Do you ever look around your apartment and think, where did all this stuff come from? Maybe some of your clothes or books or tchotchkes are unnecessary, and you could stand to de-clutter. Or, to take the very long view—the universe’s view—not only are your books not necessary, but neither are most of the elements that make up your books, your other possessions, or indeed you yourself. There was a time in the universe’s infancy when these elements didn’t exist, and yet somehow the universe managed to create them all, along with you and everything else you can see.
To de-clutter the universe, you could pare down the whole array of elements to basically two fundamental ones: hydrogen and helium, with a tiny smattering of lithium. These are the three lightest elements, and they were the only three elements generated in the Big Bang. For millions of years after the Big Bang, the universe was missing the elements it needed to build a planet, a living thing, or even a water molecule: no carbon, nitrogen, or oxygen existed. There was not a single atom of gold or silver, iron, or silicon.
All of these heavier elements were generated from just hydrogen and helium. It’s as if you left a pile of flour and a little salt on your kitchen counter, and several billion years later, the pile had manufactured apples, cinnamon, sugar, and butter, and assembled itself into an apple pie.
For a while, though, pretty much everything in the universe was made of flour and salt—that is, hydrogen and helium. But it was not evenly distributed. Some areas had slightly more dense clouds of atoms. Gravity caused these dense areas to attract more atoms, making them even denser, which made them exert even more gravitational force. Thanks to this positive feedback loop, the clouds of hydrogen and helium condensed into stars. That’s when everything changed, starting a few hundred million years after the Big Bang. It was only inside stars that the hydrogen and helium were able to transform into anything else.
Compressed more and more by gravity, the stars eventually became so hot (at least 5 million degrees Fahrenheit) that nuclear fusion reactions began to happen. The first of these to happen was the fusion of hydrogen. Nuclei of hydrogen collided with each other at such high speeds and heat that they melded together into a new element—helium. This is what’s happening inside our sun right now. In the two seconds it took you to read that last sentence, our sun transformed about a billion tons of hydrogen into helium.
The fusion of hydrogen into helium generates a huge amount of energy—it’s what powers our sun and all life on Earth. If a star has enough mass (our sun does not at this point in its life), it will get hot enough to fuse helium. The heart of our sun is about 27 million degrees. Helium fusion requires a temperature of at least 180 million degrees. At this temperature, helium atoms collide and meld to form larger elements like carbon, oxygen, and nitrogen. This will happen in our sun when it’s a red giant, later in its life.
If a star has enough mass, it gets so stupendously hot that even heavier elements are created in its core: neon, silicon, and finally iron. Iron is not good news for a star. For all the elements up until iron, fusion releases energy. This energy pushes outward to counteract the force of gravity and keep the star stable. But the fusion of two atoms to form an element heavier than iron—for example, uranium—doesn’t generate energy, it requires energy. (Incidentally, the reverse is true as well: splitting apart an atom of any element heavier than iron generates energy. This is why splitting uranium in a nuclear power plant generates energy that we can use to power an electrical grid.)
Because of this property of iron, when a star’s core has turned to iron, it no longer generates radiative energy to push outward against the force of gravity. With nothing to counteract it, gravity suddenly collapses the star in on itself so forcefully that it essentially bounces back outward from the core, and the star dies in a cataclysmic supernova. Supernova explosions generate so much heat and energy that they create elements heavier than iron.
So if you want to make an apple pie, all you really need is hydrogen and helium. They’re the only real essentials. Wait long enough, and those two elements, transformed inside the guts of living and dying stars, will take care of the rest.