At the photosphere, we can observe the properties, elements, and spectral features present at the outermost layers of the Sun. The very first stars may not have had the same elements our Sun did, as they only had the Big Bang to create their building blocks, rather than also having previous generations of stars. (NASA’S SOLAR DYNAMICS OBSERVATORY / GSFC)

What Was It Like When The Universe Made Its First Elements?

Before there were humans, planets, or even stars and galaxies, we had to make the first elements. Here’s how they happened.

From the first moments of Big Bang to the present day, the cosmic story of how our Universe evolved to become filled with stars, galaxies, and all that we can see and detect is a tale that unites us all. Although we began in an incredibly hot and dense state, the Universe expanded. That expansion spreads everything in the Universe out, reduces its energy and temperature, and compels particles to interact, decay, and freeze out.

By time the Universe is 3 seconds old, there are no more free quarks; there is no more antimatter; neutrinos no longer collide with or interact with any of the remaining particles. We have more matter than antimatter, more than a billion photons for every proton or neutron, and the Universe is a little under 10 billion K in temperature. But it cannot yet make elements. Here’s how that step happens.

In a Universe loaded with neutrons and protons, it seems like building elements would be a cinch. All you have to do is start off with that first step: building deuterium, and the rest will follow from there. But making deuterium is easy; not destroying it is particularly hard. (E. SIEGEL / BEYOND THE GALAXY)

A whole slew of things happened in the first 3 seconds of the Universe, but one of the last things to…