Galactic Collision

First you have to consider what causes the spiral arms. Despite their visual appearance, they do not contain significantly more material than the "gaps" between them. They just contain a higher percentage of bright hot stars.

They are caused by density waves passing through the galaxy, which induce star formation. The larger the star formed, the brighter it will be, but also the shorter its lifespan. Spiral arms are where you are going to find young bright stars. But smaller dimmer stars are also formed, and in greater numbers than the larger ones. They also live longer.

By the time a density wave leaves a given part of the galaxy, all the large bright stars have expended themselves, but there are still lots of smaller dimmer stars left. These are the stars that now predominate the regions between the spiral arms. The "gaps" look darker for the same reason that sun spots look black compared to the surrounding Sun surface. Sunspots can be 4500 C in temp which, if viewed by itself, would be on the Cool White region of the color temperature scale.

A collision with Andromeda will likely produce some density waves of it own, creating new regions of star formation for a time.

As to what the final result may be, it could end up as an elliptical galaxy, which will be featureless (no disk shape or spiral arms).

 

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I think a bit sooner. Four billion years. Regardless, I doubt any of our descendants will be around to see it.

 

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The question Hayden is asking here is not what happens with star-galaxy collision, but if there have been any observations of solar systems colliding within our own galaxy. Stars can collide but this typically happens when they are already gravitationally bound. Gravitational waves observed last year are suspected to be the result of a collision between two neutron stars in another galaxy. And V1309 Scorpii is a directly observed contact binary within our own galaxy.

We can rest easy though. It has been estimated that the rate of stellar collision with old Sol is once in 10^28 years.

 

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Oops! I guess this is a classic case of RTFQ.

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