When Galaxies Collide

neutron_p writes "An international team of scientists announced today, they observed a nearby head-on collision of two galaxy clusters. The clusters smashed together thousands of galaxies and trillions of stars. It is the most powerful events ever witnessed. Such collisions are second only to the Big Bang in total energy output. The event was captured with the European Space Agency's XMM-Newton observatory. Scientists are calling the event the perfect cosmic storm: galaxy clusters that collided like two high-pressure weather fronts and created hurricane-like conditions, tossing galaxies far from their paths and churning shock waves of 100-million-degree gas through intergalactic space."

Comments

Wouldn't the Galaxies just pass through each other

[Mystic0]

Since the celestial an planetary bodies are extremely far apart relative to their size, wouldn't the galaxies just pass through each other without colliding at all?

RTFA

[empaler]

Or at least look at the 'artist's impression'.

They may be small and far apart, but the rules of physics does not allow preclusion from stuff like gravity and whatnot.

No. This has been understood for decades.

[Bootsy Collins]

It the universe is expanding due to the Big Bang, then why would galaxy clusters ever meet?

The short answer is: because the Universe on these scales is not perfectly homogeneous. If it were, they wouldn't merge.

The longer answer:

Remember that the expansion of the Universe is an expansion of background space -- an expansion of the space in which everything is embedded -- rather than stuff moving through space. The rate of change of the relative separation of two hunks of matter can then be thought of as having two components: one from the expansion of space (objects staying in the same location, but the distances between objects are increasing because space is expanding), and one from the movement of objects through space (objects changing their locations in space). In the case of the latter -- the so-called "peculiar velocity" of an object -- if matter were distributed perfectly smoothly throughout the Universe, there'd be no reason for anything to change locations in space. But it isn't; and so there are net gravitational forces on objects that cause them to move. Whether the attraction of two objects "wins" over the expansion tending to separate them depends upon the situation.

For a simple way to picture this sort of thing, consider a big rubber sheet with two marbles on it. Give one a nudge towards the other (its peculiar velocity), and then start stretching the sheet (the expansion of the Universe). Will they collide? Depends on the peculiar velocity, rate of stretching (expansion), etc. But it's certainly not the case that they always won't.

I'm sorry

[b-baggins]

I just can't get my sky is falling hysteria worked up over a galactic storm that takes 100 million years to occur. I hope these folks aren't planning on watching the whole thing from beginning to end.

Re:Aha, but!

[div_B]

"All of a sudden, causality decided to give physical laws and time the finger,

Actually physical laws appear to have given causality the finger quite a while ago.

Re:Wouldn't the Galaxies just pass through each ot

[dougmc]

and so the comparison is really not a well advised one

It gets worse. The stars are generally further apart (when expressed as the ratio of their diameters) than the molecules in a gas are at STP. And the stars in a general area are usually moving in the same general direction, unlike molecules which are all moving about randomly.

Also, gravity will generally only make two stars collide under very specific conditions -- what will usually happen instead when two stars wander into the same area is that their paths will be deflected by the other, or maybe they'll go into orbit about each other. But without a way to shed the angular momentum, they're unlikely to ever collide unless their initial paths are *just* right.

But if it ever happens (two stars actually colliding, especially big ones) ... I'll bet it's one hell of a show! (as long as you're far enough away to be safe from the resulting nova/super nova/black hole -- not that a black hole would be any more dangerous than the two stars that combine to create it, of course.)

Re:Our turn is coming

[dpilot]

I thought that was the Milky Way shredding the Magellanic clouds, and that that was largely a 'gravitational' collision. I'm under the impression that we have an up-and-coming collision with the slightly-larger Andromeda galaxy, and that the Milky Way will get the shorter end of the stick on that one. Given that both galaxies are about the same size, I don't think either will feel too good, afterward.

The science fiction author Alistair Reynolds has a series of books that is partly driven by the impending collision. ("Revelation Space", "Redemption Ark", forgot the third. "Chasm City" shares the setting.)