Galaxies

[raf]

1. why do they form spirals ?

 

2. why do they attact each other ?

 

3. what happens when they collide?

[Sisyphus]

1. Actually, they don't. The spiral shape seen in some (but not all) galaxies is a result of a higher concentration of younger, brighter stars in that pattern, which makes those areas brighter. But the shaper of the galaxy itself is still a disc. The pattern itself is caused by a kind of "density wave" analogous to a traffic jam. As stars and other material enter those regions, they are slowed by gravity, and the higher density of interstellar gas leads to more stars being formed. Material passes through and continues its orbit, but the "traffic jam" itself moves slower and is left behind.

 

2. They attract each other for the same reason that all material objects in the universe attract one another: gravity.

 

3. If galaxies pass near one another, the gravity of each will distort the other, much like the moon's gravity distorts the Earth (that's what tides are), but moreso, because galaxies are more fluid and less coherent than planets. If they actually collide, they'll either merge if they weren't moving very fast relative to one another and thus can get snared by one another's gravity, or if they are moving fast they'll just pass through one another, trading some of their stars and getting their shapes greatly distorted on the way.

[Klaynos]

1. Read the question wrong, see above...

 

 

2. Gravity.

 

3. Mostly they pass through each other and become one big single galaxy with some weird angular momentum, they will eventually become "flat" again... The supermassive black holes at the centre merge as well I seem to recall...

[Radical Edward]

what happens when galaxies collide?

 

this:

 

 

just do a google image search for colliding galaxies. Hubble has taken some excellent pictures.

[Deja Vu]

Here is a decent simulation on how galaxies collide:

 

Also, there is free software you can use in order to simulate galaxy collisions of any size, type, and shape: http://www.mars3d.com/PWGravity3D.htm

[raf]

thanks for the info ,what is at the centre of a galaxy?

 

is it a black hole and if so why does it not suck all the matter with it?

[Klaynos]

It is a supermassive black hole, at least at the centre of ours, and most other galaxies where we've looked for them (if not all).

 

Everything doesn't get sucked into them for the same reason we don't get sucked into the sun. We're in stable orbits around it. Black holes, outside the Schwarzschild radius are not special.

[Sisyphus]

 

is it a black hole and if so why does it not suck all the matter with it?

 

To elaborate on Klaynos' answer, black holes don't "suck things in" any more than any other object with gravity. This is a common misconception, so it bears repeating. If, for example, the Earth were to suddenly collapse into a black hole, the Moon would continue orbiting in almost exactly the same way it does now. It would still be orbiting an object of the same mass and the same distance away. The fact that that object would now be the size of a marble instead of a planet wouldn't really matter.

 

What does make black holes different (well, one of the things), is how close you can get to them. Remember, the force of gravity from something is proportional to the square of the distance from its center. (So if you're half the distance away, the gravitational pull is four times as powerful.) Here on Earth, the closest we can the center without starting to go inside is the surface, which is still about four thousand miles away. And even here, we need huge rockets to "escape" the Earth's gravity. We would need that same kind of power if we were going to escape from an Earth-mass black hole if were four thousand miles away. But, since that black hole would be tiny, we could get much, much closer, and therefore experience much, much higher gravity. It's not a "different kind" of gravity, but their unique properties (extreme density) mean that things can get closer, and experience more.

 

But again, at the same distance away, the gravity from a black hole is no different from the gravity of a planet or a star or anything else.

[Arch2008]

If you meant to ask, “Why did the black hole stop feeding on the surrounding matter”, here is the answer:

http://jilawww.colorado.edu/www/research/blackholes.html

A large black hole creates a sort of shock wave that moves infalling matter back to a stable orbit.