Cosmology and astronomy
Intergalactic Scale. Created by Sal Khan.
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- 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. Around3:12or 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, at7:05you say the light left 13 billion light years ago. It should of course be 13 billion years, as you know. Great videos. Thanks.(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 in7:20?(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)
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.