Early Universe Stars. Or, How Did We Get Here?

Sometime after the Big Bang, the universe furiously began expanding and there was a period when everything was dense, too dense for even light to travel very far. For a while, near the beginning, it was hot and dense enough everywhere for hydrogen (which was created by pair production, or high energy light photons interacting with each other and popping out protons! Mass made from light!) to undergo fusion reactions and make some helium. This only happened until the universe expanded and cooled off, and it became too cool for these reactions after about 30 minutes. Thirty minutes after the beginning of the existence of our universe, where we exist. After about 3-500,000 more years of expansion things cooled enough for the protons and the helium nuclei to capture free electrons. This opened things up a bit, and the previously trapped light flashed free. The entire universe radiated with a characteristic temperature of about 3000 K (K = temperature scale in absolute units = Kelvin). Unbelievably we can actually see this still today! We call it the Cosmic Microwave Background radiation, or CMB for short. Because the universe has continued to expand since then, it's characteristic temperature today is now only 3 K. And then there were the Dark Ages. Not much going on for some time, but then after roughly 100 million years something did happen. Gravity began to clump together the hydrogen and helium into clumps. And thanks to some cooling mechanisms associated with molecular hydrogen (H2 instead of just H) some of these clumps condensed under the influence of gravity and when the center of clumps became hot enough, and dense enough, hydrogen fusion started in the core of these 'clumps.' In other words, stars starting lighting up the universe. It's likely that this is the cause for much of the remaining hydrogen losing their electrons again. Stars were being born, everywhere. These were not ordinary stars, like our Sun, but were likely very massive, maybe very very massive. Today, we know stars do not generally form with a mass bigger than about 100 times the Sun's mass. In fact most stars in the universe today are around the Sun's mass or less. Not so back then! The universe now has an age of about 13.7 billion years. The first stars were flashing into existence around 0.1 - 0.2 billion years after the Big Bang. And, because they were so massive, they only lasted for millions of years, before exploding! The Sun will not explode, but become unstable in a different way, however this won't happen for another 5 billion years! And we estimate that it's already been here, along with Earth, for 5 billion more years!

What's really worth pondering is that at the time that the early universe stars turned on, there was virtually nothing else in the universe besides hydrogen and helium. While the first stars lived their brief lives, they created energy in their cores by fusing elements into heavier and heavier elements until their cores filled up with Iron, which can not fuse. This is the end of the line for massive stars. Gravity will squeeze in on the core crushing it into oblivion, likely creating the first black holes. The outgoing shock wave heats up the surrounding expelled material and lights it up. It also cooks up many more heavy elements, heavier than iron. There is not any place else in the universe where these heavier elements can be made! Carbon through Iron are made in the core of massive stars, and anything heavier was and is made in the explosion of the stars!! Where else do we find carbon and iron today? Inside humans -- in our bones and in our blood! All made available courtesy of some long ago exploded star.  At some point the remains of that exploded star coalesced into our Sun, and around that a disk of debris, which clumped to form planets, and on one of those small small clumps fell every atom and molecule that eventually, 5 billion years later, would come together in the right way to form YOU.

The project I'm currently interested in is how these first stars looked, changed, and ultimately how they blew up. How long did it take before stars like the Sun could be made? A star with planets around it, with complex chemistries, maybe with water, with us.