"What does it mean when astronomers say we're all made of star dust?"

- S. Paul, NB, Canada

Our bodies are complex amalgams of cells, which are, at base, composed of molecules that, themselves, consist of various elements. These elements, such as oxygen, carbon, and iron, did not exist in the Universe's infancy. The matter in the early cosmos was primarily, though not exclusively, hydrogen and helium. As any biochemist would tell you, one cannot build life-sustaining molecules with just the first two elements. Many other heavy elements are necessary. The stars were responsible for creating heavier elements in their cores and some of them disperse these  elements into space. This dissemination of heavy elements enriches the galaxy and precipitates the formation of other solar systems.

When stars first form, their core pressures produce temperatures high enough to initiate hydrogen fusion reactions. The hydrogen nuclei fuses to produce helium. These reactions also transmute some of the initial matter into energy, which powers the star. This process is currently occurring in our Sun's core. When a star exhausts its core hydrogen reserves, the helium "ash," is compressed and also receives heat from a hydrogen-fusing shell that forms around it. When sufficiently heated, the helium core will fuse to create carbon. The Sun will reach this fusion reaction stage in about five billion years. For stars with masses comparable to the Sun's, no other fusion reactions are possible. Overcoming the powerful electrostatic repulsion carbon nuclei experience requires tremendous heat energy and pressures that only the high mass stars can generate. These more massive stars can continue the fusion process to create elements heavier than carbon, such as oxygen, silicon and, ultimately, iron.      

Iron represents the endpoint because iron fusion is an endothermic process, meaning that the star has to invest more energy to fuse iron into heavier elements than the fusion process would impart back into the star. The balance between the expansive energy pressure and compressive gravity, called 'hydrostatic equilibrium,' is violently disrupted. The outer layers collapse onto the core at nearly half the speed of light. The star then explodes outward as a supernova. This supernova explosion creates all the elements heavier than iron. It also disperses this heavy element cloud through space, where it can mix with nebulae that will often then contract to form new solar systems.  

Our solar system formed in this manner: the introduction of supernova particles enriched a gas/dust cloud (nebula) that collapsed to create the Sun and its retinue of attendant worlds, including Earth. Over billions of years, the matter within Earth coalesced into life forms of ever increasing complexity. Every life form, no matter how complicated, is fashioned of the heavy elements a supernova conveyed into a dark nebula billions of years ago.

The particles in our bodies have been in existence for billions of years. They will persist for billions of years after we move on. And, these particles were all forged in the nuclear fusion fires of stars. We truly are made of star dust.