Placeholder Image

Subtitles section Play video

  • 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