A question on galaxies.

[Phasmid]

All the planets in our solar system were formed as a result of matter attracting other matter until big clumps of it stuck together... or something. So I was wondering why don't galaxies do the same? Over time could it be possible that the Milky Way could become an enormous sun?

[Ophiolite]

This is an interesting question. Planetary systems are thought to arise because of instabilities in large gas and dust clouds. Once the initial period of planetary formation is over - and this happens quickly, almost instantaneously in astronomical terms - the bulk of the material in a system is in pretty stable orbits. So, for example, Jupiter is not going to fall into the sun.
This is because of something called the conservation of angular momentum. This has to remain constant in any system. The only way Jupiter could fall into the sun would be by transfering angular momentum to other bodies. In a stable system like ours this just wont happen.
The situation in galaxies is similar. They are also pretty stable; angular momentum has to be preserved; there is littlle chance of many stars being pulled into the the centre of the galaxy.
That said, a popular view today, is that at the centre of many galxies (perhaps all the medium to large ones) there is a black hole with the mass of 10,000s of stars. However this black hole did not form after the galaxy, but it thought to be instrumental in creating it in the first place.

[Fraggle Rocker]

Indeed. If all the mass in one galaxy coalesced in one place, there would be way too much to form a stable star. It would be a black hole.