galaxies,
439
00:28:22,900 --> 00:28:26,000
the Milky Way may have grown obese,
440
00:28:26,000 --> 00:28:30,010
and now it could be choking to death on its own dinner.
441
00:28:30,010 --> 00:28:32,270
But how?
442
00:28:32,280 --> 00:28:34,980
Once a spiral galaxy is sufficiently big,
443
00:28:34,980 --> 00:28:37,580
it's going to have an incredible gravitational force,
444
00:28:37,580 --> 00:28:39,980
so any gas that it pulls to itself
445
00:28:39,980 --> 00:28:43,550
is going to come in at an incredibly high speed.
446
00:28:43,550 --> 00:28:45,760
That gas is going to be superheated.
447
00:28:49,660 --> 00:28:52,290
The superheated gas moves so quickly
448
00:28:52,300 --> 00:28:56,400
that it's prevented from falling into the Milky Way.
449
00:28:56,400 --> 00:29:01,570
The gas is too energetic for our galaxy's gravity to pull it in.
450
00:29:01,570 --> 00:29:05,780
Instead, it stays in the halo around the Milky Way,
451
00:29:05,780 --> 00:29:10,510
and our galaxy's food supply is choked off.
452
00:29:10,520 --> 00:29:14,950
Eventually, our galaxy will starve.
453
00:29:14,950 --> 00:29:16,850
This will only happen if the Milky Way
454
00:29:16,850 --> 00:29:20,290
is over the star-building weight limit.
455
00:29:20,290 --> 00:29:24,830
But how exactly do you weigh a galaxy?
456
00:29:24,830 --> 00:29:27,300
One basic way we can weigh a galaxy
457
00:29:27,300 --> 00:29:30,000
is measure how fast the stars are moving within it.
458
00:29:30,000 --> 00:29:32,940
So the faster the stars orbit around the center of the galaxy,
459
00:29:32,940 --> 00:29:35,810
the more massive the galaxy is.
460
00:29:35,810 --> 00:29:40,110
This method of weighing the Milky Way relies on gravity.
461
00:29:40,110 --> 00:29:43,380
Fast-moving stars need more gravity to hold them
462
00:29:43,380 --> 00:29:44,710
in their orbits,
463
00:29:44,720 --> 00:29:49,590
and more gravity means more galactic mass.
464
00:29:49,590 --> 00:29:53,690
When scientists use this information to run the math,
465
00:29:53,690 --> 00:29:57,230
the horrible truth is revealed.
466
00:29:57,230 --> 00:29:59,930
We've passed kind of a critical level.
467
00:29:59,930 --> 00:30:03,500
The Milky Way is far too massive for its own health,
468
00:30:03,500 --> 00:30:06,340
and we've entered the beginning of the end.
469
00:30:06,340 --> 00:30:09,540
We're running out of gas, and I mean this literally.
470
00:30:09,540 --> 00:30:11,410
Gas clouds form stars,
471
00:30:11,410 --> 00:30:14,840
and as they form stars, they're used up,
472
00:30:14,850 --> 00:30:17,080
and so our gas tank is getting closer and closer
473
00:30:17,080 --> 00:30:19,580
to empty every day.
474
00:30:23,350 --> 00:30:26,390
The investigation into the killing of the Milky Way
475
00:30:26,390 --> 00:30:27,760
is closed.
476
00:30:29,960 --> 00:30:31,330
The verdict?
477
00:30:31,330 --> 00:30:35,130
The greedy Milky Way is killing itself.
478
00:30:37,770 --> 00:30:41,770
Over millions of years, star formation grinds to a halt,
479
00:30:41,770 --> 00:30:43,670
and the galaxy dies.
480
00:30:46,310 --> 00:30:51,450
But could the galaxy be resurrected?
481
00:30:51,450 --> 00:30:53,320
We seem to be telling a very sad story.
482
00:30:53,320 --> 00:30:55,520
We're talking about the demise of the Milky Way galaxy...
483
00:30:55,520 --> 00:30:57,290
The end of star formation...
484
00:30:57,290 --> 00:30:59,120
But maybe it's just a little bit too soon
485
00:30:59,120 --> 00:31:00,890
to write the death announcement yet.
486
00:31:00,890 --> 00:31:03,390
Hope could be just over the horizon.
487
00:31:06,230 --> 00:31:09,470
In space and astrophysics, really anything is possible.
488
00:31:30,980 --> 00:31:35,750
The shocking case of our dying galaxy has been solved.
489
00:31:35,750 --> 00:31:38,090
There was no killer.
490
00:31:38,090 --> 00:31:41,760
Turns out, the Milky Way is eating itself to death.
491
00:31:46,000 --> 00:31:49,470
But is this really the end?
492
00:31:49,470 --> 00:31:52,440
Could salvation be heading our way?
493
00:31:54,340 --> 00:31:56,210
Even if star formation is turning off
494
00:31:56,210 --> 00:31:57,570
in the Milky Way now,
495
00:31:57,580 --> 00:31:59,380
we know that it's on a collision course
496
00:31:59,380 --> 00:32:01,340
with the Andromeda galaxy.
497
00:32:01,350 --> 00:32:02,510
They're moving toward each other
498
00:32:02,510 --> 00:32:06,820
at hundreds of thousands of miles per hour.
499
00:32:06,820 --> 00:32:08,320
A collision sounds like something
500
00:32:08,320 --> 00:32:09,850
that's always destructive,
501
00:32:09,860 --> 00:32:11,860
but that's not necessarily the case.
502
00:32:14,790 --> 00:32:15,990
The Milky Way's collision
503
00:32:15,990 --> 00:32:19,930
with our giant galactic neighbor Andromeda
504
00:32:19,930 --> 00:32:23,100
won't happen for another 4 billion years.
505
00:32:25,440 --> 00:32:29,070
By then, star formation in both of these galaxies
506
00:32:29,070 --> 00:32:30,770
will have stopped completely.
507
00:32:33,550 --> 00:32:37,550
But a giant meet-up could change all that.
508
00:32:40,820 --> 00:32:42,290
As an isolated galaxy,
509
00:32:42,290 --> 00:32:45,690
the Milky Way is already in its wind-down phase.
510
00:32:45,690 --> 00:32:49,360
It's not producing as many new stars as it used to.
511
00:32:49,360 --> 00:32:53,530
But there is one way to generate a new round of star formation,
512
00:32:53,530 --> 00:32:57,230
and that's through a galactic merger event.
513
00:32:57,240 --> 00:32:59,200
When Andromeda gets close enough,
514
00:32:59,200 --> 00:33:01,300
the mutual gravity between the two galaxies
515
00:33:01,310 --> 00:33:05,610
will start to stretch them out, pulling them out like Taffy.
516
00:33:05,610 --> 00:33:08,550
Stars will be pulled out into these long, looping streamers,
517
00:33:08,550 --> 00:33:09,680
and then the galaxies
518
00:33:09,680 --> 00:33:12,850
will physically pass through each other.
519
00:33:12,850 --> 00:33:16,620
Eventually, the two galaxies will draw back together again
520
00:33:16,620 --> 00:33:19,260
and merge into one gigantic galaxy,
521
00:33:19,260 --> 00:33:21,190
and at that point, all of these gas clouds
522
00:33:21,190 --> 00:33:24,360
will flash into star formation.
523
00:33:29,070 --> 00:33:33,600
As the galaxies merge, they'll be reborn.
524
00:33:33,610 --> 00:33:36,570
Two dying spiral-shaped galaxies
525
00:33:36,580 --> 00:33:40,840
become a single living elliptical galaxy
526
00:33:40,850 --> 00:33:44,850
called Milkomeda.
527
00:33:44,850 --> 00:33:47,450
Imagine you're living in the far future of the galaxy
528
00:33:47,450 --> 00:33:49,050
and you see the night sky
529
00:33:49,050 --> 00:33:51,620
while the Milky Way and Andromeda are colliding.
530
00:33:51,620 --> 00:33:53,920
It will look like a very different place.
531
00:33:53,930 --> 00:33:56,160
Rather than one band across the night sky,
532
00:33:56,160 --> 00:33:58,900
you might have two as the two disks come together.
533
00:33:58,900 --> 00:34:00,560
It will be a miraculous sight,
534
00:34:00,570 --> 00:34:03,400
but a very, very different place than we have today.
535
00:34:06,710 --> 00:34:11,880
Our sky will light up for the first time in billions of years.
536
00:34:11,880 --> 00:34:15,310
Star formation will flare across the galaxy.
537
00:34:17,650 --> 00:34:20,550
But is it too soon to celebrate?
538
00:34:24,290 --> 00:34:26,660
This new round of star formation
539
00:34:26,660 --> 00:34:30,490
during the merger of our two galaxies...
540
00:34:30,500 --> 00:34:32,860
While it's very cool for a little bit,
541
00:34:32,870 --> 00:34:37,600
once it's over, that kind of sends the new galaxy
542
00:34:37,600 --> 00:34:39,000
into a death spiral.
543
00:34:39,000 --> 00:34:41,770
When new stars are born in this new galaxy,
544
00:34:41,770 --> 00:34:46,080
many of them are going to be hot, large, blue stars.
545
00:34:46,080 --> 00:34:49,280
Eventually, those young, hot stars are going to start to die,
546
00:34:49,280 --> 00:34:51,380
and when they do, they're going to explode
547
00:34:51,380 --> 00:34:53,380
violently as supernovae.
548
00:34:58,860 --> 00:35:00,860
And those supernovae are going to start
549
00:35:00,860 --> 00:35:03,390
blasting gas out of the galaxy.
550
00:35:07,070 --> 00:35:08,600
All of the gas is gone.
551
00:35:08,600 --> 00:35:10,900
There's no more stuff to form stars.
552
00:35:10,900 --> 00:35:12,670
And that's what kills a galaxy.
553
00:35:16,440 --> 00:35:18,740
It'll take hundreds of millions of years
554
00:35:18,740 --> 00:35:23,350
for Milkomeda to run out of star-building gas.
555
00:35:23,350 --> 00:35:27,650
And then our new elliptical galaxy will starve.
556
00:35:29,450 --> 00:35:32,060
But the final blow is still to come.
557
00:35:34,490 --> 00:35:36,730
Another issue to consider is what happens
558
00:35:36,730 --> 00:35:38,760
to the two supermassive black holes
559
00:35:38,760 --> 00:35:41,460
at the cores of the two galaxies.
560
00:35:41,470 --> 00:35:44,000
Well, initially, they're going to orbit each other,
561
00:35:44,000 --> 00:35:48,570
stirring up a lot of turbulence, and they're going to combine.
562
00:35:48,570 --> 00:35:51,510
And because there's a lot of new, hot, fresh gas,
563
00:35:51,510 --> 00:35:54,680
our new galaxy is going to be a quasar.
564
00:35:54,680 --> 00:35:57,210
And that quasar is going to turn up the heat,
565
00:35:57,220 --> 00:35:58,850
it's going to turn up the turbulence,
566
00:35:58,850 --> 00:36:02,090
and this means star formation is going to be shut off.
567
00:36:04,360 --> 00:36:07,960
The combined power of the supermassive black holes
568
00:36:07,960 --> 00:36:11,960
help create a quasar that tears through the galaxy.
569
00:36:15,070 --> 00:36:18,270
It releases ferocious beams of radiation
570
00:36:18,270 --> 00:36:21,840
that blast through Milkomeda's star-forming gas.
571
00:36:25,180 --> 00:36:27,080
It's only just been reborn,
572
00:36:27,080 --> 00:36:31,650
but our newly enlarged galaxy is once again dying.
573
00:36:34,350 --> 00:36:39,520
Vast galaxies, like Milkomeda, seem doomed from the start.
574
00:36:39,520 --> 00:36:43,630
Their size creates too many problems for star formation.
575
00:36:46,130 --> 00:36:49,400
Or... so we thought.
576
00:36:49,400 --> 00:36:51,740
The more galaxies we see, the more we realize
577
00:36:51,740 --> 00:36:53,800
there's a lot out there we haven't discovered,
578
00:36:53,810 --> 00:36:55,570
and there's a new class of galaxies
579
00:36:55,570 --> 00:36:57,970
only recently identified.
580
00:36:57,980 --> 00:37:02,580
These galaxies are more than 10 times the mass of the Milky Way.
581
00:37:02,580 --> 00:37:06,680
And, intriguingly, they're still forming stars.
582
00:37:06,690 --> 00:37:08,420
Apparently we've missed something.
583
00:37:32,240 --> 00:37:37,480
4 billion years from now, the Milky Way is no more.
584
00:37:37,480 --> 00:37:40,610
After colliding with Andromeda, it's reborn
585
00:37:40,610 --> 00:37:44,520
as a giant elliptical galaxy called Milkomeda.
586
00:37:46,450 --> 00:37:50,890
Scientists thought galaxies this big were doomed.
587
00:37:50,890 --> 00:37:53,960
But is hope on the horizon?
588
00:37:56,430 --> 00:38:00,000
The Sloan digital sky survey has spent a decade
589
00:38:00,000 --> 00:38:04,570
studying over a million galaxies.
590
00:38:04,570 --> 00:38:08,970
It's discovered a rare but enormous kind of galaxy...
591
00:38:08,980 --> 00:38:10,740
A super spiral.
592
00:38:13,650 --> 00:38:16,580
These super-spiral galaxies are spiral galaxies
593
00:38:16,580 --> 00:38:18,150
that are incredibly super,
594
00:38:18,150 --> 00:38:21,920
and by "super," I mean they have four times the size,
595
00:38:21,920 --> 00:38:23,620
10 times the mass,
596
00:38:23,620 --> 00:38:25,290
and they're weird because they exceed
597
00:38:25,290 --> 00:38:28,290
the supposed weight limit for spiral galaxies.
598
00:38:28,300 --> 00:38:31,300
So they shouldn't have new stars, but they do.
599
00:38:31,300 --> 00:38:33,630
They're very healthy galaxies.
600
00:38:39,240 --> 00:38:43,140
Scientists have found just 53 super spirals.
601
00:38:46,250 --> 00:38:50,350
Super-spiral galaxies show that in rare situations,
602
00:38:50,350 --> 00:38:54,920
massive galaxies continue to produce new stars.
603
00:38:58,790 --> 00:39:02,330
So, is this a lifeline for Milkomeda?
604
00:39:05,170 --> 00:39:07,200
When we think about two galaxies colliding,
605
00:39:07,200 --> 00:39:08,830
a lot of our computer models suggest
606
00:39:08,840 --> 00:39:10,440
that they really mess each other up.
607
00:39:10,440 --> 00:39:12,140
Things get very chaotic.
608
00:39:12,140 --> 00:39:15,970
But over time, could they settle back down into a spiral shape?
609
00:39:15,980 --> 00:39:18,840
And, in fact, that may be what happens with super spirals.
610
00:39:18,850 --> 00:39:21,410
One of the clues is that many super-spiral galaxies
611
00:39:21,410 --> 00:39:22,950
have double cores.
612
00:39:22,950 --> 00:39:25,420
Instead of there just being one supermassive black hole,
613
00:39:25,420 --> 00:39:29,250
there are actually two orbiting each other.
614
00:39:29,260 --> 00:39:31,860
The fact that we see spiral galaxies with two cores
615
00:39:31,860 --> 00:39:33,890
makes it possible that you could have a collision
616
00:39:33,890 --> 00:39:36,430
and still survive as a spiral galaxy.
617
00:39:36,430 --> 00:39:39,400
So maybe there's hope that even the Milky Way will be a spiral
618
00:39:39,400 --> 00:39:41,430
once it collides with Andromeda.
619
00:39:45,640 --> 00:39:49,440
Picture the scene... 6 billion years in the future.
620
00:39:51,640 --> 00:39:54,250
Milkomeda drifts through the universe...
621
00:39:56,220 --> 00:40:00,250
not as an elliptical galaxy, but as a super spiral.
622
00:40:02,820 --> 00:40:07,490
This shape means the galaxy is far more stable.
623
00:40:07,490 --> 00:40:09,530
The damaging heat and turbulence
624
00:40:09,530 --> 00:40:13,030
generated by Milkomeda's supermassive black holes
625
00:40:13,030 --> 00:40:19,070
can't disrupt star-building gas way out in the spiral arms.
626
00:40:19,070 --> 00:40:24,910
Far from dying off, our galaxy lives on...
627
00:40:24,910 --> 00:40:28,610
Larger than ever before.
628
00:40:28,620 --> 00:40:31,880
But that isn't the end of the story.
629
00:40:31,890 --> 00:40:34,990
Tens of billions of years from now,
630
00:40:34,990 --> 00:40:38,960
could the galaxy continue to grow?
631
00:40:38,960 --> 00:40:41,190
Our local group of galaxies...
632
00:40:41,190 --> 00:40:43,730
Milky Way, Andromeda, Triangulum...
633
00:40:43,730 --> 00:40:47,470
And then a collection of dwarf satellite galaxies...
634
00:40:47,470 --> 00:40:50,540
Is gravitationally bound together,
635
00:40:50,540 --> 00:40:53,140
and eventually, we're all glued together
636
00:40:53,140 --> 00:40:55,340
into a single massive object.
637
00:40:57,080 --> 00:40:58,540
What does this mean?
638
00:40:58,550 --> 00:41:00,980
This means we might be part of one of the largest structures
639
00:41:00,980 --> 00:41:03,180
in the universe.
640
00:41:03,180 --> 00:41:05,450
During its billion years of life,
641
00:41:05,450 --> 00:41:10,050
the Milky Way changes beyond recognition.
642
00:41:10,060 --> 00:41:13,390
It suffers countless collisions,
643
00:41:13,390 --> 00:41:17,560
feasts on many smaller galaxies,
644
00:41:17,560 --> 00:41:20,630
and gives birth to innumerable stars.
645
00:41:22,840 --> 00:41:24,900
We talk about the life cycle of galaxies...
646
00:41:24,900 --> 00:41:28,110
How they're born, how they live healthy lives making new stars,
647
00:41:28,110 --> 00:41:30,210
and eventually how they die away.
648
00:41:30,210 --> 00:41:32,380
It's really not as depressing as that.
649
00:41:32,380 --> 00:41:34,480
Everything in the universe changes.
650
00:41:36,550 --> 00:41:41,850
Galaxies like ours are in a constant state of flux.
651
00:41:41,860 --> 00:41:44,660
So when it comes to the Milky Way,
652
00:41:44,660 --> 00:41:47,290
death really isn't the end.
653
00:41:49,200 --> 00:41:50,760
What we see in our universe
654
00:41:50,760 --> 00:41:54,530
is that there's always a process of birth and rebirth,
655
00:41:54,530 --> 00:41:56,100
so the future of the Milky Way
656
00:41:56,100 --> 00:41:58,400
is that it's going to keep on doing what it does.
657
00:42:01,040 --> 00:42:02,740
Galaxies are ever-changing.
658
00:42:02,740 --> 00:42:04,180
10 billion years ago, the Milky Way
659
00:42:04,180 --> 00:42:06,010
was nothing like what it is today,
660
00:42:06,010 --> 00:42:08,010
and certainly, 10 billion years in the future,
661
00:42:08,010 --> 00:42:09,850
it'll be a very different place.
662
00:42:12,420 --> 00:42:14,790
Look, I live in this galaxy.
663
00:42:14,790 --> 00:42:17,390
I hope that it can find a way to rejuvenate itself
664
00:42:17,390 --> 00:42:19,520
through collisions or some other process
665
00:42:19,530 --> 00:42:21,260
because that gives me some hope
666
00:42:21,260 --> 00:42:24,130
that it'll go on for a long, long time.
53925
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