Friday, 23 September 2016

We're all made from stardust

We're all made from the same 96 elements (like carbon, nitrogen, etc). Actually humans are only made up of about 11 of those 96, but they're the same 11 as are found throughout the Universe. It's all the same stuff!

Have you ever thought about where all these elements come from? Maybe not! The answer is they all come from space, either being formed right back at the very beginning of the Universe or in stars that have born and died through the aeons. Quite literally we're all made from stardust.

I feel some astronomy coming on...

The entire Universe is one

To all intents and purposes, the theory and our latest observations agree that the beginning of the Universe started with a Big Bang. At the point of the big bang, everything was compressed into a single point of infinite density and infinite heat (which is really just energy). Everything in the entire Universe was once compressed together into a single point. Isn't that amazing – science shows we are all one.

As you might imagine, the explosion that followed was immense. The physics implies that the Universe underwent an incredible period of what's known as "inflation", where it increased in size by an enormous amount in a very very short time.

As the Universe then continued to expand and cool after this rapid inflation, the pure energy began to condense out into subatomic particles. In 1905, Einstein published his theory of special relativity, stating that energy is equivalent to mass (multiplied by the speed of light). Energy is equivalent to mass – it's another incredible concept! Essentially, mass (basically all "things") is just condensed, solidified energy. It blows my mind every time I think of it!

The Big Bang element factory

So as the Universe was cooling from it's infinitely high temperature, pure energy started solidifying into particles – first into things like quarks and other exotically named particles, then into more common or garden protons and neutrons, etc. At these high temperatures, a series of reactions started to convert single protons into atoms of hydrogen (two protons together) and helium (four protons and some neutrons), and a small fraction of lithium and beryllium. At the end of this cooling off period, the Universe contained about 75% hydrogen and about 24.99% helium.

So we've now got 4 of our elements (albeit not much of the other two). The big bang couldn't produce any elements heavier than beryllium due to a bottleneck in the reaction (as it happens, the absence of a stable nucleus with 8 or 5 nucleons).

The stellar element factory

Now we need to fast-forward about 200 million years to when the first stars formed. (This is a mere blink of the eye for the Universe, bearing in mind it is currently 13.5 billion years old.) So these large clouds of hydrogen and helium were floating around, getting bigger because gravity was pulling in more material, and gently cooling. Eventually, one clump somewhere deep inside one of these clouds got big enough to start collapsing in on itself. One of the things about gravitational collapse is that it really starts to heat things up. Right in the middle it got hot enough to start a totally new type of reaction – that of compressing 4 hydrogen atoms together to form helium. Stellar nuclear fusion was born and indeed the first star was born.

Stars can be thought of as giant furnaces that convert lighter elements to heavier elements and in the process release energy that radiates out (some of which we see). The reaction bottleneck that the plain Universe got stuck on was overcome in this fusion reaction.

Stars spend the majority of their life converting hydrogen into helium because it's extremely efficient, and there's so much hydrogen (even in a star like our own Sun) that this reaction can continue for billions of years. There are two main fusion reactions that are important inside of stars. For stars of small-medium size with (comparatively) low core temperatures, a reaction known as the p-p chain dominates. For medium-big starts with much higher core temperatures, a process known as the CNO cycle (standing for carbon-nitrogen-oxygen) dominates.

Both the p-p chain and the CNO cycle have the same effect though. Four protons are combined to form a helium atom, liberating energy (and a few other bits). The difference is that the CNO cycle requires the presence of carbon, nitrogen and oxygen to act as catalysts, thus producing energy more efficiently than in the p-p chain.

So where does this carbon, nitrogen and oxygen come from if all stars do is convert hydrogen into helium? Some of it is produced very near the end of a small-medium size star's life. The majority, however, is formed in a supernova.

The supernova element factory

A nova is a burst or explosion in a star. Some stars undergo regular nova outbursts. A supernova, as you might imagine, is a much bigger version of a nova, and it tends to be catastrophic. There are two main types of supernovae. The first happens when there are two stars in orbit around one another, and the more massive star is sucking material from the smaller one. At some point so much material accumulates on the bigger star that it can't cope with the pressure (literally) and it implodes. The second type concerns very massive single stars (typically bigger than around 8 times the mass of our Sun). These massive stars consume their hydrogen fuel fast. At the end of their lives, as the fuel runs out, the outward force holding the star up fades and again the star implodes. A supernova happens when the imploding material rebounds to produce one of the Universe's most energetic explosions.

Binary star where one star is sucking material from the other

It's in this second type of (so-called "core collapse") supernova that many of the heavier elements we know and love are formed. In fact all the elements from carbon up to and including iron are produced in supernovae through various processes. For example, large amounts of radioactive (and therefore unstable) nickel can be produced, which would quickly decay into stable iron.

The periodic table indicating the main origin of elements found on Earth (source).

For the elements heavier than iron another process is needed, and this is called r-process neutron capture (r for rapid). This can only work in the super-high density and high temperature conditions of a supernova. Lighter elements rapidly accumulate neutrons to create particular very heavy isotopes, which then decay to the first stable isotope. All the "heavy" elements from iron up to about plutonium are made this way.

The remnant of a core-collapse supernova explosion
When the supernova explodes, it blasts all this enriched material out into its environment. It stirs up the gas and mixes everything together. This is another reason why supernovae are so important – they get the newly enriched material back out into space.

Generations of stars forming and exploding

So let's say you have a big gas cloud that initially forms a few million stars. After some time a few of the big ones will go supernova. Leave it another few million years and some of that expelled gas will come together again and form a new generation of stars enriched by the previous generation of supernovae. After a few times through the cycle you collect up enough material to start forming rocky planets – and eventually (maybe) life.

Our Sun is thought to be a 3rd-generation star and has been shining for about 5 billion years.

Look around you right now. Everything you see is made of material that was forged in the furnace of past stars and supernovae. And who's to say that all this stuff may be swept up in some future event and incorporated in a new star and planet system...

As the Buddha said, all things are subject to change, even if it takes millions of years.

I am a member of the Zenways sangha led by Zen master Daizan Skinner Roshi, and I teach meditation, mindfulness and yoga at the ZenYoga studio in Camberwell, London. See my website for further details.

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