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Thread: Weird Question

  1. #1
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    Weird Question

    I havent read anything about it before, and I didnt even know it happened until recently.

    But what would happen if two massive galaxies, one of them having a massive black hole in the center, collided?

  2. #2
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    They Combine, and form an Elliptical Galaxy, even if they aren't already.

    As for ONLY One of them having a Black Hole at its Center, that would be a Major Oddity, as it is Currently Thought that ALL Galaxies contain a Super-Massive Black Hole at their Centers, typically at a Proportional Size to the Galaxy itself.

    It'll actually happen to our own Galaxy, in 4 Billion Years ...

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    They Combine, and form an Elliptical Galaxy, even if they aren't already.
    I thought they would form an irregular galaxy.

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    Irregular galaxies are galaxies in chaos. The mass will be irregular when the two galaxies interact. But ultimately they will form into an elliptical galaxy.

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    Quote Originally Posted by ZaphodBeeblebrox
    It'll actually happen to our own Galaxy, in 4 Billion Years ...
    Oh great, something else for the woowoos to worry about.

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    Quote Originally Posted by Quartermain
    Irregular galaxies are galaxies in chaos. The mass will be irregular when the two galaxies interact. But ultimately they will form into an elliptical galaxy.
    I think this all depends on relative center of mass and other ratios in the system. If they were to be "balanced", i.e., the black hole and the surround matter, then I don't think there would be as much rotation and thus you would get an elliptical galaxy. Otherwise you could get an irregular, and then maybe a spiral, depending on rotation and angular momentum.

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    Thanks guys. I can only imagine the grandeur of such an event in my head. I mean imagine being an outsider looking at such an event.

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    Quote Originally Posted by Tranquility
    Thanks guys. I can only imagine the grandeur of such an event in my head. I mean imagine being an outsider looking at such an event.
    It's kinda like going down to the beach and watching the tectonic plates collide.

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    mili360 wrote:

    Tranquility wrote:
    Thanks guys. I can only imagine the grandeur of such an event in my head. I mean imagine being an outsider looking at such an event.

    It's kinda like going down to the beach and watching the tectonic plates collide.
    Yep, when poetry takes over from science and we step back and marvel at how a butterfly exists in all the randomness about us.

    In the great 'there' of everything planet Earth isn't even a grain of sand

  10. #10
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    Quote Originally Posted by milli360
    Quote Originally Posted by Tranquility
    Thanks guys. I can only imagine the grandeur of such an event in my head. I mean imagine being an outsider looking at such an event.
    It's kinda like going down to the beach and watching the tectonic plates collide.
    Just bring along a picnic lunch and make a day of it. :wink:

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    I would bring along my cryo unit and make an era of it.

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    I once heard that the because the stars are so far appart they could actualy pass through each other without hitting.

  13. #13
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    Re: Weird Question

    Quote Originally Posted by Tranquility
    But what would happen if two massive galaxies, one of them having a massive black hole in the center, collided?
    Hmmm, having black hole in their centers? Well, I remember reading in a book by Carl Sagan (Cosmos?) that if two galaxies collided, one galaxy would effortlessly pass through the other because of the immense distances between stars. There may be distortion in their shapes, but I don't remember reading two colliding galaxies would form a single one.

    I appreciate it if someone can refresh my memory.

    I don't have that book anymore - after reading and rereading the pages fell off and I dropped the book off at a local library. :-?

  14. #14
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    Re: Weird Question

    Quote Originally Posted by SiriMurthy
    Quote Originally Posted by Tranquility
    But what would happen if two massive galaxies, one of them having a massive black hole in the center, collided?
    Hmmm, having black hole in their centers? Well, I remember reading in a book by Carl Sagan (Cosmos?) that if two galaxies collided, one galaxy would effortlessly pass through the other because of the immense distances between stars. There may be distortion in their shapes, but I don't remember reading two colliding galaxies would form a single one.

    I appreciate it if someone can refresh my memory.

    I don't have that book anymore - after reading and rereading the pages fell off and I dropped the book off at a local library. :-?
    It depends on a lot of things: the angle at which they collide, their relative momenta, their gravitational fields, etc. They might pass through each other if they're moving fast enough to overcome their collective gravity or if they graze each other, as opposed to hitting each other head on. On the other hand, they might get entangled thanks to their gravity and form one larger galaxy. The latter seems to be the case with the galaxies in the Antennae and the Mice.

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    While it is true that stars will pass each other without colliding, the same is not true of gas and dust clouds. At a minimum, there will be a tremendous burst of star formation and the resultant galaxy or galaxies (depending on initial conditions) will be, for all practical purposes, bereft of gas and dust.

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    I wonder though, if Andromeda and our Milky Way were to collide, what would happen to life on Earth? Will we survive? Will new life spawn elsewhere in either of the galaxies?

    PS: For a moment forget that this would happen in millions (billions?) of years. Assume that the collision has begun.

  18. #18
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    I read that when galaxies collide, they merge because of the great distances between stars.... If humanity was still around we would probably still survive.

  19. #19
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    Quote Originally Posted by SiriMurthy
    I wonder though, if Andromeda and our Milky Way were to collide, what would happen to life on Earth? Will we survive? Will new life spawn elsewhere in either of the galaxies?

    PS: For a moment forget that this would happen in millions (billions?) of years. Assume that the collision has begun.
    Since the collision itself takes millions of years, and stars are so far apart that one may not even pass close to our sun, we probably wouldn't notice much. If we were unlucky enough to have another star pass close to the sun, however, that could be pretty disasterous.

  20. #20
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    plus ive read that when galaxies do "colide" or pass thru each other the odds of 2 stars hitting each other are less than the odds of 2 knats starting at each end of the grand canyon flying into each other

  21. #21
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    Quote Originally Posted by kenneth rodman
    plus ive read that when galaxies do "colide" or pass thru each other the odds of 2 stars hitting each other are less than the odds of 2 knats starting at each end of the grand canyon flying into each other
    There, that summerizes that. I have read it as well - can't remember where.

  22. #22
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    There are several issues hiding in this thread - the fate of central black holes, timescales, what is leftover after a galaxy collision, whether individual stars collide. A few random bits:

    Indeed the expected number of stellar collisions is tiny (not sure whether it's actually zero for two Milky Way-like system...). There was a painting in the old Time-Life "World we Lve In" series that embodied an old view that this meant that colliding spirals would fly right through one another, leaving their gas behind in some kind of glowing blob. It was already known from Holmberg's analog experiments (I'll let folks stew over that one for a while) in the 1940s that this was wrong - that interactions between the internal motions and orbits of the galaxies would have the net effect of transferring energy to someof the stars, shrinking the galaxies' orbit (which is what drives merging). Numerical simulations suggest that the merger takes a couple of orbital times at the disk edge of oneof the
    incoming galaxies (for disks) - which would be half a billion years for us, starting from the time the stellar disks overlap. This timescale is borne out by ages of the bright "super" star clusters oten formed during strong interactions.

    As this happens, the dense cores of the galaxies retain their identities longest, like marbles roling in a big bowl. Central black holes are most
    extreme, and will fall together until they dominate the mass within the relative orbits. This gets interesting - once that happens, they may stick in that orbit until something else (a small infalling galaxy?) perturbs the gravitational field and may drive them to a spectacular merger (best viewed from a safe distance).

    Numerical simulations again suggest that ellipticals results from all but
    the most special (like both face-on and corotating) spiral-spiral mergers as long as the incoming galaxies are within about a factor 3 in mass. Larger mass ratios are more like the Milky Way eating the Sagittarius dwarf, and their traces can be very subtle if surprisingly long -lasting.
    These include kinematically and chemically distinct groups of halo stars, as found in Heather Morrison's "Spaghetti Survey".

    I'll shamelessly mention my own collection of pictures of interactions and mergers: www.astr.ua.edu/pairs2.html
    and a web lecture with more detail than anybody wants at
    www.astr.ua.edu/keel/galaxies/mergers.html

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