- For much of human history people believed that the planet earth was the center of the universe.

That's understandable, earth is pretty big.

We now know that it's a speck compared to the universe.

To get a sense of that scale consider this grain of sand held at arm's length.

In that tiny area there are almost 10,000 entire galaxies.

That's a quadrillion stars and as many planetary systems.

In fact there are surely more entire worlds in the observable universe than there are grains of sand on this one.

(gentle cosmic music) Less than a century ago we didn't know that a universe existed beyond our own Milky Way Galaxy.

In the last 100 years our perspective and our understanding has grown enormously to include countless galaxies to the cosmic horizon and almost to the beginning of time.

But before we could map the universe, first we had to discover the universe.

We're at the Mount Wilson Observatory on the outskirts of Los Angeles California.

This is where Edwin Hubble made a paradigm shifting discovery in 1924.

He proved that the universe exists beyond the Milky Way.

Oh my God.

This is the Hooker Telescope at Mount Wilson Observatory.

It is beautiful.

The whole dome is built of Carnegie iron in the years when most of the iron was being sent to the front for tanks.

This thing was built in 1914.

That's not to say we'd never seen other galaxies.

Andromeda is visible to the naked eye if you have good vision.

And many nearby galaxies were visible as fuzzy blobs in our early telescopes.

But as far as we knew they were just clouds of gas inside our own galaxy.

The philosopher Immanuel Kant guessed that these nebulae might be entire island universes of their own.

In 1915 the American astronomer Vestos Slipher reported that some of those fuzzy blobs were zipping away from us so quickly that they would surely escape the Milky Way's gravitational field.

It was a clue that they were in fact never within the Milky Way in the first place.

But to prove that these spiral nebulae were really island universes, we needed to find their distances.

And distance is one of the hardest things to measure in astronomy.

So that's the desk and chair where Edwin Hubble conducted his observations of Andromeda to calculate his distance.

He would spend nights up there taking photographs, sensitive photographic plates using a camera much like this one.

Photographic plates go right in here.

See, when you look through a telescope it flattens 3D space into a two dimensional image, a dome above our heads that the ancients called the celestial sphere.

All sense of distance is lost.

For example, consider two stars, one bright and one dim, to our eyes.

Now perhaps those stars are identical and the bright one is just closer to us.

Or perhaps the bright one is truly much more luminous and is at the same distance or even further away than the dim star.

Now, there are some clever ways to measure distances to stars when they're within the Milky Way Galaxy.

But in another galaxy, not likely.

At least not until the brilliant work of astronomer Henrietta Levitt.

A decade before Hubble made his discovery, Levitt discovered that a certain type of star, cepheid variables, brightened and dimmed with a repeating period that is mathematically related to the star's absolute brightness.

They were the first standard candles.

If you time how many days or weeks it took a given cepheid to fluctuate, you can determine its true brightness.

So you can also figure out how far away it is just by seeing how much its true brightness has been dimmed by that distance.

That means, if you spot a cepheid variable in a distant spiral nebula, you can find the distance to both.

And that's exactly what Hubble did, right here.

He looked for cepheid variables in the great Andromeda Nebula, as it was known back then.

He found that Andromeda is around two million light years away, or about 20 times the entire diameter of the Milky Way disk.

The Andromeda Nebula became the Andromeda Galaxy.

And with more distance measurements it became clear that all spiral nebulae were island universes of their own.

Of course Hubble didn't stop there.

He combined his distances with Vestos Slipher's velocities to discover that the galaxies all appear to be racing away from the Milky Way, paving the way for the discovery that the universe is expanding.

Meaning it once must have started with the Big Bang.

With this telescope Edwin Hubble not only discovered the universe beyond the Milky Way, but he opened the door to discovering its very origin, and to our exploration of the true vastness of the universe that followed.

To see where nearly 100 years of exploration has led us, we'll need a little help from the Digital Universe built by the Hayden Planetarium at the American Museum of Natural History.

We're on our way up.

This is where it all happens.

What a view.

We're looking down at the planetarium from the astrophysics department.

FYI, this is pretty much my favorite place in the world, and I spend as much time here as I can.

Folk here at the Hayden have turned that dome into a spaceship.

It can fly us through a virtual universe built from the most complete 3D atlas of our universe ever compiled.

And unlike regular spaceships, this dome can fly at many times the speed of light.

It's the perfect place to explore the scale of the vast universe that Edwin Hubble unlocked for us.

Let's take a journey.

We're now flying through the American Museum of Natural History's Digital Universe, rendered by the OpenSpace software, leaving earth, the sun, and our solar system behind, we're zipping past the hundreds of billions of stars of our Milky Way Galaxy at a few hundred billion times the speed of light.

While we've only mapped the locations and velocities of around 1% of those stars, that gives us an incredible understanding of the shape and motion of our home galaxy, once imagined to be the entire universe and now our very familiar island home.

As we zoom out, now to several trillion times the speed of light, our local group of galaxies comes into view.

There's Andromeda, it's incredible distance first revealed to Hubble through its pulsing cepheid variable stars.

That's the Virgo Cluster, the nearest big city, of which our local group is an outlying suburb.

Every dot we see is a known galaxy, it's position, velocity, and even stellar content, measured and cataloged.

The latest studies of galaxy motion reveal that Virgo, the Milky Way, and a 100,000 more galaxies belong to Laniakea, a cluster of superclusters, some 500 million light years across.

As we accelerate to 100 quadrillion times the speed of light we see the extent of our modern mapping of the universe.

Galaxies assembled into many vast filaments flowing together on rivers of dark matter to form the cosmic web, of which Laniakea is just a part.

Edwin Hubble never imagined such a structure.

Those smaller dots at the edge of our surveys are quasars.

They look small from here, but each is a maelstrom of gas falling into a giant black hole.

And it shines from the core of its own galaxy.

That distant light comes to us from a much younger universe.

And so we just flew through an atlas of some of the first galaxies to ever form.

To see these most distant quasars, as well as the earliest galaxies, or even black holes, or worlds around other stars, Mount Wilson's Hooker Telescope wouldn't cut it.

For that we need state of the art modern observatories, like the Gemini Observatory, and the Event Horizon Telescope, and LIGO, which we've visited throughout this series.

Yet, it was Edwin Hubble's observations that opened the door to our current appreciation of the immensity of our universe, knowledge that is revealed here at the Hayden Planetarium.

And hopefully our modern perspective will in turn be the gateway to future knowledge if we continue exploring, if we keep looking deeper and further, who knows what we'll find in the expanding horizons of space time (gentle cosmic music)