I was just thinking earlier about how a spiral galaxy rotates, and I was wondering how does it stay in a spiral shape? It should be that the farther away from the center it is, the longer it takes to orbit (like in the solar system) So it should be uneven and turn into an elliptical galxy. Any ideas?

Based on our current understanding of physics and particles, there are a couple of reasons why a spiral galaxy holds its shade.

First, and probably most importantly, the visible matter in any galaxy makes up only about 10% of the matter within the gravitaional influence of the galaxy. The remaining 90% is made up of dark matter and/or dark energy.

Secondly, it appears that the spiral arms of spiral galaxies are influenced by gravity waves. The compression of the waves is what causes the galaxy's arms to hold their shape.

__________________
All civilizations become either spacefaring or extinct.~ Carl Sagan ~

Humanity must rise above the Earth, to the top of the atmosphere and beyond, for only then will we fully understand the world in which we live.~Socrates, 500 B.C. ~

Let every man judge according to his own standards, by what he has himself read, not by what others tell him. ~Albert Einstein~

Or third, imagine that the gas in the atmosphere (perhaps Venus), because there is no H2O to dissolve the gas which means that they have to remain there forever or it takes some times to evaporate... and besides, spiral galaxies are not perfectly spiral shape....

What Duane said.

The arms of a spiral galaxy don't orbit around the galaxy intact. It's the density waves that we see, and only because that's where the brightest stars live.

Matter on different orbits winds up temorarily creating these denser areas - there was a recent Cassini photo of Saturn's rings showing tight spiral density waves in them, caused by the gravity of one of Saturn's moons - same principle. Except in a galaxy, (if it contains any gas) when an area gets higher density, it becomes a star forming region. And the biggest and brightest stars don't live long, so they never get far from the star forming regions. The stars we can see the farthest are the big, bright, young ones that are found preferrentially near the star forming regions that are found in the denser areas that happen to have a spiral shape.

Rather than like planets orbiting the sun, spiral arms are more like the spiral spokes of one of those pinwheel fireworks. If you took a photo of one it would look like a spiral, and it stays in that form, even though not one single spark in it follows a spiral path - they all move almost straight out from the center.

If the Saturn Ring analogy holds, then the spiral density waves might be caused by a very massive object orbiting our galaxy just outside the rim of the galaxy? Or maybe within the galatic plane itself? Not likely or we would have observed it. Just a thought.

Ok, thanks for helping to correct my misunderstanding.