So, a year and some change ago I did an absurdly wordy post that may have had slightly too many run-on sentances about the early history of the universe (If you didn't read it, here's A Brief History of the Universe, Part I). I then promised to follow it up with more history up until present day or so, and never did. Until now. And while you may think it took me a long time to follow up with a part II to that post, that time span is just peanuts to the universe. In fact, it's been around for about thirteen billion times longer than the wait between the first part of this series and this follow-up. So, cosmologically speaking, the delay was perfectly acceptable. Now, let's get on with it.

When I left off last time, we were around 300,000 years into the universe's lifetime, and "recombination" had just happened. The universe had become transparent to light, and the light scattering from that time is still visible today. But I had mentioned that it will takes hundreds of millions of years for star to first begin appearing. In fact, the time between recombination, when the universe became transparent to light, and somewhere between 150 and 200 million years, when the first stars began to shine through the universe, is called the dark ages. During this time, matter started really condensing, and gravity began to become a major player in the universe, pulling clumps of lighter elements together to eventually form giant balls of hydrogen and other light elements that ignited into the first stars, called Population III Stars.

Population III Stars are stars that lack almost any heavier elements. These stars burned bright and fast, and none are around today. Instead, the ejecta from their supernovae created heavier elements and metals, which went on to allow the formation of Population II Stars. We know these stars started appearing around 200 million years after the Big Bang, as the oldest star we know of, SMSS J031300.36-670839.3 is about that old, and it seems to be a Population II Star. These stars have more heavy metals than Population III Stars, but still not that many. The best kind of stars for us are the Population I Stars (which are longer-lived and more moderate), but those are still a ways off. For now, the next big events are the formation of other huge astronomical objects that we can observe, such as quasars (which are nuts, and I'll have to talk more about them some other time), at 300 or 400 million years in. For the really big stuff, like galaxies, we're still going to be waiting a while longer.

At sometime around 600 million years or so, galaxies to begin to form (estimates for when our own galaxy, the Milky Way, started to form put it around there), and we start seeing some so-called dwarf galaxies. However, it's not until about a billion years after the Big Bang that we find the oldest "normal"-looking galaxy, HCM-6A. Around now, we enter the galaxy epoch (the dark ages are long behind us).

A lot of stuff happens in the galaxy epoch. Many stars we see today form, we get comets, other nebulae, Andromeda, our own galaxy, the works. But really, the next interesting thing we're going to care about is when our very own sun starts shining. For that, we have to speed ahead, through eight billion years of history. The most significant thing we missed along the way, at seven or eight billion years after the Big Bang, was the beginning of the domination of dark energy. From that point onward, the acceleration of the universe becomes a major thing, and we're still speeding up more and more to this day as things grow further apart and dark energy becomes more of an influence (remember that, it will come up in the future).

But the sun! At 9.23 billion years after the Big Bang, a bit over 9 billion years after the very first stars started shining, our very own star, named Sol begins its life as a Population I Star (the first Population I Stars forming was another thing we skipped over, they were about 3 billion years in, and were still very metal-poor, at about 10% of the heavier elements found in our sun—side note: when astronomers say "metal," they mean any element that isn't Hydrogen or Helium. I'm trying to stick with a more understandable "heavy elements," but I do slip up here and there, and that's what I mean when I say that). Over the next 20 million years, the remnants of the supernova that formed the sun which weren't used in its creation coalesce into the planets (and dwarf planets) we see orbiting it today. About 15 million years later, the Earth is hit by a (hypothetical) planetoid named Thea, and from that cataclysm the moon is formed. Finally, about a billion years later, when the Earth has cooled enough, we start to see the first life. And at that point we're no longer talking physics, but biology and chemistry (which I am significantly less qualified to speak about). At some point, we evolved, and 3-and-some-odd billion years after the first life appeared on Earth, you're sitting here reading this (unless you're reading this really far in the future, in which case I'm astounded that my physics blog survived. What's the internet like now? Oh, wait, you can't tell me, because you're in the future). It's been a very long road getting here, and there's an even longer road ahead of us. Join me next week (for real this time) as we near Halloween and I get spookier and talk about how the universe will end.