If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Intergalactic scale

Intergalactic Scale. Created by Sal Khan.

Want to join the conversation?

  • leaf green style avatar for user Alejandro Artiles
    how can the universe be expanding if there is no space out of the universe for it to ocupy...tell me if u understand my question because like when i wrote it my mind just shoked :)
    (183 votes)
    Default Khan Academy avatar avatar for user
    • leaf green style avatar for user Luis Ruiz
      SPACE is expanding. If space is expanding, then why would it need more space to expand to? it's counter intuitive for us humans to think of it since we live inside space. so when we observe something expand, we observe it expand INSIDE space. so when someone says, "space is expanding" our natural question is what is space expanding inside of? But if space would expand INSIDE something, that something would be itself space. it's not a good question. it' like asking, what happen before the big bang? if time started at the big bang then how can you ask what happened BEFORE time? It's the same when you ask, what is space expanding inside of? space is not expanding inside anything, space is just expanding.
      (244 votes)
  • leafers ultimate style avatar for user Kamila Zdybał
    Sal gave the distance 2.5 milion l.y. from Milky Way to Andromeda but the space between galaxies is constantly expanding, so how should I treat this distance?
    (86 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user yumitori
      In order to answer this, you need to understand the idea of significant digits. Say that you ask me the distance from New York to London, England, and I tell you it's 3470 miles. If you were to measure it yourself with highly accurate instruments, and came up with a distance of exactly 3468 miles, 33 feet and 7 inches, would you call me a liar? Most likely not, because you would recognize that I gave you a distance rounded to the nearest ten miles. At such distances a mile or two one way or the other doesn't really matter.

      Now consider that North America and Europe are moving away from one another due to plate tectonics. A couple of years later you again measure the distance between New York and London with extreme accuracy and find that it is now 3471 miles, 118 feet and 3 inches. (I'm just making up numbers here, don't assume I'm suggesting such rapid changes in distance are actually occurring.) Would my initial answer of "3470 miles" now be incorrect? No, because rounding to the nearest ten miles still gives the same answer.

      So even though the galaxies are moving away from one another at great speeds, the distance of "2,500,000 light years" is just an approximation. Sal is in no way claiming that the stated distance is accurate down to the inch, any more than I was with my answer. For the immediate future it's close enough.
      (136 votes)
  • aqualine seedling style avatar for user Dipesh Chaudhary
    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.
    (13 votes)
    Default Khan Academy avatar avatar for user
    • female robot grace style avatar for user Mari
      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)
  • duskpin sapling style avatar for user FluffyBolt
    What is the oort cloud? He never really explained what it is.
    (6 votes)
    Default Khan Academy avatar avatar for user
    • old spice man green style avatar for user Matt B
      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.
      (17 votes)
  • leaf grey style avatar for user Saad Shahid
    are there things in space whose light has yet not reached us
    (2 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user merikemartsepp
    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?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • marcimus purple style avatar for user Ralph Kepler
      My mind is blown by the distances. Just a couple corrections. Around or 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.
      Also, at you say the light left 13 billion light years ago. It should of course be 13 billion years, as you know. Great videos. Thanks.
      (1 vote)
  • aqualine ultimate style avatar for user Cephalopoda Sapiens
    Is there more than one universe?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • piceratops ultimate style avatar for user abdulrahman akram
    why is the universe expanding so fast and not slowly in ?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user Samiha Rahman
    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?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • boggle blue style avatar for user Davin V Jones
      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.
      (3 votes)
  • blobby green style avatar for user Carlos de Jesus
    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?
    (2 votes)
    Default Khan Academy avatar avatar for user
    • aqualine ultimate style avatar for user ĐαniΣl βaranowski
      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.
      (4 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.