The Sun is a star, a dazzling, luminous ball of gas more than 100 times bigger in diameter than the Earth. Although to the naked eye it gleams peacefully, a telescope reveals the Sun's surface to be violently agitated, with rising fountains of incandescent gas and a twisted magnetic field. Even greater violence wracks its core. There, a nuclear furnace every second burns 600 million tons of hydrogen into helium, producing in one heartbeat the energy of 100 billion nuclear bombs. How that energy is released and how the Sun manages to hold itself together is the main theme of this chapter. We will begin by describing the Sun: its radius, mass, and so on. Then we will discuss how the crushing force of its gravity balances the explosive power in its core. Finally, we will see how energy escaping from the core stirs its atmosphere into the inferno we see.