Making Elements

Every time you take a breath, you’re filling your lungs with elements created in the heart of a star. If you take an iron supplement, you’re ingesting an element forged in the heart of a massive star. And if you put on a silver necklace, you’re wearing an element created in the death of a massive star.
In fact, almost all the elements were created by stars. The main exceptions are the lightest and simplest elements — hydrogen, helium, and a smidgen of lithium.
Hydrogen came first — or at least hydrogen nuclei: single protons created in the first fraction of a second after the Big Bang. After a few minutes, the universe had cooled enough that two protons and two neutrons could stick together to form the nuclei of helium.
After a few hundred thousand years more, the universe cooled enough to allow electrons to stick to the bare nuclei, turning them into complete atoms.
Over time, the hydrogen and helium started clumping together to make stars. The cores of these stars were hot enough to