Exploring the Universe: Crash Course Big History #2


In which John Green, Hank Green, and Emily Graslie teach you about what happened in the Universe after the big bang. They’ll teach you about cosmic background radiation, how a bunch of hydrogen and helium turned into stars, formed galaxies, created heavy elements, and eventually created planets.


Transcript Provided by YouTube:

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Hi, I’m John Green and this is Crash Course Big History. Today we’re gonna be exploring
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what happened to the universe after the Big Bang, particularly how you and I and everyone
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you know, emerged from stars. And we’ll also be investigating the burning question of why
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anyone who studies history has to care about chemistry.
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Mr Green! Mr Green! I’m sorry, but I hate chemistry. Why can’t we just learn about like,
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English kings stabbing each other!?
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Sorry, me from the past. The thing is, if you look far back enough in your family tree,
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you’re going to find not just like farmers and foragers and fish and microbes… You’re
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gonna find stars. And I don’t mean stars like Kim Kardashian, who is actually not a star
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— she is a person! I mean actual stars, Me From the Past. And to understand how we got
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from stars to people, you’re gonna need some chemistry.
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[Theme Music]
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So, last episode, we went from the very beginning of everything to the release of Cosmic Background
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Radiation, and CBR is a major piece of evidence that the Big Bang happened. Studying it closely
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also tells us the age of the universe and it allows us to see the minute variations in temperature
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and density of the early universe. And it turns out that those tiny differences are a really big deal.
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So when the universe inflated from much, much smaller than an atom to the size of a grapefruit
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in a split second, there were quantum fluctuations — tiny little blips on the unpredictable
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quantum scale — and they created those little variations that we see in Cosmic Background
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Radiation. And as the universe continued to expand — I mean, it is currently larger than
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a grapefruit — those variations in density were inflated to such a scale that gravity
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was able to take hold and start clumping together clouds of hydrogen and helium gas.
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So 380,000 years after the Big Bang, the universe was becoming an increasingly cold and increasingly
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boring place. Like, temperatures were no longer high enough to forge new elements, and if
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hydrogen and helium hadn’t clumped together, nothing would have ever happened, ever again!
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Our universe would just be a dull, homogeneous place with some clouds of hydrogen and helium
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gas floating around. Dull and gassy, just like North Dakota. I’m just kidding, North
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Dakota! You do have a lot of natural gas, but you’re very interesting! I mean, you have
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Mount Rushmore! What’s that? Oh. Oh, I see. Sorry. Yeah, mmm. Ugh!
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But what happened is that while the universe on the whole continued to cool, thanks to those
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tiny variations that emerged during inflation, certain pockets of the universe were about to get very hot!
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Indeed! A liberal dose of hot sauce was yet to come! Hydrogen and helium, though, are light gasses.
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They are the lightest two elements — so light that they require very little
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encouragement to escape the Earth’s atmosphere. But while the explosive force of the Big Bang
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flings matter and energy apart, gravity has the ability to pull tiny pockets of the cosmos
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back together — provided it has some wrinkles in the universe to work with.
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As gravity sucked hydrogen and helium atoms together, enormous thick clouds began to form.
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While the expansion of the universe continued to increase the gaps between these clouds,
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the density of these pockets also increased. The vastness of empty space began to be filled
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with tiny islands where atoms of hydrogen and helium were increasingly squished together.
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Despite being the lightest of all the elements, the immense amount of all that gas built pressure
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up in the center. Increasing pressure meant increasing temperatures, just like after your
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2am taco run, suddenly these gassy pockets were burning inside. It was in this rather uncomfortable
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state of heartburn that the first stars flared into life, roughly 100 million years after the Big Bang.
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By a billion years after the big bang, the universe was starting to look like what we
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think of as a universe — an immense vastness littered with hundreds of billions of galactic
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islands, containing hundreds of billions of stars. And as recent work with the Kepler
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space telescope has revealed, a mind-numbing number of planets.
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So the universe is big! It’s really big! But it’s not so big that it’s impossible for the
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average person to get a mental picture of like, our neighborhood.
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Our galaxy, the Milky Way, formed from these galactic mergers with other galaxies that
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stopped like, around 10 billion years ago. Our galaxy’s about 100,000 light years across,
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which means that it takes, you know, 100,000 years for light to get across it.
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And even if humans become like, technologically capable of colonizing the galaxy in the next millions of
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years, our little galactic island is probably where we’re gonna stay, just peeping out on the rest of the universe.
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So there are between 200 and 400 billion stars in the Milky Way, with huge distances between them.
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There hasn’t been a merger between our galaxy and another for a long time, but our
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neighbor, Andromeda — which has closer to a trillion stars — is actually set to collide
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with us in 3.75 billion years. But don’t worry, this isn’t gonna be like a car crash, because
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the vast distances between stars make it very unlikely that stars will actually hit each
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other in such an event, although many new stars will form.
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Instead of a car crash, think of like a three billion year long tango of two graceful galactic
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dancers. This is gonna totally mess up the constellations that we’re familiar with now,
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but the good news is that by that time, the sun will have wiped out life on Earth regardless,
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so we won’t have to worry about it! And the even better news is that, let’s face it, there’s
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no way our species is making it until the sun wipes us out.
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As far as these galactic islands go, ours is a modest size. Like, Malin 1 is a spiral
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galaxy like ours, but it’s a whopping 680 thousand light years across. And the giant
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elliptical galaxy, excitingly named M87 — because astronomers are so good at naming things — is
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980 thousand light years across. And with its radio jets, the elliptical galaxy Hercules A —
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that’s a slightly better name — is a whopping 1.5 million light years across from end to end.
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Galactic islands are separated by millions and millions of light years, and the Virgo
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Supercluster of galaxies, to which the Milky Way belongs, is roughly 110 million light
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years in diameter, and that’s only one of many, likely infinite, Superclusters in the universe.
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Wait, literally infinite? Wow!
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Unfortunately, we can’t know whether the universe truly is infinite or not, because of a little
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thing called the cosmic horizon. We can only see the light that has reached us from the
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start of the universe 13.8 billion years ago. Simply looking into the sky is an act of investigating
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history, and the farther we look back, we begin to see more primitive things —
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the first stars and galaxies.
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Mind you, the light we observe billions of years after it first shone, and the continued
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expansion of the universe, means that the cosmic horizon is approximately 46 billion
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light years away by now. Roughly double that, and you know that our little cosmic bubble
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is about 92 billion light years across. I mean, compare that to our already-huge 100 thousand light year galaxy!
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Just for a little bit of context, the circumference of the earth is less than one fifth of one
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light second. But beyond our little cosmic bubble, there is more universe eternally inflating.
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And where our universe is sort of one hole in a block of Swiss cheese, other holes might
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exist in that block of cheese, multiple universes with laws of physics completely different from ours!
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John from the Past: What!?
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I know, right?! It’s nuts! It’s actually more like cheese, but it’s nuts!
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But our cosmic bubble, while it’s very large, is not such an intimidating place. Like it’s
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pretty easy just to get a mental picture of it — a vast bubble with a lot of empty space
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and a light dusting of galaxies.
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To further this point, and don’t take this too seriously, but in 2002, Karl Glazebrook
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and Ivan Baldry added up the light from 200,000 galaxies and determined that if you were able
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to stand outside our cosmic bubble and look at it with human eyes, the color of our universe
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would be — wait for it! — beige. That’s a bit of an anti-climax so they tried to dress
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it up by calling it Cosmic Latte. But I don’t mind beige. I mean, look. This stuff is gigantic
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and somewhat scary, but you can’t be scared of beige!
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And a lot of cosmologists infuse their lessons with a sense of awe at this vast expanse.
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And that awe is certainly justified — I mean, the universe is literally awesome.
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But let me ask you this: if you lived in New York City, would you feel bashful or depressed
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about the size of your city compared to say, the miles and miles of the plains of Saskatchewan?
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So yeah, there are millions and millions of light years of empty space, but it’s empty
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space! One thing we find out about the rising complexity in Big History is just how unique
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some of these tiny areas of the universe can be. This is where the action is!
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Enough of the pontification! Let’s get back to those gassy heartburn-suffering stars!
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As core regions of the gas clouds heat up, the atoms get jumpy, move faster and faster
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and collide with ever-increasing ferocity. Eventually it’s ferocious enough to overcome
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the electric repulsion between the atoms, they fuse and the cloud officially becomes a star.
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Hydrogen atoms fuse into helium atoms at about ten million degrees, releasing yet more energy.
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The sun is a massive hydrogen bomb in the sky, and the release of energy in just the
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right amounts is very good for us, provided we don’t mess up the ozone layer too bad or
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spend too much time tanning on the beach.
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When it comes to stars, size matters. If an initial cloud is smaller than 8% of the size
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of our sun, it’ll never form a star — maybe only a brown dwarf. If the initial cloud is
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60-100 times our sun, it will probably split into two or more regions of stellar formation.
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If the cloud is between 8% and eight times the size of our sun, it has a longer lifespan.
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Our sun is middle-aged and will last for about another 5 billion years. Much smaller stars
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may have lifespans of hundreds of billions of years. Large stars sometimes only live
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for a few hundred million years.
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As all stars run out of hydrogen and helium as fuel, the outer edges of the star swells
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up. Fusion of heavier elements occurs, requiring higher and higher temperatures, creating heavier
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and heavier elements, all the way up to iron. But elements heavier than iron can’t be created
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in the stars – there simply isn’t enough energy to fuse those heavier nuclei together.
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So how is the rest of the periodic table formed? When giant stars, 8-60 times the size of our
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sun, exhaust their fuel, they collapse. This may last no longer than a second, but it will
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be followed by a huge explosion. These explosions shine with the energy of billions of stars,
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and combined with proton and neutron capture, supernovae are responsible for creating the
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heavier elements of the periodic table. Flinging out these elements, the rest of the cosmos
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is fertilized and nourished by the ashes of dead stars.
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Carl Sagan said it best: “We are made of star stuff.” And he really meant that! I mean,
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you see this globe? It was made in the belly of a star! You see your computer? Made in
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the belly of a star. Your dog? Made in the belly of a star. Your right hand — made in
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the belly of a star. Your left hand? Potentially made in the belly of a different star. Stellar
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evolution bridges the gap between the mind-boggling origin of our universe and the tangible material
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stuff that you see around you. And in fact, the tangible material stuff that you are!
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Humans haven’t just appeared out of nowhere. We’ve changed form. We used to be much hotter
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of course — I mean temperature-wise. This is why chemistry is important to understanding
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the grand narrative of 13.8 billion years, and it’s also why we look at the big history
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of individual objects — something we call “little big histories”.
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Like, see this ring on my finger? I bought it in a jewelry store — a nice person sold
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it to me, a jeweler crafted it and miners dug it out of the ground. But it got there
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by being flung out of a huge star in a massive explosion billions of years ago. It wound
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up in our solar system, was part of the tiny .1% of matter that didn’t get sucked into
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the sun, accreted from the dusty debris in the one sliver of the solar system where the
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Earth was, and because gold is an iron-loving element, it was more prone to sink to the
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center of the Earth, making it even more unlikely that it should be found on the Earth’s crust!
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Rare and shiny things are valued by a lot of human social orders, and during the agrarian
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era, gold became a sign of social-standing and wealth. And in marriage tradition, giving
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someone an expensive gift can be a sign of esteem, hence Sarah and I spent $450 on this
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which we could have spent on an Xbox!
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Tiny wrinkles in the early universe had a major impact on one of the unifying themes
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of 13.8 billion years — rising complexity. Wrinkles created stars, stars created elements,
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and some of those elements came together to form life, and of course, us.
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Gradually, we see an increase in the number of connections in building blocks present
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in the universe. For instance, a star’s comprised primarily of two elements, hydrogen and helium.
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But here’s the thing — if there had been no wrinkles in the early universe, energy would
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have been evenly distributed across the cosmos. Without flow of energy, like say, that through
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a star, no complexity can arise — none whatsoever. This state of existence is called thermodynamic
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disequilibrium, which means that energy is not evenly distributed.
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A simple structure like a star is big, but it’s just a large pile of the lightest elements
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and doesn’t score very high in energy flow density. Your brain is 75,000 times more complex
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than a similar sized chunk of the sun. Its building blocks and nodes are way more intricate.
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Your brain has way more connections than there are stars in the galaxy.
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You wouldn’t think a story than spans cosmology, geology, biology, and human history would have
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a unifying theme, but rising complexity is something that stretches across all 13.8 billion years.
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And it began with those tiny wrinkles in the early cosmos.
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So now, moving past, I hope you see why a basic understanding of chemistry is important
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to understanding our ancestry. I mean, stars are pretty much your great-great-great-great-great-great-great-great-great-great-great
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etc grandparents! And you wouldn’t ignore your grandparents, would you?!
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Mr Green, Mr Green! No — no way! I mean, they’re a big part of my plan to get a car for my sixteenth birthday!
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That’s really touching, Me From the Past. Also I’ve got bad news for you.
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So in today’s episode, we’ve learned that everything around us, everything that we can
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touch and feel and see, even us, is debris floating around enormous stars in the vacuum of space.
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We clump into specks, we change form, but we owe our entire existence to these burning,
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gassy balls that we see in the night sky. We may just be the ashes of dead stars, but
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those ashes hold the potential to arrange themselves in increasingly complex ways, from
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which the Earth and all it contains, can arise. But more on that next time!


This post was previously published on YouTube.

Photo credit: Screenshot from video.