A star is an extremely hot ball of gas, with hydrogen fusing into helium at its core. Stars spend most of their lives fusing hydrogen, and when the hydrogen fuel runs out, stars fuse helium into carbon. The more massive stars can fuse carbon into some heavier elements, which is where a lot of the heavy elements in the universe comes from. Throughout this whole process is a fight between gravity and gas pressure, known as equilibrium. It’s very important to keep this battle in your mind as you try to understand how stars live and die. Stars live out a lot of their lives in a phase termed as the Main Sequence. Once achieving nuclear fusion, stars radiate (shine) energy into space. The star gradually contracts over billions of years to compensate for the heat and light energy lost. As this slow contraction continues, the star’s temperature, density, and pressure at the core continue to increase. The temperature at the core of the star slowly rises over time because the star radiates away energy, but it is also slowly contracting. This battle between gravity pulling in and gas pressure pushing out will go on over the entire life span of the star.