"The Earth is not gonna run out of heat."<br><br><b>laughs in geological time scale</b>,(reply>), Voldy356 just watch,(reply>), @Boter Bug lots of heat is produced from the decay of uranium in the earth's crust/mantle, enough to compensate for half the lost heat,(reply>), @Berkeloid Perfectly said!,(reply>), If you look it up they've done simulations that show that very few stars are impacted by a galactic collision because as @jetison333 says, the distances are just too large.  The solar systems just move around but there are very, very few collisions.  If you look at the comets that come through our solar system, they all just orbit the sun and leave again without colliding, so the stars would also move around each other in a similar way but at great distance, and not collide.  However as @Edward Philibin says, it doesn't take much gravity to disturb the Oort cloud which is where most of our comets are believed to come from, so while the stars might not collide when galaxies come together, a huge number of new comets are likely to appear from the leftovers at the edge of each solar system, which may well cause devastating impacts on most of the system's planets.,(reply>), David Bergmann Not soon at all we still have billion of years left