Nucleosynthesis

Big Bang nucleosynthesis

About three minutes after the Big Bang, the universe had cooled enough for protons and neutrons to fuse into the first atomic nuclei. In a window of only about twenty minutes, this produced almost exclusively hydrogen and helium — plus trace amounts of lithium. After that, the universe had already expanded and cooled so much that nuclear fusion ground to a halt.

Why didn't it continue? There are no stable atomic nuclei with mass number 5 or 8 — every attempt to fuse ⁴He with an additional proton or neutron, or with a second ⁴He, immediately decays back apart. Without these intermediate steps, and without the vastly higher densities and time spans found inside stars, the early universe had no path open to heavier elements.

Fusion inside stars

Everything heavier than lithium first forms in stars. In a star's core, under enormous pressure and at temperatures of millions of kelvin, four hydrogen nuclei first fuse into a single helium nucleus — the process that has made the Sun shine for 4.6 billion years. Once the core's hydrogen supply is exhausted, the star contracts, heats up further, and ignites the next stage of fusion.

In the so-called triple-alpha process, three helium-4 nuclei fuse into carbon-12 — a remarkably improbable intermediate step that only works because carbon happens to have a matching excited energy state. Massive stars then ignite further fusion stages up to silicon and finally into the region of Iron-56 and Nickel-62 — there the binding-energy curve reaches its maximum (exactly at Nickel-62, only fractionally ahead of the far more abundant Iron-56) and fusion no longer releases energy.

With each fusion stage, the required temperature rises and the burn time shrinks dramatically — in a massive star, the final stage before the iron core often lasts only hours.

Beyond iron: neutron capture

Since fusion beyond iron would cost energy instead of releasing it, all heavier elements — from cobalt to uranium — form by a different route: capturing free neutrons, which don't need to overcome any electrostatic repulsion. After each capture, an unstable nucleus can undergo β⁻ decay, turning a neutron into a proton and moving up to the next element. Depending on how fast the neutrons hit, two fundamentally different processes are distinguished.

The cosmic origin of the elements

Putting all these formation pathways together, it's possible to trace the cosmic birthplace of practically every element in the periodic table — the result of decades of astrophysics, which can be shown vividly as a map of origins.