Question

What will happen if the galaxy will stop moving?

Answer 1

• All planets fall into their stars. They are destroyed even before they would be incinerated as they pass the Roche limit and are torn apart by the tidal forces of the star. Without the aid of the Earth's velocity, no spaceship would be able to reach heliocentric orbit, meaning humanity is doomed as well.
• Most stars survive (initially). Centrifugal force is negligible compared to the pressures holding up the star, so the fact that rotation is stopped doesn't have a noticeable effect. The impact of one of the gas giants is noticeable: Neptune would strike with a speed of 600 km/s (0.2% the speed of light). However, the huge mass of the Sun absorbs the blow easily and any material blasted off into space will eventually fall back into the Sun. However...
• Binary stars may not survive. It depends on the specific situation: two similar stars will probably just combine into a more massive star. However, star systems with a compact object like a white dwarf or neutron star will probably supernova or collapse into a black hole due to the added mass. Speaking of black holes...
• Many stars are swallowed by a black hole. Specifically, the black holes residing in the centers of galaxies. It takes the Sun about 10 million years to fall to the center of the galaxy, and by then most of the stars in the galactic core have been consumed. With billions of stars, the gravitational interactions are complex enough that some stars are deflected and go into highly elliptical orbits around the black hole. There are probably no stellar collisions, since stars are so small.
• The evolution of the universe continues almost as normal. There are now many fewer stars in each galaxy, but there is no real change to larger-scale structures.