Main content
Cosmology and astronomy
Course: Cosmology and astronomy > Unit 1
Lesson 1: Scale of earth, sun, galaxy and universeIntergalactic scale
Intergalactic Scale. Created by Sal Khan.
Want to join the conversation?
- Why is light years a measure of distance and not time? Because in my head the phrase "light year" would refer to something related with time, because you would measure how fast light takes to travel.(16 votes)
- Good question! We actually use light years to measure the distance light travels in a vacuum within one Julian calendar year, or 365.25 days. So in a way, it's kind of like we are measuring distance with time. In one Julian year, light in a vacuum would travel 9.461 trillion km (almost 6 trillion miles).
We use light years to measure distance in space because it is so huge that trying to measure with a unit like a mile or a kilometer would be like trying to use a grain of rice to measure the circumference of a football field - it gets confusing and easy to lose track of the number of grains of rice it would take to get around the field. The nearest star to us is 4.2 light years away. Which would be roughly 4 x 9,461,000,000,000 = 37,844,000,000,000 km away. And that's the closest star. (Besides the sun, of course!)
So you can kind of think of a light year like a big ruler that we use to measure distance in space.
I hope this helps!(31 votes)
- What is the oort cloud? He never really explained what it is.(6 votes)
- Great and important question! The Oort cloud is an immense spherical cloud surrounding the planetary system and extending approximately 3 light years, about 30 trillion kilometers from the Sun. It is believed to be a thick bubble of icy debris that surrounds our solar system.(20 votes)
- are there things in space whose light has yet not reached us(3 votes)
- yup! why not, there would more beautiful things that we haven't saw this. According to me, there would be things that would be very far from us and whose would be on the way .There would be on the edge of our universe or other universe.(12 votes)
- Wow, really amazing video, but it really raises a question - how those diagramms are made of, how someone knows what`s that far from us, and make even a card ... and how accurate these cards are, approximately?(7 votes)
- My mind is blown by the distances. Just a couple corrections. Aroundor so, you say 4 million times the distance to the nearest star. It should be 1 million times the distance since Proxima Centauri is 4 million LY away. 3:12
Also, atyou say the light left 13 billion light years ago. It should of course be 13 billion years, as you know. Great videos. Thanks. 7:05(3 votes)
- I have seen some videos where astrophysicist predict that our galaxy will colide with Andromeda (by the way what is our galaxy's name) when that happens, if there is still life on earth will that have any affects on life as we know it?(4 votes)
- Four billion years from now, our galaxy, the Milky Way, will collide with our large spiraled neighbor, Andromeda.
The galaxies as we know them will not survive.
In fact, our solar system is going to outlive our galaxy. At that point, the sun will not yet be a red giant star – but it will have grown bright enough to roast Earth’s surface. Any life forms still there, though, will be treated to some pretty spectacular cosmic choreography.(6 votes)
- Is there more than one universe?(5 votes)
- People have talked about different dimensions, but I don`t think that there is more than one universe.(6 votes)
- Is the space is expanding faster than the speed of light or the BOUNDARIES of the space is expanding faster than light? Or neither of them is expanding faster than light? If the answer is yes how can it expand faster than the speed of light?(4 votes)
- Expansion is a rate, not a speed. The current rate of expansion is about 68 km/s/Mpc. This can result in two points far enough apart to have a relative velocity to each other of greater than the speed of light. But neither object is actually moving through space at greater than light velocities, so no violation occurs.(5 votes)
- why is the universe expanding so fast and not slowly in? 7:20(4 votes)
- Expansion isn't a speed. As long as there is some rate of expansion, if you get two points far enough apart, their relative speed to each other will be greater than the speed of light.(4 votes)
- Apparently, the Milky Way will collide with the Andromeda galaxy in about 4 billion years.
How is this possible since space is expanding and stars are constantly moving away from each other?(3 votes)- The expansion rate is very small so it take a very large distance to make it great enough to overcome the gravitational pull of objects. Everything within our local group of galaxies and clusters is being pulled together with more force than they are being pulled apart by the expansion of space.(3 votes)
- Could you tell me, what is the point (star or galaxy) the farthest of earth and visible without telescope (north and south hemisphere)?
Sorry for my english.(2 votes)- The furthest visible object with the naked eye is probably the Andromeda galaxy. It is 2.5 million ly away. The Triangulum is harder to see, most likely on visible in the best conditions, and is a little further at about 3 million ly away.(3 votes)
Video transcript
Where we left off
in the last video, we were just kind
of staring, amazed, at this Earth's view of
the Milky Way galaxy, just making sure we
understood how enormous and how many stars
we were looking at. And even if each of
these dots were a star, this is a huge amount of stars. But a lot of these dots
are thousands of stars. So our mind was already blown. But what we're going
to see in this video is that in some ways, this is
kind of just the beginning. And to some degree,
I'm going to stop doing these particles of sand
and a football field analogy because at some point,
the particles of sand become so vast
that are our minds can't even grasp
it to begin with. But let's just start
with our Milky Way. And we saw in the last video,
the Milky Way right here, we're sitting here about 25,000
light years from the center. It's roughly 100,000
light years in diameter. And then, let's put
it in perspective of its local neighborhood. So let's look at
the Local Group. And when we talk
about Local Group, we're talking about the
local group of galaxies. So this right here is the
Milky Way's Local Group. That's us right
there, sitting right over here, about
25,000 light years from the center
of the Milky Way. You have some of these
"small"-- and I use "small" in quotation marks-- because
these are also vast entities, also unimaginable entities. But we have these
satellite galaxies around, under the gravitational
influence, some of them, of the Milky Way. But the nearest
large galaxy to us is Andromeda right over here. And this distance
right over here. And now, we're going
to start talking in the millions of light years. So this distance right here
is 2.5 million light years. And just as a bit of reference,
if that's any reference at all, one light year is roughly
the radius of the Oort Cloud. Or another way to think about
it, one radius of the Oort Cloud is about 50,000 or
60,000 astronomical units. And that's the distance
from the Sun to the Earth. So you could view this
as 2.5 million times 60,000 or so times the distance
from the Sun to the Earth. So this is an unbelievably
large distance we're talking about here. And that's to get to the
next big galaxy over here. But even these things are huge
things with many-- I mean, just unfathomably many-- stars. But Andromeda, in
particular, we said that the Milky Way has
200 to 400 billion stars. Andromeda, people
believe, has on the order of 1 trillion stars. So these just start
to become numbers. It's hard to grasp. But we're not
going to stop here. So in this, over here, this
whole diagram right here, it's about four
light years across, if you go from point to point. If you go from one
side to the other side, this is about-- not
four light years. Sorry. This is 4 million light years. Four light years is just
the distance from us to the Alpha Centauri. So that was nothing. That would only take that a
Voyager 1 80,000 years to get. This is 4 million light years. So 4 million times the
distance to the nearest star. But even this is-- I mean I'm
starting to stumble on my words because there's really no words
to describe it-- even this is small on an intergalactic scale. Because when you zoom out more,
you can see our Local Group. Our local group is
right over here. And this right over here
is the Virgo Super Cluster. And each dot here is
at least one galaxy. But it might be more
than one galaxy. And the diameter here is
150 million light years. So what we saw in the Local
Group, in the last diagram, the distance from the
Milky Way to Andromeda, which was 2 and 1/2
million light years, which would be just a little
dot just like that, that would be the
distance between the Milky Way and Andromeda. And now, we're looking at
the Virgo Super Cluster that is 150 million light years. But we're not done yet. We can zoom out even more. We can zoom out even
more, and over here. So you had your
Virgo Super Cluster, 150 million light years
was that last diagram, this diagram right over here. I want to keep both of them
on the screen if I can. This diagram right here, 150
million light years across. That would fit right about
here on this diagram. So this is all of the super
clusters that are near us. And once again, "near" has to be
used very, very, very loosely. Here, this distance is about
150 million light years. A billion light years
is-- two, three, four, five-- a billion light
years is about from here to there. So we're starting to talk
on a fairly massive-- I guess we've always been
talking on a massive scale. But now, it's an even
more massive scale. But we're still not done. Because this whole
diagram-- now these dots that you're seeing now, I
want to make it very clear. These aren't stars. These aren't even
clusters of stars. These aren't even
clusters of millions or even billions of stars. Each of these dots are
clusters of galaxies, each of those galaxies
having hundreds of billions to trillions of stars. So we're just on an unbelievably
massive scale at this point. But we're still not done. We're still not done. This is roughly about a
billion light years across. But right here is
actually the best estimate of the visible universe. And in future
videos, we're going to talk a lot more about what
the visible universe means. So if you were to
zoom out enough, this entire diagram right here,
about a billion light years, would fit just like that. So we're talking about a super
small amount of this part right here. And this is just the
visible universe. I want to make it clear. This is not the entire universe. And we say it's the
visible universe because think about
what's happening. When we think about
the a point out here, and we're observing
it, and that's let's say 13 billion light years away. Let's say that point 13 billion. We're going to talk
more about this in future videos, 13
billion light years. And I feel it's
almost a sacrilege to be writing on this because
this complexity that we're seeing here is
just mind boggling. But this 13 billion light
year away object, the light is just getting to us. This light left some point
13 billion light years ago. So what we're actually doing
is observing that object close to the beginning of
the actual universe. And the reason why it's
the visible universe is there might have been
something a little bit further out. Maybe it's light hasn't reached
us yet or maybe the universe itself, and we'll
talk more about this, it's expanding so fast that the
light will never, ever reach us. So it's actually a
huge question mark on how big the
actual universe is. And then some people might
say, well, does it even matter? Because this by itself
is a huge distance. And I want to make it
clear, you might say, OK, if this light over here, if
this is coming from 13 billion light years away, or if this
is 13 billion light years away, then you could say, hey, so
everything that we can observe or that we can even
observe the past of, the radius is about 26
billion light years. But even there, we
have to be careful because remember the
universe is expanding. When this light was
emitted-- and I'll do a whole video on this
because the geometry of it is kind of hard to visualize--
when this light was emitted, where we are in the
Virgo Super Cluster, inside of the Milky Way
Galaxy, where we are was much closer to that point. It was on the order
of-- and I want to make sure I get this
right-- 36 million light years. So we were super close by, I
guess, astronomical scales. We were super close, only
36 million light years, to this object, when
that light was released. But that light was coming to us
and the whole time the universe is expanding. So we were also moving away from
it, if you just think about all of the space, that everything is
expanding away from each other, And only 13 billion
years later did it finally catch up with us. But the whole time that
that was happening, this object has
also been moving. This object has also
been moving away from us. And so our best estimate of
where this object is now, based on how space is
expanding, is on the order a 40 or 45 billion
light years away. We're just observing
where that light was emitted 13
billion years ago. And I want to be very clear. What we are
observing, this light is coming from something
very, very, very primitive. That object or that area of
space where that light was emitted from has now
condensed into way more, I guess, mature
astronomical structures. If you take it from the
other point of view, people sitting where
in this point of space now, and they've now moved
46 billion light years out, when they observe
our region of space, they're not going to see us. They're not going to
see Earth as it is now. They're going to see
the region of space where Earth is at a
super primitive stage, shortly after the Big Bang. And when I use words
like "shortly," I use that also loosely. We're still talking about
hundreds of thousands or even millions of years. So we'll talk more about
that in a future video. But the whole
point of this video is it's beyond mind numbing. I would say the last
video, about the Milky Way, that alone
was mind numbing. But now, we're
going in a reality where just the Milky Way
becomes something that's almost unbelievably
insignificant when you think about this picture right here. And the really
mind numbing thing is, if someone told me that
this is the entire universe, this by itself would certainly
put things in perspective. But it's unknown
what's beyond it. There's some estimates
that this might be only be 1 times 10 to the 23rd
of the entire universe. And it might even be the
reality that the entire universe is smaller than this. And that's an interesting
thing to think about. But I'll leave you there
because I think no matter how you think about it,
it's just-- I don't know. I actually, before
doing this video, I stared at some of these
photos for half an hour. This is my least productive day
just because it's just so awe inspiring to think about what
these dots and dots of the dots really are.