The heat and light radiated by stars is the result of fusion, the process in which energy is released when hydrogen nuclei (protons) smash into each other to form helium. Fusion is not easy to achieve since the protons' like charges means that there is a huge electric repulsion between them when they are at the small distances (on the order of 1 fm) required to form a nucleus. To see why fusion only occurs at very high temperatures (like at the center of stars), consider this basic problem: Two protons are initially very far apart from each. What minimum speed must one of the protons be launched in order for it to overcome the electric repulsion and to end up within 1 fm of the other? (For simplicity, assume the other proton is being held in place.)