I'm Star Stuff

Yesterday, (well, logical yesterday) I said where I came from, the center of the universe.

Today I'll tell you what I am: I'm star stuff.

It's actually a pretty cool thing to meditate on. I can sit here, close my eyes, put my attention out in all directions, back in time, to the start of time, and know that where I am is where it all started; the center of the earth. The location of the Big Bang.

And I can think about the history of the universe as we know it that led to my arriving at this precise spot at this precise time. Think about how the atoms that make up my body, apart from the Hydrogen ones, were made in stars; my body is the built from the corpses of many, many stars.

Here's how it all came about.

If God said "Let there be light!" the universe didn't hop right to it. According to our best models, It took a full ten seconds for things after the Big Bang for the universe to cool enough for photons to appear. That was the start of the Photon Epoch. Even though there were lots of photons there wasn't what you'd call light. For one thing the photons kept bumping into stuff. It was about 380,000 years before the universe started to become transparent enough for photons to travel any distance.

But even though there were photons, the universe was still a pretty dark place. There were no concentrated sources of photons beaming out. No stars. Near perfect blackness. What you see between the stars today, only more so.

Stars didn't appear until around 400 million years after the Big Bang. Finally there was light. But those early stars were very different from most of the stars we can see today. Scientists divide stars into three populations: ones like our sun are Population 1 stars. They are relatively young and fairly hot. In other parts of the universe are Population 2 stars. They are generally much older and almost always much cooler. The very oldest stars are population 3 stars. They are the ancestors of all population 1 and 2 stars. The difference between Population 1, 2 and 3 stars is explained by metalicity--the proportion of matter other than Hydrogen and Helium in s star.

When the first stars were formed, the universe was made almost entirely out of hydrogen (92% by number of atoms, 75% by mass) and Helium (8% by number of atoms, 25% by mass) along with really tiny trace amounts of very light nuclei, up through lithium. There was no oxygen, so no water; no carbon, so no carbohydrates; no nitrogen, so not proteins; no calcium, nitrogen, sodium, so no possibility of me.

To the extent that I could be said to exist at all, I was just gas. Eventually some of the hydrogen and helium in the universe came together--dark matter is implicated in the process--and after that happened gravitational forces pulled more and more Hydrogen and Helium into the growing mass until, at some point--there's debate on exactly when and how--the object became dense enough and hot enough to trigger a nuclear reaction, like the one that lights our sun and the other stars that we see.

Those nuclear reactions produce all the heavier elements. Some, are produced the normal course of a star's lifetime, but most are produced when stars explode, go nova. Eventually some of the Hydrogen, Helium, and--importantly--the other elements form new stars, and some of them go through the same process, producing even more of the heavy elements.

The first stars, called Population 3 stars had no heavy elements--because there were none. The second kind of stars have few heavy elements. They are mostly older stars and formed before metals were abundant. As more and more stars produced more and more heavy elements the composition of the universe changed.

So I'm the result of exploding Population 3 and Population 2 and maybe some Population 1 stars.

Cool!