ok the question is a spherical star expands to 6 times its volume but its mass remains constant and is uniformly distributed - how does the period of rotation change? obviously it rotates slower and thus the period goes up, but i dont know how to solve it mathematically. can someone give me some pointers and get me going in the right direction? is there a main equation i should be using and do i need to find the ratio of the radii before and after the star expands? thanks.