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### Course: Cosmology and astronomy > Unit 1

Lesson 1: Scale of earth, sun, galaxy and universe# Scale of the large

Attempting to comprehend the scale of the large. Created by Sal Khan.

## Want to join the conversation?

- At6:13Sal wrote 1 Au underneath the tiny sun and earth. What does Au mean?(142 votes)
- AU stands for 1 astronomical unit which is the distance between the Sun and the Earth. The distance of other planets from the Sun can be measured in AU which gives a helpful comparison between distance of two objects compared to the distance between the Sun and Earth.(315 votes)

- Why does Jupiter have a great big red spot and Neptune have a great dark spot and how did they get there?(35 votes)
- I searched it up for you and I found this link which would just about answer all your questions about the red spot on Jupiter.

http://www.universetoday.com/15163/jupiters-great-red-spot/(21 votes)

- If we're 200,000 distances from Earth to the Sun to the nearest star, what state are the objects between? Are there planets? Are they absurdly cold?(12 votes)
- There ARE planets zooming around in interstellar space, ejected from their home star systems. They are called "rogue planets".(29 votes)

- At4:30Sal says that Earth would be destroyed if it was too close to Jupiter, but how would that happen?(6 votes)
- The gravity of Jupiter is so intense, that if our planet was right next to Jupiter we'd be smashed into Jupiter. The same would happen if our planet was right up next to any large body. If the earth did smash into Jupiter like that, it might not be completely destroyed, though: probably the earth's crust would break off, but the core might bounce off and become a moon of Jupiter. That's a theory I once read of where the moon came from, that a smaller planet collided with earth (billions of years ago), that planet lost it's crust into proto-earth, and the remaining core began orbiting the earth as our moon.(28 votes)

- Is there a black hole in the center of every galaxy?(9 votes)
- We can safely
**predict**that there are supermassive black holes at the center of say, spiral galaxies but we have no proof to believe that every single spiral galaxy has a supermassive black hole at its center. There are just too many galaxies to survey.(16 votes)

- i really cant understand why the universe is 13.7 billion ly old

can somebody help me?(19 votes)- I don't know if there is an answer to
*why*the universe is 13.7 billion years old, nor will there ever be. This is similar to asking "*Why*did the Big Bang occur?"

If you mean "how we know", it's based primarily on cosmic microwave background (CMB) radiation that originated with the Big Bang. This radiation is still around today (but it's low energy radiation, so it's not dangerous) and is measurable.

This may help:

http://simple.wikipedia.org/wiki/Cosmic_microwave_background_radiation(8 votes)

- Hey, i got a question: In theory, nothing can pass infinite. Applying this rule to the universe, there must be an end to the universe's expansion right?(9 votes)
- Infinity is not a specific value. There are different sizes of infinity. For example there are countable infinities like the number of integers or rational numbers are the same size infinities since there is a way to map one of them 1 to 1 onto the other. There are uncountable infinities like the irrational numbers.

Also if the universe is infinite and expanding there is nothing that says it can't expand infinitely. For example lets say that there is a hotel with an infinite number of rooms that is full so that there is someone in every room.

Now someone comes to the full infinite hotel and is looking for a room. How are they able to get a room? Just have the person in room 1 move to room 2 and the person in room 2 move to room 3 and so on so the person in room N moves to N+1. This frees up the 1 room needed.

Now a bus with a million people arrives, you can still accommodate them by having the person in room N move to room N + 1,000,000. This will free up the million rooms needed.

What if an infinite buss of people shows up? You just need to have the person in room N move to room 2 * N. This puts the people in the current rooms into all of the even rooms and not the infinite people in the bus can go into the infinite odd numbered rooms.

You can do this over and over and still not run out of rooms. This is the same as "expanding" the universe over and over.(12 votes)

- I can't understand what lightyears are, pls help(5 votes)
- Light always travels at the same speed in a vacuum, which means we can measure distance by saying how long it takes light to go that far. A light-year is just the distance that light travels in one Earth year. 1 light year is about equal to 9,461,000,000,000 kilometers or 5,879,000,000,000 miles.(13 votes)

- why the planets are round ?(11 votes)
- planets are spheres because the gravitational force is from the centre of the planet so the distance from the surface to the core should be equal a sphere is the only shape that allows this to happen that is why most celestial bodies eventually turn to spheres.(4 votes)

- Is our universe the only universe out there or are there more?(5 votes)
- It really depends on what you mean by a universe but in general we do not know since we only have access to observe this universe.(13 votes)

## Video transcript

The purpose of this
video is to just begin to appreciate how vast and
enormous the universe is. And frankly, our brains
really can't grasp it. What we'll see in this
video is that we can't even grasp things that are
actually super small compared to the size of the universe. And we actually
don't even know what the entire size of
the universe is. But with that said,
let's actually just try to appreciate
how small we are. So this is me right over here. I am 5 foot 9 inches, depending
on whether I'm wearing shoes-- maybe 5 foot 10 with shoes. But for the sake of this
video, let's just roughly approximate around 6
feet, or around roughly-- I'm not to go into the details
of the math-- around 2 meters. Now, if I were to lie
down 10 times in a row, you'd get about the
length of an 18-wheeler. That's about 60 feet long. So this is times 10. Now, if you were to put an
18-wheeler-- if you were to make it tall, as opposed to
long-- somehow stand it up-- and you were to do
that 10 times in a row, you'll get to the height of
roughly a 60-story skyscraper. So once again, if you took me
and you piled me up 100 times, you'll get about a
60-story skyscraper. Now, if you took that
skyscraper and if you were to lie it down
10 times in a row, you'd get something of the
length of the Golden Gate Bridge. And once again, I'm not
giving you the exact numbers. It's not always going
to be exactly 10. But we're now getting to
about something that's a little on the
order of a mile long. So the Golden Gate Bridge is
actually longer than a mile. But if you go within the twin
spans, it's roughly about a mile. It's actually a little
longer than that. But that gives you
a sense of a mile. Now, if you multiply
that by 10, you get to the size of a large city. And this right here is
a satellite photograph of San Francisco. This is the actual
Golden Gate Bridge here. And when I copy and
pasted this picture, I tried to make it roughly
10 miles by 10 miles just so you
appreciate the scale. And what's interesting here--
and this picture's interesting. Because this is the first
time we can relate to cities. But when you look at
a city on this scale, it's starting to
get larger than what we're used to processing
on a daily basis. A bridge-- we've
been on a bridge. We know what a
bridge looks like. We know that a bridge is huge. But it doesn't
feel like something that we can't comprehend. Already, a city is
something that we can't comprehend all at once. We can drive across a city. We can look at
satellite imagery. But if I were to
show a human on this, it would be unbelievably,
unbelievably small. You wouldn't actually
be able to see it. It would be less than
a pixel on this image. A house is less than
a pixel on this image. But let's keep
multiplying by 10. If you multiply by 10
again, you get to something roughly the size of the
San Francisco Bay Area. This whole square over here
is roughly that square right over there. Let's multiply by 10 again. So this square is about
100 miles by 100 miles. So this one would be about
1,000 miles by 1,000 miles. And now you're including a
big part of the Western United States. You have California here. You Nevada here. You have Arizona
and New Mexico-- so a big chunk of a
big continent we're already including. And frankly, this
is beyond the scale that we're used to operating. We've seen maps, so maybe
we're a little used to it. But if you ever had to walk
across this type of distance, it would take you a while. To some degree, the fact that
planes goes so fast-- almost unimaginably fast
for us-- that it's made it feel like things like
continents aren't as big. Because you can fly across
them in five or six hours. But these are already
huge, huge, huge distances. But once again, you
take this square that's about 1,000
miles by 1,000 miles, and you multiply that by 10. And you get pretty
close-- a little bit over-- the diameter of
the Earth-- a little bit over the diameter of the Earth. But once again,
we're on the Earth. We kind of relate to the Earth. If you look carefully
at the horizon, you might see a little
bit of a curvature, especially if you were
to get into the plane. So even though this
is, frankly, larger than my brain can
really grasp, we can kind of relate to the Earth. Now you multiply the
diameter of Earth times 10. And you get to the
diameter of Jupiter. And so if you were to sit
Earth right next to Jupiter-- obviously, they're
nowhere near that close. That would destroy
both of the planets. Actually, it would
definitely destroy Earth. It would probably just
be merged into Jupiter. So if you put Earth
next to Jupiter, it would look something
like that right over there. So I would say that Jupiter is
definitely-- on this diagram that I'm drawing
here-- is definitely the first thing that I
have I can't comprehend. The Earth, itself,
is so vastly huge. Jupiter is-- it's 10
times bigger in diameter. It's much larger in terms
of mass, and volume, and all the rest. But just in terms of diameter,
it is 10 times bigger. But let's keep going. 10 times Jupiter
gets us to the sun. This is times 10. So if this is the Sun-- and
if I were to draw Jupiter, it would look
something like-- I'll do Jupiter in pink-- Jupiter
would be around that big. And then the Earth
would be around that big if you were to put them
all next to each other. So the Sun, once again, is huge. Even though we see
it almost every day, it is unimaginably huge. Even the Earth is
unimaginably huge. And the Sun is 100 times
more unimaginably bigger. Now we're going to start getting
really, really, really wacky. You multiply the diameter
of the Sun, which is already 100 times the diameter
of the Earth-- you multiply that times 100. And that is the distance
from the Earth to the Sun. So I've drawn the Sun
here as a little pixel. And I didn't even draw
the Earth as a pixel. Because a pixel would
be way too large. It would have to be a
hundredth of a pixel in order to draw
the Earth properly. So this is a
unbelievable distance between the Earth and the Sun. It's 100 times the distance of
the diameter of the Sun itself. So it's massive, massive. But once again, these
things are relatively close compared to where
we're about to go. Because if we want to get to
the nearest star-- so remember, the Sun is 100 times the
diameter of the Earth. The distance between the Sun
and the Earth is 100 times that. Or you could say it's
10,000 times the diameter of the Earth. So these are
unimaginable distances. But to get to the nearest star,
which is 4.2 light years away, it's 200,000 times-- and
once again, unimaginable. It's 200,000 times the distance
between the Earth and the Sun. And to give you a rough sense of
how far apart these things are, if the Sun was roughly
the size of a basketball-- if the average star was about
the size of a basketball-- in our part of the galaxy in a
volume the size of the Earth-- so if you had a big volume
the size of the Earth, if the stars were the
sizes of basketballs, in our part of the
galaxy, you would only have a handful of
basketballs per that volume. So unbelievably sparse. Even though, when you
look at the galaxy-- and this is just an
artist's depiction of it-- it looks like something
that has the spray of stars, and it looks
reasonably dense, there is actually a huge
amount of space that the great, great,
great, great, great majority of the volume in the galaxy
is just empty, empty space. There's no stars, no
planets, no nothing. I mean, this is a huge jump
that I'm talking about. And then if you
really want to realize how large a galaxy,
itself, can be, you take this distance
between the Sun, or between our solar system
and the nearest star-- so that's 200,000 times the
distance between the Earth and the Sun-- and you multiply
that distance by 25,000. So if the Sun is right
here, our nearest star will be in that same pixel. They'll actually be
within-- you'd actually get a ton of stars
within that one pixel, even though they're
so far apart. And then this whole thing
is 100,000 light years. It's 25,000 times the
distance than the distance between the Sun and
the nearest star. So we're talking about
unimaginable, unfathomable distances, just for a galaxy. And now we're going
to get our-- frankly, my brain is already
well beyond anything that it can really process. At this point, it almost just
becomes abstract thinking. It just becomes playing with
numbers and mathematics. But to get a sense of
the universe, itself, the observable universe--
and we have to be clear. Because we can
only observe light that started leaving from its
source 13.7 billion years ago. Because that's how
old the universe is. The observable universe is about
93 billion light years across. And the reason why it's
larger than 13.7 billion is that the points
in space that emitted light 13.7 billion
years ago, those have been going away from us. So now they're on the order of
40 billion light years away. But this isn't about cosmology. This is just about
scale and appreciating how huge the universe is. Just in the part of the universe
that we can theoretically observe, you have to get--
and that we can observe, just because we're getting
electromagnetic radiation from those parts
of the universe-- you would have to
multiply this number. So let me make this clear. 100,000 light years-- that's
the diameter of the Milky Way. You would have to
multiply not by 1,000. 1,000 would get you to
100 million light years. This is 100,000 times
1,000 is 100 million. You have to multiply
by 1,000 again to get to 100
billion light years. And the universe,
for all we know, might be much, much,
much, much, larger. It might even be infinite. Who knows? But to get from just the
diameter of the Milky Way to the observable universe, you
have to multiply by a million. And already, this is an
unfathomable distance. So in the whole
scheme of things, not only are we pretty small,
and not only are the things we build pretty small, and not
only is our planet ultra small, and not only is our
Sun ultra small, and our solar
system ultra small, but our galaxy is
really nothing compared to the vastness of the universe.