Transcript: Stars are stable. For most of their lives, fusion provides the energy source. Even though the Sun and other stars are fusing hydrogen into helium, it does not mean that they are bombs. The Sun will be stable for billions of years. Stars also do not cool off. Energy flows continuously from the core where fusion occurs to the outer cooler regions. At every point within a stable star there’s an energy balance between two forces: the inward force of gravity and the outward pressure caused by energy release from nuclear reactions. This balance is called hydrostatic equilibrium.