The First Stars

As the universe expanded, plasma grew cooler and cooler. The universe cooled down further to 3000 K, making it possible for the nuclei in the plasma to gain electrons and become full-fledged neutral Hydrogen and Helium atoms. 

Photons of light tangled in the plasma could now move freely and were released. The universe became transparent for the first time. This is known as cosmic microwave background that we can see even today.

Due to gravity, hydrogen and helium clouds began to clump together. Tiny imperfections. Little knots, wrinkles, and flaws began attracting nearby particles of matter. The clumps grew, became more massive, and attracted more particles. Hence, it made them denser, and as a result, gravity became more intense. As the clouds compress more and more, they became hotter, until the center became so hot that nuclear fusion began. This released a tremendous amount of energy, which caused the earliest stars to light up. The gravity that was trying to squash the star together was pushed back by the fusion at the center which created a balance. Within a few hundred million years after the Big Bang, the first generation of stars, known as Population III stars, were formed. After CMB, these stars were the first source of visible light in the universe.