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