Every breath of oxygen you take was forged inside the core of a dying star.

Immediately after the Big Bang approximately 13.8 billion years ago, the universe consisted almost entirely of hydrogen and helium. Heavier elements like oxygen, carbon, and iron did not yet exist. These elements were only formed much later through a process called stellar nucleosynthesis within the high-pressure cores of massive stars.Inside these stars, temperatures reach over 100 million degrees Celsius. This extreme heat allows helium nuclei to fuse together to form carbon, which then fuses with more helium to create oxygen-16. This process continues until the star exhausts its fuel and collapses under its own gravity.The life of a star at least eight times more massive than our Sun ends in a spectacular supernova explosion. This event is so powerful that it ejects the star's enriched outer layers into the interstellar medium at speeds of thousands of kilometers per second. NASA's Chandra X-ray Observatory has captured images of these debris clouds, such as the Cas A supernova remnant, which contains enough oxygen to support thousands of solar systems.Over billions of years, this cosmic dust coalesced to form new star systems and planets like Earth. Roughly 21 percent of Earth's atmosphere is now composed of this recycled stardust. Every molecule of oxygen in your lungs today is a direct relic of a stellar explosion that occurred long before our solar system was born.