How Did We Get Here? – by Julian

How did us carbon based life forms get here? Let’s start with a checklist of things we might need. Carbon, oxygen, hydrogen, nitrogen, a handful of other useful elements. What we actually need is:

Oxygen (65%) – mostly in the form of water

Carbon (18%) – in lots of things, proteins, food, CO2

Hydrogen (10%) – again in water and proteins and, well everywhere.

Nitrogen (3%) – A vital part of amino acids, which make up proteins

Calcium (1.5%) – Bones and signalling in cells

Phosphorus (1.0%) – Nucleic acids, ATP and some other things

Potassium (0.35%) – used in nerves and inside cells for keeping
electrical potential

Sulfur (0.25%) – a popular bonding element in proteins

Sodium (0.15%) – good for nerves and osmotic control

Magnesium (0.05%) – used in various guises to help form nucleic acids,
proteins and ATP.

Tiny bits of Copper, Zinc, Selenium, Molybdenum, Fluorine, Chlorine, Iodine,
Manganese, Cobalt, Iron (0.70%)
Traces of Lithium, Strontium, Aluminum, Silicon, Lead, Vanadium,
Arsenic, Bromine

So – one fine non-day, a minute particle of something exploded. But this was no ordinary explosion. It wasn’t like putting a firecracker in a pot of M&Ms, lighting the touch paper and retiring with your mouth open hoping to catch one. Oh no, this explosion did not happen in space–it was the actual space that exploded. Let’s just consider that again: it wasn’t something exploding out into space, because there was no space to explode into. Come to that, there wasn’t any time for it to happen either at that point. This explosion was so special it created space and time together.


Now, when all this set off it was incredibly hot, and very weird things happened in the first few nanoseconds.  I’m going to skate over the complicated bits that happened up to the first second–but basically that’s where *stuff* gets made. For some reason *stuff* isn’t made in exactly equal quantities like you might expect, and what’s left over is the building blocks of what we’ll be calling matter a few hundred years down the line. All this is happening at somewhere around 10,000,000,000 degrees (C, K or F – it really doesn’t matter). Now at this point there is nothing we would recognize around, except perhaps light in the form of photons. It is far too hot for anything else but elementary particles to hang together. The light is very energetic though, and you would need more than factor-50 to survive it. The universe is also opaque. All those photons which we use everyday to see things are continually running into electrons and protons and getting absorbed, and then reradiated. They can barely leave one particle before running into another. This radiation is what will give us the cosmic background radiation by the time space has stretched out far enough that its lengthened to microwave wavelengths.

Now–at about 3 minutes, the universe has grown big enough that things start to cool down, and finally when protons happen to bang together they are moving slowly enough that they can sometimes stick – this is called fusion, and still happens today in the Sun, stars and H-bombs. This continues for a full 17 minutes, and then the universe has cooled down to such an extent (because of its continual expansion) that now it is not hot enough for those protons to bang into another fast enough to overcome their electrical repulsion, and fusion ends.

What is the result of this? Well, the vast majority of all protons, 75%, have not managed to bang into anything hard enough, and they just hang around on their own. They will become hydrogen one day. Most of the other nuclei that have formed are helium, nearly 25%, and a tiny bit of element 3, lithium. So–not much carbon or oxygen then …

After this, it all gets a bit boring. Not much happens of significance for the next 300,000 years. There is still plenty of light around, but its still getting absorbed by all the free electrons hanging around, which are still too hot to get themselves attached to nuclei and form atoms. Then, it finally happens. The universe has cooled down enough that electrons are moving slowly enough to stick to nuclei and stay there, and we get hydrogen and helium – and suddenly there is the sort of light we might recognize. Light can now make it long distances and the universe has cleared and we can see what’s going on! If there was a first day, this is probably a good candidate, except for the fact it’s also the first night you can see the night sky (although it will probably be pretty empty). We also have our first required building block, hydrogen. Helium isn’t much use for anything chemically, and isn’t part of your body, except for the times you inhale it from party balloons to make your voice go squeaky.

Some other things start to happen, too. As things continue to cool, these clouds of particles start to attract each other because of gravity–until now they’ve been too hot to get together. In actual fact the contribution of matter is probably pretty small in this, as the more exotic dark matter is far more influential. Normal matter we can see and hold accounts for about 4% of the universe. Dark matter comes in at about 23% and even more mysterious dark energy the remainder. Both matter and dark matter attract things gravitationally, so the result is sort of the same. Clumps of matter and dark matter start to form what we will later call galaxies. Occasional dense spots of matter start to form too, clumping together into proto-stars. While the rest of the universe is cooling down, these proto-stars are heating up as the particles all fall into a thick mass. As they get hotter and hotter, they eventually get hot enough so that fusion can occur again for the first time since 20 minutes after the start.


These first galaxies were probably pretty nasty places, if distant observations are anything to go by. They probably had very active centers throwing out extreme amounts of radiation in jets and generally throwing their weight around. Similarly, the stars at this stage were very boring. They would be composed of the original matter in the universe, so 3 parts hydrogen to 1 part helium. These stars (of which we’ve never seen any yet) are known as population III stars, and they would burn hydrogen deep in their core into helium.


Now what happens next depends a lot on how big a star happens to be. Big stars live life in the fast lane, and they cling desperately to what they can get, but it’s all over in a few million years for them. After somewhere between 1 and 30 million years they will have shone their last. Small stars like our own live much longer, maybe 10 billion years, but they don’t do very much that’s exciting compared to these big boys, unless you count letting us live. In the early universe it is likely that massive stars were by far the most common. Anyway, big stars are where it’s at, and they can get to be 100 times more massive than our own sun. Above that size they tend to rip themselves apart before they get going.

After these big stars race through their supply of hydrogen, burning it like there is no tomorrow, things start to come unstuck. With all the easily fused hydrogen turned into helium in their cores, they resort to burning hydrogen in a thin shell around this core of helium. But at the same time, their cores start to shrink, and this causes them to heat up again. Now it gets hot enough for helium to fuse. However the obvious reaction of two heliums banging together to form beryllium doesn’t happen. Well actually it does, but beryllium is so unstable, it tends to decay back as fast as it forms. Just occasionally though, you get a helium banging into a beryllium just as it’s about to fall apart and you’ll get carbon, which does hang around. Hey – we have one of the ingredients for life at last! Known as the triple alpha process this let the star shine again, at least for a while, but the writing is on the wall.

Things get increasingly desperate, as the helium runs out larger stars can switch to burning the carbon into neon, magnesium, sodium and oxygen. Hello, we have another few ingredients we’re going to need later on! Burning carbon will keep a star going for maybe 10,000 years or so. Then they can try neon burning (good for a year), oxygen burning (6 months) and silicon burning. Things are by this point really, really desperate. Silicon is the last chance to get energy out, as this ends up as nickel and iron, and fusing anything from iron upwards requires you to put energy in. The entire silicon burning phase lasts about a day and then it’s curtains. Whilst these last few stages are going on, some heavy elements are being built up by the s-process. The s stands for slow, and it adds a neutron at a time to some of the elements in the star, and builds up small quantities of heavy elements. A typical s-process reaction takes about a 1000 years, so it’s not a great way to make stuff. There are also only certain elements that can be built this way, as many of the products are radioactive and decay in less than 1000 years.

When one of these big stars reaches the end of the fuel, it’s like they’ve been holding up an increasingly heavy load, and finally they buckle. The star collapses in on itself, and there is nothing half hearted about this phase. The outer portions race into the center at nearly a quarter the speed of light and there is an almighty explosion. This is a supernova, and suddenly there are vast amounts of neutrons around and every element under (well over actually) the sun can be built up in literally seconds. This is the r-process – r standing for rapid and in just one or two seconds vast amounts of heavy nuclei are made and blasted out into space.


This outflow of material percolates into interstellar space and mixes with the clouds of hydrogen hanging around there and causes them to stir around somewhat, and in places this causes them to get dense enough to contract, and we’re off again to form another star. These stars we do see around, and are called population II stars, because we can detect in them quantities of things heavier than helium. Our own star, the sun, is a population I star, and is therefore probably composed of bits of previous population II stars. So we’re on the third wave of cosmic recycling at least. Most of our atoms have probably been in the center of two burnings stars at least.


So when the Earth formed there was more than just hydrogen and helium for it to form from, and we get a rocky surface to walk on with all that iron and nickel so desperately created in the death throes of early stars at or center. A world for… well for what? What happened next?



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