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  • Hi, I'm John Green, and this is Crash Course Big History.

  • Today, we're going to be exploring what happened

  • to the universe after the Big Bang, particularly,

  • how you and I and everyone you know emerged from stars.

  • And we'll also be investigating the burning question

  • of why anyone who studies history

  • has to care about chemistry.

  • Mr. Green, Mr. Green!

  • I'm sorry, but I hate chemistry.

  • Why can't we just learn about like English kings

  • stabbing each other?

  • Sorry, me from the past, the thing is if you look far back

  • enough in your family tree, you're going to find not just

  • like farmers, and foragers, and fish, and microbes,

  • you're going to find stars.

  • And I don't mean stars like Kim Kardashian,

  • who is actually not a star, she is a person.

  • I mean actual stars, me from the past.

  • And to understand how we got from stars to people,

  • you're going to need some chemistry.

  • So last episode we went from the very beginning of everything

  • to the release of cosmic background radiation.

  • And CBR is a major piece of evidence

  • that the Big Bang happened.

  • Studying it closely also tells us the age of the universe

  • and it allows us to see the minute variations in temperature

  • and density of the early universe.

  • And it turns out that those tiny differences

  • are a really big deal.

  • So when the universe inflated from much, much smaller

  • than an atom to the size of a grapefruit in a split second,

  • there were quantum fluctuations, tiny little blips

  • on the unpredictable quantum scale.

  • And they created those little variations that we see

  • in cosmic background radiation.

  • And as the universe continued to expand--

  • I mean it is currently larger than a grapefruit--

  • those variations in density were inflated to such a scale

  • that gravity was able to take hold and start clumping together

  • clouds of hydrogen and helium gas.

  • So 380,000 years after the Big Bang,

  • the universe was becoming an increasingly cold

  • and increasingly boring place.

  • Like temperatures were no longer high enough

  • to forge new elements, and if hydrogen and helium

  • hadn't clumped together,

  • nothing would have ever happened ever again.

  • Our universe would just be a dull, homogeneous place

  • with some clouds of hydrogen and helium gas floating around.

  • Dull and gassy, just like North Dakota.

  • I'm just kidding, North Dakota.

  • You do have a lot of natural gas,

  • but you're very interesting.

  • I mean you have Mount Rushmore.

  • What's that?

  • Oh, oh I see.

  • Sorry, yeah, then...

  • But what happened is that while the universe on the whole

  • continued to cool, thanks to those tiny variations

  • that emerged during inflation, certain pockets of the universe

  • were about to get very hot.

  • Indeed, a liberal dose of hot sauce was yet to come.

  • Hydrogen and helium though are light gases;

  • they are the lightest two elements.

  • So light that they require very little encouragement

  • to escape the earth's atmosphere.

  • But while the explosive force of the Big Bang flings matter

  • and energy apart, gravity has the ability to pull

  • tiny pockets of the cosmos back together,

  • provided it has some wrinkles in the universe to work with.

  • As gravity sucked hydrogen and helium atoms together,

  • enormous, thick clouds began to form.

  • While the expansion of the universe continued to increase

  • the gaps between these clouds,

  • the density of these pockets also increased.

  • The vastness of empty space began to be filled

  • with tiny islands where atoms of hydrogen and helium

  • were increasingly squished together.

  • Despite being the lightest of all the elements,

  • the immense amount of all that gas built pressure

  • up in the center.

  • Increasing pressure meant increasing temperatures,

  • just like after your 2:00 a.m. taco run,

  • suddenly these gassy pockets were burning inside.

  • It was in this rather uncomfortable state of heartburn

  • that the first stars flared into life,

  • roughly 100 million years after the Big Bang.

  • By a billion years after the Big Bang, the universe

  • was starting to look like what we think of as a universe.

  • An immense vastness littered with hundreds of billions

  • of galactic islands containing hundreds of billions of stars.

  • And as recent work with Kepler space telescope has revealed,

  • a mind-numbing number of planets.

  • So the universe is big, it's really big.

  • But it's not so big that it's impossible

  • for the average person to get a mental picture

  • of like our neighborhood.

  • Our galaxy, the Milky Way, formed from these

  • galactic mergers with other galaxies that stopped

  • like around ten billion years ago.

  • Our galaxy is about 100,000 light years across,

  • which means that it takes,

  • you know, 100,000 years for light to get across it.

  • And even if humans become like technologically capable

  • of colonizing the galaxy in the next millions of years,

  • our little galactic island is probably where

  • we're going to stay just peeping out on the rest of the universe.

  • So there are between 200 and 400 billion stars in the Milky Way

  • with huge distances between them.

  • There hasn't been a merger between our galaxy and another

  • for a long time, but our neighbor Andromeda,

  • which has closer to a trillion stars, is actually set

  • to collide with us in 3.75 billion years.

  • But don't worry, this isn't going to be like a car crash

  • because the vast distances between stars make it

  • very unlikely that stars will actually hit each other

  • in such an event.

  • Although many new stars will form.

  • Instead of a car crash,

  • think of like a three-billion-year-long tango

  • of two graceful galactic dancers.

  • This is going to totally mess up the constellations

  • that we're familiar with now, but the good news is that

  • by that time, the sun will have wiped out life on earth

  • regardless, so we won't have to worry about it.

  • And the even better news is that let's face it, there's no way

  • our species is making it until the sun wipes us out.

  • As far as these galactic islands go, ours is a modest size.

  • Like Malin 1 is a spiral galaxy like ours, but it's a whopping

  • 680,000 light years across.

  • And the giant elliptical galaxy excitingly named M87--

  • because astronomers are so good at naming things--

  • is 980,000 light years across.

  • And with its radio jets, the elliptical galaxy Hercules A--

  • that's a slightly better name-- is a whopping 1.5 million

  • light years across from end to end.

  • Galactic islands are separated

  • by millions and millions of light years.

  • And the Virgo super cluster of galaxies to which

  • the Milky Way belongs is roughly 110 million light years

  • in diameter, and that's only one of many likely infinite

  • super clusters in the universe.

  • Wait, literally infinite?

  • Wow.

  • Unfortunately, we can't know whether the universe

  • truly is infinite or not because of a little thing

  • called the cosmic horizon.

  • We can only see the light that has reached us

  • from the start of the universe 13.8 billion years ago.

  • Simply looking into the sky is an act of investigating history,

  • and the farther we look back we begin to see

  • more primitive things.

  • The first stars in galaxies.

  • Mind you, the light we observe billions of years after it

  • first shown and the continued expansion of the universe

  • means that the cosmic horizon is approximately 46 billion

  • light years away by now.

  • Roughly double that and you know that our little cosmic bubble

  • is about 92 billion light years across.

  • I mean compare that to our already huge

  • 100,000 light year galaxy.

  • Just for a little bit of context...

  • But beyond our little cosmic bubble there is more universe,

  • eternally inflating.

  • And where our universe is sort of one hole in a block

  • of Swiss cheese, other holes might exist

  • in that block of cheese, multiple universes with laws

  • of physics completely different from ours.

  • I know, right, it's nuts!

  • It's actually more like cheese, but it's nuts!

  • But our cosmic bubble, while it's very large,

  • is not such an intimidating place.

  • Like it's pretty easy just to get a mental picture of it.

  • A vast bubble with a lot of empty space

  • and a light dusting of galaxies.

  • To further this point, and don't take this too seriously,

  • but in 2002, Karl Glazebrook and Ivan Baldry

  • added up the light from 200,000 galaxies and determined

  • that if you were able to stand outside our cosmic bubble

  • and look at it with human eyes, the color of our universe

  • would be-- wait for it: beige.

  • That's a bit of a anticlimax, so they tried to dress it up

  • by calling it cosmic latte.

  • But I don't mind beige.

  • I mean look, this stuff is gigantic and somewhat scary,

  • but you can't be scared of beige.

  • And a lot of cosmologists infuse their lessons with a sense

  • of awe at this vast expanse.

  • And that awe is certainly justified.

  • I mean the universe is literally awesome.

  • Though let me ask you this, if you lived in New York City

  • would you feel bashful or depressed about the size

  • of your city compared to, say, the miles and miles

  • of the plains of Saskatchewan?

  • So yeah, there are millions and millions of light years

  • of empty space, but it's empty space.

  • One thing we find out about the rising complexity in Big History

  • is just how unique some of these tiny areas

  • of the universe can be.

  • This is where the action is.

  • Enough of the pontification, let's get back to those

  • gassy, heartburn-suffering stars.

  • As core regions of the gas clouds heat up,

  • the atoms get jumpy, move faster and faster and collide

  • with ever-increasing ferocity.

  • Eventually it's ferocious enough to overcome

  • the electric repulsion between the atoms,

  • they fuse and the cloud officially becomes a star.

  • Hydrogen atoms fuse into helium atoms at about

  • ten million degrees, releasing yet more energy.

  • The sun is a massive hydrogen bomb in the sky.

  • And the release of energy in just the right amounts

  • is very good for us, provided we don't mess up the ozone layer

  • too bad or spend too much time tanning on the beach.

  • When it comes to stars, size matters.

  • If an initial cloud is smaller than 8% of the size of our sun,

  • it'll never form a star.

  • Maybe only a brown dwarf.

  • If the initial cloud is 60 to 100 times our sun

  • it will probably split into two or more regions

  • of stellar formation.

  • If the cloud is between 8% and eight times the size of our sun

  • it has a longer lifespan.

  • Our sun is middle-aged and will last for about

  • another five billion years.

  • Much smaller stars may have lifespans of hundreds

  • of billions of years.

  • Large stars sometimes only live for a few hundred million years.

  • As all stars run out of hydrogen and helium as fuel,

  • the outer edges of the star swells up.

  • Fusion of heavier elements occurs,

  • requiring higher and higher temperatures,

  • creating heavier and heavier elements,

  • all the way up to iron.

  • But elements heavier than iron can't be created in the stars.

  • There simply isn't enough energy

  • to fuse those heavier nuclei together.

  • So how is the rest of the periodic table formed?

  • When giant stars, eight to 60 times the size of our sun

  • exhaust their fuel, they collapse.

  • This may last no longer than a second,

  • but it will be followed by a huge explosion.

  • These explosions shine with the energy of billions of stars

  • and combine with proton and neutron capture,

  • supernova are responsible for creating the heavier elements

  • of the periodic table.

  • Flinging out these elements, the rest of the cosmos

  • is fertilized and nourished by the ashes of dead stars.

  • Carl Sagan said it best, "We are made of star-stuff,"

  • and he really meant that.

  • I mean you see this globe?

  • It was made in the belly of a star.

  • You see your computer?

  • Made in the belly of a star.

  • Your dog, made in the belly of a star.

  • Your right hand, made in the belly of a star.

  • Your left hand, potentially made in the belly

  • of a different star.

  • Stellar evolution bridges the gap between the mindboggling

  • origin of our universe and the tangible material stuff

  • that you see around you,

  • and, in fact, the tangible material stuff that you are.

  • Humans haven't just appeared out of nowhere.

  • We've changed form.

  • We used to be much hotter, of course--

  • I mean temperature-wise.

  • This is why chemistry is important to understanding

  • the grand narrative of 13.8 billion years.

  • And it's also why we look at the big history

  • of individual objects, something we call little big histories.

  • Like see this ring on my finger?

  • I bought it in a jewelry store.

  • A nice person sold it to me.

  • A jeweler crafted it and miners dug it out of the ground.

  • But it got there by being flung out of a huge star

  • in a massive explosion billions of years ago.

  • It wound up in our solar system, was part of the tiny

  • .1% of matter that didn't get sucked into the sun,

  • accreted from the dusty debris in the one sliver

  • of the solar system where the earth was.

  • And because gold is an iron-loving element,

  • it was more prone to sink to the center of the earth,

  • making it even more unlikely that it should be found

  • on the earth's crust.

  • Rare and shiny things are valued by a lot of human social orders,

  • and during the agrarian era, gold became a sign

  • of social standing and wealth.

  • And in marriage tradition, giving someone an expensive gift

  • can be a sign of esteem.

  • Hence Sara and I spent $450 on this,

  • which we could have spent on an Xbox.

  • Tiny wrinkles in the early universe had a major impact

  • on one of the unifying themes of 13.8 billion years:

  • rise in complexity.

  • Wrinkles created stars, stars created elements,

  • and some of those elements came together to form life,

  • and, of course, us.

  • Gradually, we see an increase in the number of connections

  • and building blocks present in the universe.

  • For instance, a star's comprised primarily of two elements:

  • hydrogen and helium.

  • But here's the thing, if there had been no wrinkles

  • in the early universe, energy would have been

  • evenly distributed across the cosmos.

  • Without flow of energy, like say that through a star,

  • no complexity could arise.

  • None whatsoever.

  • This state of existence

  • is called thermodynamic disequilibrium,

  • which means that energy is not evenly distributed.

  • A simple structure like a star is big, but it's just

  • a large pile of the lightest elements and doesn't score

  • very high in energy flow density.

  • Your brain is 75,000 times more complex than a similar size

  • chunk of the sun.

  • Its building blocks and nodes are way more intricate.

  • Your brain has way more connections than there are stars

  • in the galaxy.

  • You wouldn't think a story that spans cosmology, geology,

  • biology, and human history would have a unifying theme,

  • but rise in complexity is something that stretches

  • across all 13.8 billion years.

  • And it began with those tiny wrinkles in the early cosmos.

  • So now moving fast, I hope you see why

  • a basic understanding of chemistry is important

  • to understanding our ancestry.

  • I mean stars are pretty much

  • your great-great-great-et cetera grandparents.

  • And you wouldn't ignore your grandparents, would you?

  • >> Mr. Green, Mr. Green, no, no way, I mean

  • they're a big part of my plan to get a car for my 16th birthday.

  • That's really touching, me from the past.

  • Also I've got bad news for you.

  • So in today's episode, we've learned that everything

  • around us, everything that we can touch, and feel, and see,

  • even us, is debris floating around enormous stars

  • in the vacuum of space.

  • We clump into specks, we change form, but we owe

  • our entire existence to these burning, gassy balls that we see

  • in the night sky.

  • We may just be the ashes of dead stars, but those ashes hold

  • the potential to arrange themselves in increasingly

  • complex ways from which the earth and all it contains

  • can arise, but more on that next time.

Hi, I'm John Green, and this is Crash Course Big History.

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大歷史速成班。探索宇宙 (Crash Course Big History: Exploring the Universe)

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    Li 發佈於 2021 年 01 月 14 日
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