1. The problem statement, all variables and given/known data Dear all, This is my first post and I need some help. The exercise I am trying to solve is this one: A star has a radius of 6 × 10^8 m and a period of rotation of 30 days. Eventually it becomes a neutron star with a radius of 10^4 m and a period of 0.1 s. If the mass has not changed, find the ratio of initial and final (a) angular momentum and (b) kinetic energy. 2. Relevant equations I know that I must use for angular momentum L = Iω and for kinetic energy K = 1/2 Iω^2, where I is the moment of inertia. I assumed the body geometry as a sphere. 3. The attempt at a solution I made all the substitutions and in fact I got the right answers (Lini/Lfin = 139 and Kini/Kfin=5.4x10^-6). My question: in this exercise, why the angular momentum is not conserved? Can anybody provide some physical explanation?