WATCH: Threshold 2 — Stars Light Up

NARRATOR: Right after the Big Bang, our young Universe was what scientists call a plasma. This was basically an incredibly hot mush of charged particles without much structural complexity. About 380,000 years later, things began to change. By then, temperatures had fallen low enough for protons, which have positive charges, to link up with electrons, which have negative charges. And together, they formed electrically neutral atoms, very simple ones like hydrogen, some helium, and a few slightly heavier atoms thrown in for good measure. The Universe now contained vast clouds of these atoms. Add gravity and now you have the ingredients for our second threshold, the formation of stars. Here's what happened next. Wherever there was slightly more matter, gravity is more powerful. So, tiny variations in the density of matter became the first Goldilocks condition for this second threshold. Gravity packed slightly denser regions ever closer together, squashing them so tightly that they began to heat up. This growing pressure and heat created our second Goldilocks condition. Eventually, the clouds got so hot, the protons and electrons split apart once more, recreating a plasma. And when temperatures in these hot spots got to about 10 million degrees Celsius, protons began to fuse together and part of them turned into energy as they did so. This huge release of heat from the center of each cloud of matter stopped the cloud from collapsing any further. And this is how the first stars lit up. Soon, the Universe had billions of hot spots pouring energy into the cold of deep space. Each star would continue releasing energy into space for millions or even billions of years until it had no more protons to fuse. And as these stars formed, so did galaxies, each containing billions of stars. Galaxies in turn grouped together into huge clusters and chains of galaxies, the largest structures in the Universe. Suddenly, the Universe seemed to have a lot more variety and a lot more structure. Now, what new things could happen in a Universe filled with stars?