Shortly after hydrostatic equilibrium is achieved the star must ignite hydrogen fusion within its core in order to maintain the balance and resist gravity's grasp. If it does not have the mass to do this it will continue to collapse under gravity and cool as it does. Such failed stars are called Brown Dwarfs (3a). Stars like our Sun live for 10 billion years on the main sequence. Stars with higher masses live shorter lives, and stars with less mass live longer lives. The main sequence is where all stars spend the vast majority of their active existences.