According to the Wikipedia article Elliptical galaxy, elliptical galaxies have a sparse interstellar medium. I know that probably sometimes two spiral galaxies collide then exponentially approach the state of being an elliptical galaxy. For a spiral galaxy that will never collide because if it hasn't collided before all the other galaxies went past its cosmic horizon, I wonder whether its interstellar medium will slowly get thinner until it's so thin that stars are no longer confined to the plane of the galaxy by friction and then as soon as a few of them start to go slightly outside the plane, they will bring with them their gravitational field causing stars to go further outside the plane and then after it gets wide enough, the interstellar medium will be even faster to get consumed by stars moving in random directions.
1 Answer
First of all, stars are not confined to the plane of a spiral galaxy's disk by friction with the interstellar medium. Stars are very massive objects and the interstellar medium is extremely thin (comparable to the best laboratory vacuums on Earth), so there is effectively no friction involved. (And if there were, it would cause stars to spiral into the center of the galaxy.) Collisions between gas clouds do tend to keep the gas confined to a thin disk.
In practice, an isolated spiral galaxy will gradually turn into a lenticular (a.k.a. S0) galaxy, not into an elliptical. As it uses up its gas in forming stars, star formation will then gradually cease, and the visible spiral arms will weaken and fade away; you will be left with a smooth disk of stars. (This process can happen more quickly in galaxy clusters, where the high-speed motion of a spiral through the hot, high-pressure gas of the cluster rapidly strips out the gas, leaving behind just the stars.)
Stars in spiral-galaxy disks do tend to increase their vertical motions a bit over time, due to the gravitational effects of minor mergers, spiral arms, and massive molecular clouds (not because of friction, but because the gravity of the molecular cloud shifts the orbits of nearby stars a bit). Ironically, this means that the thinning out of the interstellar medium will probably weaken the tendency of stars to increase their vertical motions, because there won't be any more molecular clouds to perturb the stellar orbits.
Given sufficient time, close encounters between pairs of stars will tend to randomize their orbits, and the lenticular galaxy will eventually evolve into something more like an elliptical galaxy. However, because the average distances between stars are so large, the frequency of close encounters is very low, and the whole process will take trillions of years.
