One thing I didn't understand about the SUN

The new sun that formed was an LED sun, not nuclear. Much cooler and Earth friendly..

Stars can have all kinds of surface temperatures. Granted, to get one that is as bright as Jupiter is in the closing earth shots would likely be pretty hot.

Our solar system would have been turned into a binary star system. It would have rearranged all the planets, moons and asteroids. Normally this would create total havoc and destroy everything. But we must assume the intelligent design behind the operation worked all the celestial mechanics out. But it's very unlikely Europa maintained its close proximity to the new sun.

I am in no way an expert or even a layman on the subject of gravity, space etc. But I have wondered about this part of the movie.

I don't think it's physically possible to turn a planet into the sun. You need hydrogen to act as a fuel because it's the most basic of elements and easiest to fuse together to give off the light and heat needed. Once you have the heavier planet-building elements around, you can't fuse them together, not even the sun can do that. If you squashed Jupiter down in size so it was the same density as the sun I can only assume it would collapse under its own gravity into a black hole.