Galactic collisions | Stars, black holes and galaxies | Cosmology & Astronomy | Khan Academy

By | December 2, 2019

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.

96 thoughts on “Galactic collisions | Stars, black holes and galaxies | Cosmology & Astronomy | Khan Academy

  1. sanoonbs Post author

    AHhhh-maazzing! and they say why u wana lie forever! to witness this!! 😛

  2. fdpsrib Post author

    @DeluxeWarPlaya Oh NO!!!! I beter start doing everthing I want right now or I wont have enough time!!! Hahahaha…. nice

  3. Emily5396 Post author

    wowwwwwwwwwwwwww prettyyyyyyyyyyy (and pretty f#$&ing awesome!!!!!)

  4. JBH27 Post author

    Where do you learn this stuff??!
    I think people who wanted to delve deeper might want to know some good resources (and for the other subjects you explicate so clearly, math, finance etc)

  5. Alexandros Katechis Post author

    I've heard that the collision is already underway, but because the "collision" is on such a massive scale, we don't realize that it is happening. Can you confirm/deny this?


  6. Horsehand Post author

    Makes me want to put down my hist102 hw and play starcraftII

  7. ThatGuyTheyareAfter Post author

    @MITCHsaysHI When that happens life will most likely already be dead because our sun has turned into a red giant. Our sun's volume increased 11% since the age of the dinosaurs. it would probably only need to expand to the size of mercury's orbit before all life is extinguished

  8. Jonathan Espinoza Post author

    @MITCHsaysHI all of the planets in our system are not affected by the pull of the blackholes in the center of the galaxies, because we are gravitationally attached to our sun, like a leash, we would go where it goes, our local environment will not change, but the sky shall look spectacularly different. that unless, we do encounter another object and a collision occurs, then we screwed=]

  9. Melthornal Post author

    @MITCHsaysHI I'm pretty sure it would barely effect the stars themselves let alone the planets. Its more the groups of stars and whatnot being redistributed. The sun may move a lot, but it wouldn't stop earth from moving around it. I think at this scale the solar system can be considered a single point in space as opposed to a bunch of bodies.

  10. Rich Homie Quarkk Post author

    I remember going to an establishment as a child and hearing that the sun would eventually engulf the planets as it goes through its life stages.

    Could I also assume galactic collisions are likely to occur to galaxies already merged?

  11. qasimisonline Post author

    Hmm.. how come this can happen when everything is moving away from everything else?

  12. CaptainFoxButt Post author

    It isn't an actual star, but actually a meteoroid that enters our Earth's atmosphere and ignites because of the friction with the air.

  13. superdau Post author

    What "breaks" the galaxies so they can form a new single one? Is it the friction of interstellar material?

  14. 8bitpineapple Post author

    @superdau I don't understand what you are trying to ask, nothing really breaks it is just that the galaxies are attracted to each others centers via gravity, since we assume that the centers of galaxies are super black holes, when a superblackhole is within the event horizon of the other black hole, it will become a part of that blackhole (by definition of event horizon). The gravity from this "larger" super massive blackhole is so huge it keeps most of the "stuff" from travelling into space.

  15. 8bitpineapple Post author

    @Gtonicity Sorry to say this but as far as answers to questions go, yours is pretty bad. Unless you believe that gravity acts on one flat plain (In which case you also believe the earth is flat). Either you are incredibly intelligent and understand gravity so well you feel that it implies flat solar systems with no need for explanation, and the idea of two singularities with large bodies of mass orbiting combining a giggle, or you're an idiot. I will go for the latter.

  16. Dyslexic Artist Theory on the Physics of 'Time' Post author

    Great galaxy collision simulation!!

  17. superdau Post author

    I refer to conservation of momentum. If nothing collides (because most space is empty), the galaxies should move right through each other (of course stars would be thrown of because of gravity).
    Also there's conservation of energy. The kinetic energy of two galaxies moving towards each other has to go somewhere, otherwise this would behave like a spring or pendulum (moving -> stationary -> moving -> …).

  18. genobahamut1337 Post author

    @tjobtjobtjob Another galaxy colliding with our own doesn't have to mean we'll die. If our group of stars is thrown out into space or sticks around to orbit the new center, we might still be safe. We'll especially be safe if our group of stars is thrown out into space. Of course, this is all assuming that our sun will still be alive during the collision.

  19. genobahamut1337 Post author

    @superdau A key thing to point out here is that the Andromeda galaxy has an even larger black hole than our own, so it's not exactly two equal gravitational forces. Even if there is no friction in space (which, I'm sure there probably is some), we would eventually merge with the Andromeda galaxy just because of gravity. Also, the black holes are very likely to collide on one of the passes and that would make a new center of gravity. I'm sure there are other factors as well.

  20. genobahamut1337 Post author

    @0HellBlaze0 There are around the center. Also, not all galaxies have a black hole in the middle, and are very globular, such as the two small galaxies that orbit ours.

  21. genobahamut1337 Post author

    @0HellBlaze0 As for why they are "mostly" in a disk, it's the same reasons solar systems usually are. As something becomes locked in orbit, it tends to slowly conform to the rotation of the center of gravity. At least, that's what I've heard. It could also have it's orbit influenced by where most of the matter is orbiting.

  22. 8bitpineapple Post author

    @superdau Oh I understand now, Ok the majority of the stars do not collide but the centers of each galaxy are much more dence and contain the majority of the mass, these indeed do collide and combine, so treating the galaxy centers as two large objects that collide and combine, momentum is concerved. The resulting object attracts the stars to it via gravity that's almost twice as strong as before, some stars have a velocity too high to orbit, and some do infact follow the new galaxy center.

  23. 8bitpineapple Post author

    @Gtonicity hehe slightly, but it was a bad answer "gravity, same reason why when solar systems coalesce they take on a disk shape", If you think about it, you never answered the quesiton. If the answer is because it was made via gravity, I must assume the earth is flat because it is was made via gravity, wrong. so the answer is "same reason why when solar systems coalesce they take on a disk shape", Then you've answered the question with a question as complex as the original question.

  24. ThatGuyTheyareAfter Post author

    @TheKturner05 I can't find the source. It was a featured article on Yahoo! about 3 years ago. I'll keep looking but this is a good start:

    h t t p ://en .wikipedia. org/ wiki/Stellar_ evolution

  25. Judicial78 Post author

    These are literally the biggest collisions in the universe.

  26. PiChild619 Post author

    andrilky way, however, none of us will be alive if that happened.. or would it be crazy if humans still were alive and had all these plants to look at in the sky lol

  27. jim kuj Post author

    Very informative,thank you..I have a question(theoretically).Can the universe be
    just as micro as it is macro(in size)?Would the LHC help to determine this?

  28. Vibol Lee Post author

    is it possible that a really really really big star whose diameter is equal to that of the galaxy exist? tell me plz i really wanna know this one.

  29. Rayan Khan Post author

    @vibol03 im not sure but dont give up if on finding the correct answer to that, also why would u want to know

  30. eliezer Post author

    @vibol03 there is a limit to how big a star can be before it collapse in of itself due gravity/and or other forces(such as running out of fuel). But there are some massive stars out there. Google: VY Canis Majoris the largest know star in the universe.

  31. Vibol Lee Post author

    @3li3z3r1 lol thanks thats a really big star, but i bet theres even a much larger one

  32. sonnyhe2002 Post author

    I alway wonder why galaxies are in a flat spiral shape and not in a spherical shape. Can anyone answer that?

  33. liquidstl Post author

    @sonnyhe2002 some galaxies are sherical such as the galaxy M87

  34. Green Marshmallow Post author


    By the time you finish writing your will, your great grandchildren would have their great grandchildren and beyond

  35. hamo Post author

    loved the video and the commentary, you have very good talking skills and it seems like you have alot of knowledge, subscribed!

  36. Pål Oskar Engen Post author

    1:33 – Actually the closest star to us is the sun…

  37. Paul Kero Post author

    @poengen i lost 2 marks in an exam for saying that once 😛 But you get what he meant; closest star outside our solar system.

  38. Liz Bathory Post author

    I wonder where our Solar System would be by the end of all of that…

  39. 666alberto Post author

    O M G
    Actually, the nature in the universe it's even better than inside our Earth

  40. amozoness6 Post author

    @wmuir2009 Good question. I assume it has to do with the bodies orbiting the most massive object (for example, stars around their supermassive black hole in a galaxy) pulling toward each other. You know, the gravity of the black hole is pulling everything toward it in initially an amorphous 3 dimensional cloud that eventually smooths out, resulting in the 'disk' shape.

  41. danagamer Post author

    My daughter is worried the sun will go out some day… wait till she sees this!

  42. digerpaji Post author

    did the super computer predict to a certain degree of accuracy what will happen to earth?

  43. TheInspectorwho Post author

    they should have something like this happen in Doctor Who

  44. b0uGhTbYChrist Post author

    @danagamer dont worry this will only happen is like 4,000,000,000 years!

  45. Harbinger Post author

    1 person is afraid they might witness a galactic collision!

  46. Benjamin Swan Post author

    Khan you seem somewhat out of your depth with cosmology.

  47. spilot14 Post author

    @XxSpaCeUnIcOrNxX The Andromeda Galaxy is approaching the Milky Way at about 100 to 140 kilometres per second (62 to 87 mi/s) (400 lightyears every million years). The Andromeda Galaxy and the Milky Way are thus expected to collide in about 4.5 billion years.

  48. topgun767 Post author

    I'm amazed why our daughters are so focused on the sun. Mine was worried for about 1 week when she saw a video of the sun expanding and turning into a dwarf 😛

  49. Notlad Post author

    you dumass it wont expand into a dwarf it was split apart dwarfs are smaller

  50. Notlad Post author

    like if u say super nova dislike if u say super nove

    For the word super nova

  51. topgun767 Post author

    there are many theories of what would happen after the sun (or star in general) expands/explodes…an acceptable one is that the sun will turn into a dwarf..investigate and read "dumass"

  52. Notlad Post author

    the owner of this ur pretty smart but when it comes to galaxy's colliding they don't collide thy form into a small ball of mass over millions of years turns into a super nova that will form a LARGE black hole.

  53. carolinacat80 Post author

    Oh Khan, the great and powerful, how we worship your mighty videos. Blind us with your boundless display of scientific and mathematical knowledge. 😉

  54. apolicum Post author

    Before this will happen, our sun will have become so much of a red giant, life on earth won't be possible anymore (Not to mention the hundreds of killer asteroids that will erase all life on earth before the galaxy collision).
    Really, this should be one of your least worries lol.

  55. BokanProductions Post author

    I just hope that if this happens the planets won't be tossed out of orbit and into the cold depths of space.

  56. F l o • Post author

    is there any other planet with life near milky way or in?>!

  57. Muhammad Hussain Sarhandi Post author

    May be when the time comes when the Great Andromeda and our Milky Way Galaxies are about to collide, at that time our Physics has become able to stop the collision of these two galaxies,

  58. Elements of a Dark Universe dm6Productions-LLC Post author

    They are more likely to converge

  59. Rahima Aktar Post author

    actually if they merge it will have a new name called Milkomeda

  60. Larry Tolan Post author

    Why do I need to worry about something that won't happen for another 4 or 5 billion years?? (Like when he mentioned about stars getting thrown off into space)


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