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Galactic collisions

Collision of the Milky Way Galaxy with Andromeda (forming "Milkomeda"). Created by Sal Khan.

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  • blobby green style avatar for user Jay Varela
    Wow, what an amazing and beautiful video! I'm struggling with one thing though - if everything is moving away from everything, how could we collide with another galaxy? Have the galaxies been orbiting around something bigger and are crossing paths? Is the Milky Way traveling faster than the Andromeda galaxy? What exactly makes this possible?
    (187 votes)
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    • blobby green style avatar for user matt.reynolds
      If you're close enough then gravity pulls you together a lot faster than the universal expansion spreads you apart. Just think about you and the Earth... technically, every instant the expansion of the universe makes you a little bit further from the center of the Earth, but your gravitational attraction to the Earth is a LOT stronger and keeps moving you back toward it.
      (191 votes)
  • male robot hal style avatar for user Elijah Foster
    Are there any pictures of galaxies that are halfway through colliding? i mean with 200-300 billion galaxies there have got to be some.
    (53 votes)
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  • blobby green style avatar for user Christopher Flugel
    hi! Please clarify for me: our galaxies are set on a collision course of some sort yet our distance is substantial enough for the Hubble's law equation? Just confused a little bit regarding this collision and the notion of all objects moving away relative to us.
    (16 votes)
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    • duskpin ultimate style avatar for user Jesse William Olson
      Someone else answered this already in a different question, but yeah -- everything is moving away from everything because of expansion, but gravity is a stronger force when you have enough mass at a small distance (which is why, among other things, we individual people are not expanding away from the earth). So these galaxies have become close enough to be attracted to each other gravitationally. Does that help?
      (16 votes)
  • piceratops ultimate style avatar for user Ferret
    But wouldn't the black holes burn out like other stars over that time?
    (5 votes)
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    • piceratops ultimate style avatar for user AegonTargaryen
      Black holes take time scales much longer than that to burn out. The video spans maybe 2,000,000,000 years. The average stellar black hole will take 10^66 years or 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 years to decay. A supermassive black hole (like the ones at the centers of the galaxies) will take 10^92 years to decay.
      (26 votes)
  • spunky sam blue style avatar for user Doug likes Stew Art
    If we do collide with Andromeda, and lets say our sun did hit another star what would happen? Also what if a farther away star say Beetlejuice collided with another star? 3rd question. Would we on Earth not really feel anything from such a huge collision? Not even an increase in some radiation or something?
    (6 votes)
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    • ohnoes default style avatar for user shrikarsv2000
      If our sun hit another star when the galaxy clashes with the Andromeda, then we're dead. You might think this is a stupid answer, but this is basically the simplest answer. If a star clashes with the sun, there would have been something like a mega nuclear explosion because of major nuclear fusion interruption. If that didn't kill us, the cold would have. And if a star like Betelgeuse (not Beetlejuice) hit another star, there would have been an incredibly large explosion, that even we would have been affected, the speed of the Earth would have been increased, which is major (at least by a day).
      (17 votes)
  • duskpin ultimate style avatar for user Hyperdark
    Are there any other known galaxies that are predicted to collide in the future?
    (5 votes)
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  • male robot hal style avatar for user Enn
    Will the overall mass of the galaxy formed after the collision of the Milky Way and Andromeda be more or less than the present mass of the Milky Way ?
    (3 votes)
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  • blobby green style avatar for user 218321
    what I dont understand is why is the black hole dosent suck things up because i thought black holes always did that and dose anybody agree with me that the video was beautiful
    (5 votes)
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  • winston default style avatar for user Rino
    Despite this process taking more than 1 billion years, would there be any moments of a large and instant event that would be visible? For example, a dramatic shift in positioning that could be examined within a year.
    (3 votes)
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  • purple pi purple style avatar for user louisaandgreta
    So wait, does that mean that Earth would found itself with more planets within it’s own galaxy, since it “merges” with the Andromeda galaxy?
    Also why would the two galaxies collide? I thought everything is getting further away from each other, under the hypothesis that the universe is expanding.
    (2 votes)
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    • male robot hal style avatar for user Charles LaCour
      On very large scales the universe is expanding but within the local grouping of galaxies there is enough gravitational attraction to be pulled apart.

      When galaxies merge most solar systems will not be affected much. There is a large amount of space between stars except for near the center of the galaxies. Any stars that come close enough to interact with each other any of the orbits of the planets will be disrupted and possibly thrown out of the solar system.
      (3 votes)

Video transcript

In the last video on quasars, I think I sparked some interest when I threw out the idea of the Milky Way galaxy actually colliding with the Andromeda galaxy, which people think will happen in 3 to 5 billion years. And I threw out in the context of maybe, maybe the super massive black holes at the core, the galactic cores of each of those galaxies will start getting a little bit more material when that collision happens, and maybe quasars will happen. I don't know. But given the interest in that, what I wanted to do here is kind of an unconventional thing for the Khan Academy, and actually show a video. And before I play the video, I have to give credit where credit is due. This is a supercomputer simulation made at the National Center for Supercomputing Applications in NASA, and it's by B. Robertson of Caltech and L. Hernquist of Harvard University. And what I want you to remember, this is super sped up in time. Just to give an idea, the amount of time it takes for a star about as far away as the sun to make one orbit around the galactic core is 250 million years. And you're going to see that this is happening multiple times over the course of this video. So this video is actually spanning billions of years. But when you actually speed up time like that, you'll see that it really gives you a sense of the actual dynamics of these interactions. The other thing I want to talk about before I actually start the video is to make you realize that when we talk about galaxies colliding, it doesn't mean that the stars are colliding. In fact, there are going to be very few stars that actually collide. The probability of a star star collision is very low. And that's because we learned, when we learned about interstellar scale, that there's mostly free space in between stars. The closest star to us is 4.2 light years away. And that's roughly 30 million times the diameter of the sun. So you have a lot more free space than star space, or even solar system space. So let's start up this animation. It's pretty amazing. And what you're gonna see here, so these are just the-- obviously-- so one rotation is actually 250 million years, give or take. But now you see these stars right here are starting to get attracted to this core, and then they're actually attracted to that core. and then some of the stuff in that core was attracted to those stars, and they get pulled away. That was the first pass of these two galaxies. Some stuff is just being thrown off into intergalactic space. And you might worry maybe that'll happen to the Earth, and there's some probability that it would happen to the Earth, but it really wouldn't affect what happens within those stars' solar systems. This is happening so slow, you wouldn't feel, like, some type of acceleration, or something. And then this is the second pass. So they passed one pass. And once again, we're doing this-- this is occurring over hundreds of millions, or billions of years. And on the second pass, they finally are able to merge. And all of these interactions are just through the gravity over interstellar-- almost you could call it intergalactic distances. You can see they merge into what could be called as a Milkomeda, or maybe the Andromedy Way. I don't know. Whatever you want to call it. But even though they've merged, a lot of the stuff has still been thrown off into intergalactic space. But this is a pretty amazing animation to me. One, it's amazing to think about how this could happen over galactic space scales and time scales, but it's also pretty neat how a supercomputer can do all of the computations to figure out what every particle, which is really a star, cluster of stars, or group of stars is actually doing to actually give us a sense of the actual dynamics here. But this is pretty neat. This is pretty neat. Look at that. I mean, these are-- every little dot is whole groups of stars, thousands of stars, potentially.