Milky way and andromeda galaxy collision #facts #shorts

Milky way is

Milky way is

The Sun, which is 2.2 xx 10^20 m from the center of the Milky Way galaxy, revolves around that center once every 2.5 xx 10^8 years. Assuming each star in the Galaxy has a mass equal to the Sun's mass of 2.0 xx 10^30 kg, the stars are distributed uniformly in a sphere about the galactic center, and the Sun is at the edge of that sphere, estimate the number of stars in the Galaxy.

Anwer carefully, with reasons: a) In an elastic collision of two billiard balls, is the total kinetic energy conserved during the short time of collision of the balls (i.e. when they are in contact)? Is the total linear momentum conserved during the short time of an elastic collision of two balls? c) What are the answers to a) and b) for an inelastic collision? d) If the potenital energy of two billiard balls depends only on the separation distance between their centers, is the collision elastic or inelastic? (note we are talking here of potential energy corresponding to the force during collision, not gravitational potential energy).