Life on Earth could not exist without carbon. But carbon itself could not exist without stars. Nearly all elements except hydrogen and helium—including carbon, oxygen and iron—only exist because they were forged in stellar furnaces and later flung into the cosmos when their stars died. In an ultimate act of galactic recycling, planets like ours are formed by incorporating these star-built atoms into their makeup, be it the iron in Earth's core, the oxygen in its atmosphere or the carbon in the bodies of Earthlings.