By 200 million years after the Big Bang, the Universe had become a very dark and cold place. Then things started to change. First, galaxies and nebulae formed. These were the earliest structures in the Universe. Then stars – “hot spots” of light and energy – emerged from these clouds of dust and gas. Why did they form and how did they change everything? Stars, the first complex, stable entities in the Universe, have the capacity to generate energy for millions, even billions of years. The first stars, which passed through their entire life cycles relatively quickly, produced many of the chemical elements of the periodic table. In this unit, you’ll learn how stars first formed and how the lives and deaths of stars provided the chemical diversity necessary for even more complex things.