I'd posted the following on physicsforums but this is probably a better place (found this after listening to some astronomy cast episodes which are awesome).
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I was reading the wiki page on dark matter, and have trouble visualizing how galaxies keep their shape if most of the stars rotate at the same rate. If I try to imagine this (ie: without any sort of math modelling of the gravity interactions involved) I picture everything just merging into a big amorphous disk instead of shapes like spirals.

Does anybody know of a nice animation or visualization of long term effects of a gravity model of a spiral galaxy? Ideally I'd like to see how the rotation would look over time with and without the dark matter distribution.

The spiral pattern is not a result of stars holding their places relative to
each other in spiral arms as they orbit around and around and around
the center of the galaxy.

Instead, the spiral pattern is in large part due to a wave of compression
which causes gas clouds in a small region to become slightly more dense
than usual, which causes stars to form in them. Some of those stars
are massive, blue, luminous and short-lived. They light up the regions
in which they formed for just a few million years, then die. Those few
luminous stars define the spiral arms in many galaxies. Because they
only live for 5-20 million years, while it takes 200 million years to complete
one orbit around the center of the galaxy, the pattern they make doesn't
really rotate with the longer-lived stars.

If you were to ride in a helicopter far above New York City, you'd
see patterns of red brake lights in the streets below. Those patterns
would appear to move over time. However, the pattern is not due
to a small number of cars which always have their brake lights on,
and which slowly cruise down the street at a constant speed.
Instead, one car hits the brakes, which causes the one behind it
to hit the brakes, which causes the one behind IT to hit the brakes,
and so on. The pattern of red lights is really not tied to any
particular cars.

This is a very big subject. If you really want to understand it,
try reading books or articles on "spiral density waves in galaxies".

Very interesting, and thank you also for the pointers. Googling "spiral density waves in galaxies" provided some interesting starting points for reading:

http://www.astrophysicsspectator.com...piralWave.html
http://www.astrophysicsspectator.com...eOrbitSim.html

The second of these also has a little java orbit simulator to play with which is probably as close to the visualization I was hoping to see without trying to write one myself.

Another fine answer from StupendousMan. I had no idea.

Welcome to BAUT Peeter.

Though watch out Peet, StupendousMan has been known to get cranky with folks. Especially those that persist in believing in thier personal woo even after being shown they were dividing by zero somewhere. It can be funny to watch sometimes.

Another good one is Cougar when he's had enough of some fool. More than once I've picked up my drink and stepped out from between him and someone, you know, just in case. (If you are an Iron Sunner, wear a big hat. He probably won't notice the pointy head and just pass you by. Don't draw attention to yourself and it should all be cool.)

And you just know it's going to be an interesting week when the physics equivalent to a "scratch DJ" shows up claiming to be a musician and his big guitar solo is a cut and paste job of unrelated papers done by people he couldn't tell what Universities they work for that proves...

Damn, you know, a whole menagerie of wierdness just flowed through my head and I wanted to put the weirdest in my example, but after a year or so here my mind just reeled at trying to pick out the weirdest, counter observational thing grown men have tried to pass off in this forum as true. (Without maths, I add in the reread)



Which sounds like a good topic for its own thread.

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Gimme a minute to read through Jay's latest observations...