[MUSIC]
At the Lawrence Hall of Science planetarium,
we're lucky to have students
from UC Berkeley as presenters.
Even though the planetarium is closed for now,
they can't resist sharing with
you the excitement of astrophysics.
Welcome! I'm Ellen from the
Lawrence Hall of Science planetarium.
I've had a lot of extra time on my hands lately,
and I've been wondering about
some big questions about the universe.
Do you ever find yourself
wondering, how old is the universe?
Or like one of my favorite planetarium
programs likes to ask: How big is the universe?
Well lucky for us curious minds,
there is a special branch of astronomy
all about answering these big questions.
It's called cosmology.
Scientists who focus on cosmology
try to figure out how the universe started,
how it's grown over time,
and what it'll be like way, way in the future.
This is what scientists today think the
history of the universe up to now looks like.
It kind of looks like a
tipped-over cup spilling out Milky Way.
We'll talk more about this
funky timeline in future videos.
Here are some pictures of some of the
different ways scientists think the universe could end.
Again, we'll talk more about
what all these pictures mean later.
But I will say now, don't worry
about the universe ending anytime soon.
The world might feel like it's
ending sometimes but it's definitely not.
The study of cosmology has
been a thing for a long, long time.
Really, people all across time and all around the world
have always asked the same questions as you and me.
A long, long time ago people thought that all
there really was to the universe was the Earth
and everything they could see in the sky.
And people used to think that the Earth was flat.
This sounds silly to us now,
but if you go outside and look
around it's easier to imagine that
you're at the flat bottom of a big
snow globe, rather than on a big ball.
There are lots of clues though
that the Earth is actually round.
For example, the Earth casts a curved
shadow on the moon during a lunar eclipse.
People noticed clues like this and started to
like thinking that the Earth was round instead of flat.
For a while, the most popular model of
the universe was called the geocentric model,
which says that the Earth is round, and also
says that the Earth is at the center of the universe.
I'm using the desktop planetarium program Stellarium,
where I can make time go by faster than real life.
I see that the Sun moves
across the sky and so do the planets.
After the Sun sets, I see the stars come out,
and they seem to move across the sky as well.
Just looking at the way things move around in the sky,
it's easy to believe that the
Earth is at the center of the universe.
But just like there are clues
that the Earth is round instead of flat,
there are plenty of clues that the Earth
goes around the Sun, instead of the other way around.
The scientist Copernicus said that you
could prove the Earth went around the Sun
using something called parallax.
Here's an example of parallax:
Hold your finger out at eye level about
an arm's width away, and close one eye.
Now when I say go, close your open eye,
and at the same time, open your closed eye.
Ready ... go!
Did you see anything interesting?
Try doing that back and forth
a few times as fast as you can.
Left wink, right wink, left wink, right wink.
Did you see anything interesting?
If you said, "My finger's moving," you're right!
Well, your finger isn't really moving but it looks like it is.
Copernicus's idea was that you
could look at a star, say this star,
at a certain time of year, say the month of January,
and that star would be
somewhere over here in this picture,
closer to the bottom stars than the top stars.
Then you would wait six months, and
the Earth would go around its orbit around the Sun,
and you could look at that star in July, only
this time it would look closer to these top stars.
Now doing Copernicus's experiment, you would
only actually see the star move a teensy-tinsy bit.
And the telescopes during
Copernicus's time weren't all that great,
so the other scientists didn't
actually see the parallax effect,
and they kept believing that the
Earth was at the center of the universe.
But lots of clues kept coming up over
time that said the Earth wasn't at the center.
For example, the astronomer Galileo's
discovery of Jupiter's four largest moons
were the first objects anyone had seen that
clearly orbited around something other than the Earth.
So people eventually agreed 
that the Earth goes around the Sun,
and this model, called
the heliocentric model,
became the most popular idea
of what the universe looked like.
Over time, people learned that the stars
are much further away than they used to think —
even the ones we call nearby are super far.
We realized that the
universe wasn't just our solar system,
that our sun is just one of
many stars in the Milky Way Galaxy,
and we are not at the center.
Astronomers wondered
how big the Milky Way Galaxy was
and whether the galaxy was the whole
universe, or if the universe was even bigger.
An astronomer named Henrietta Swan Leavitt
found a way to tell how far
away certain very distant stars are.
People learned from this
how enormous the Milky Way is.
But when they saw little, fuzzy
blobs of light in their telescopes,
they did not know whether those were
cloudy objects inside the Milky Way Galaxy
or whether they were other galaxies much farther away.
Then, only about a century ago, the
astronomer Edwin Hubble made a discovery
that made people's idea of the
universe get a whole lot bigger.
In one of those fuzzy blobs
that people called the Andromeda nebula,
he was able to see individual stars in his telescope.
Using Swan Leavitt's method, he learned that the stars
in the Andromeda nebula are far outside the Milky Way.
Now, people call it the Andromeda Galaxy,
and it's one of the many galaxies that
has been observed outside the Milky Way.
Scientists have also discovered that other
galaxies are moving away from the Milky Way
because the universe is actually expanding.
So the universe is growing all the time,
and it's actually full of
billions and billions of galaxies.
And, of course, the Milky Way is not at the center.
It might be kind of sad that we
aren't at the center of everything,
and it may seem scary that the universe is so big.
But just because you aren't at the center of the universe,
that doesn't mean you aren't special.
And just because the universe is too big to imagine,
you're not too small to matter.
Even though you and I are
small compared to how big the universe is,
we still have big ideas and big questions.
So, keep wondering and
keep asking those big questions.
