So far we’ve discovered two sources of helium in the universe. First (which came second) is the magic furnace of the stars, in which primordial hydrogen is transformed into helium via nuclear fusion, in the process lighting up the world and making peanut butter and jelly sandwiches possible. The second (which came first) is the fireball that began our universe, hot enough to cause helium to form in the first second of the universe’s existence.
But those two sources of helium don’t account for the helium we have here on Earth. Almost every bit of that helium has escaped into space by now. Why? Because it’s so light, and because it doesn’t react with anything else. With nothing to hold it here, that helium has long ago returned to the void.
You might wonder how we know that. It’s because the helium we detect today on Earth is almost entirely an isotope called helium-4. The helium produced in both the Fireball and inside stars is a mixture of helium-4 with another isotope, helium-3. We find almost no helium-3 on Earth, but there is, possibly, a nearby source that will become important as this entire story comes to a close. Stay tuned!
So why do we have helium on Earth at all? Because there’s a third source. And its been right under your feet all the time.
To find the source of that source, we need to go back to that dying star. With its hydrogen running low, that star began to collapse. That drove the temperature up, resulting in the fusion of helium into carbon. Just like hydrogen, helium can’t last forever, so eventually the star collapses again. As the temperature rises, carbon begins to fuse. It fuses with helium to make oxygen. It fuses with itself to make magnesium. It fuses with oxygen to make silicon. Oxygen fuses with helium to make neon.
All this nuclear cooking, by the way, shows why atoms with even numbers of protons are generally more common than atoms with odd numbers of protons. He (2) makes C (6). C(6) and He (2) make O (8). O(8) and He (2) make Ne (10). And so on. One exception is nitrogen (7 protons), which is actually formed in a different process that happens when a star is hot, but still contains hydrogen.
This process goes on as the star gets hotter and hotter, until the star produces iron. This is a dead end. No amount of fusion can wring energy out of the iron nucleus. Any change from iron is a net user of energy instead of a producer. That’s bad news for the star; with no energy source left, gravity takes over, and the star collapses. But just like a brick falling from a bridge, this gravitational collapse releases energy itself. In the collapse, and in the rebound that we see as a supernova explosion, vast amounts of energy are suddenly available. No longer constrained by the economics of nuclear fusion, wild collections of atoms, never before seen, are created in a moment. Among these are nearly all the gold in the universe and every bit of two very important atoms called uranium and thorium. All these atoms drifted about in space after their tumultuous birth and a few of them found themselves, quite by accident, falling into the cloud of materials that condensed to form the young planet Earth.
Uranium and thorium are rather gaudy elements. They are right at the edge of what is possible. The lighter elements are almost elegant in their balance of protons and neutrons – the most common isotope of carbon has 6 of each, oxygen 8 of each, nitrogen 7 of each, and so on. Even iron is close to being balanced, with 26 protons and 30 neutrons. But by the time you get to thorium (90 protons, 142 neutrons) and uranium (92 protons, most commonly 143 or 146 neutrons), things are wildly out of balance. In fact, these two elements have so many protons that even these wildly excessive neutrons are not enough to permanently hold the element together. Eventually, something happens.
Let’s watch one of these isotopes and see what happens. We wait, and we wait, and we wait. For thorium 232, we might wait 13 billion years or so. Or we might get lucky. The amazing thing about this process is that nobody knows when thorium will do what it does. Why? Patience, grasshopper. That answer will come.
There! The atom decayed! Shooting out of the thorium atom we spot a tiny something, traveling faster than a bullet from a gun. Where did all that energy come from? Though we haven’t discussed the details, we know the grand outline now. It’s star energy, the energy stored by that dying star, like a note in a bottle, in its final deadly explosion. You just watched a little bit of an exploding star!
http://www.youtube.com/watch?v=ItdSjJKmyDY (sorry about the music)
So what comes shooting out like a bullet? You’ve probably guessed by now. It’s helium. More specifically, it’s the nucleus of a helium atom, two protons and two neutrons. Now it just gathers a couple pieces of fluff called electrons, and we have a fully-formed helium atom, ready to fill balloons or make you talk funny. Virtually every atom of helium on Earth today was born in just this way, in the mini-explosion of a large and unstable atom inside the Earth, an atom that itself was created in the last, dying moments of an exploding star.
How cool is that?