How did the first stars formed

Yesterday I was preparing for my Saturday meeting with my ex supervisor and I found the article about first stars numerical Simulation. So I can simply explain what was going on many many years ago.....(200-400 millions years after Big D... Bang)

When the Universe first began to evolve, almost everything in it was hydrogen and helium. These were the primordial elements, and there was so much of them that, in the end, the first stars would form from them.

But how exactly did stars begin to appear, and why is it so important?

At the very beginning of the Universe, there was just cold and sparse gas. As it gradually compressed under the influence of gravity, its density increased and its temperature began to rise. However, because the gas lacked heavy elements (like carbon or oxygen), it was hard to cool down. Without cooling, it couldn’t compress enough to form stars. (If a star is too hot, molecules just scatter because of the strong kinetic energy.)

For the gas to cool, there needed to be hydrogen molecules in it. They acted as cooling agents, helping to dissipate excess heat, which allowed the gas to keep compressing.

As a result of the gas compressing, the first stars began to form and they were very massive. These stars could be tens, even hundreds, of times bigger than our Sun (our star formed a generation later). Why? Because in the early Universe, the gas was much denser, and therefore the stars that formed were much larger.

And when these first stars ran out of fuel, they exploded as supernovae, spreading heavy elements throughout the Universe.

You might ask, where did these heavy elements come from?
They were the result of chemical reactions during the star's life. Heavier elements formed from hydrogen and helium. These elements became the building blocks for subsequent stars and planets. And for us too: we are the children of stars.