The heart of every star is a forge for the building blocks of the universe. Since the day the universe began, stars have been taking the materials spit out by its formation and transforming them into the elements that make up everything we know: deserts, dinosaur bones, oak trees, apartment buildings, giant squids, jellybeans, and you.
It all started with hydrogen and helium, the two most common elements in the universe; they’ve been here since the beginning, appearing just minutes after the Big Bang. For some time after the universe formed, these two light elements (along with a smattering of lithium) were the only ones in existence. To make everything else, the universe needed a spark.
That spark is called nuclear fusion, a reaction that fuses two separate atoms into one to form an entirely different element. Fusion doesn’t just happen on its own; in order to generate a new element, two atoms need to collide, and they need to collide fast. The only way to get an atom moving fast enough to fuse with another atom is to make it very, very hot, and so far, the only place in the universe hot enough to create these temperatures is within the high-pressure furnace of a star.
Collide fast enough, and the atoms’ protons and neutrons will smash together to create a brand-new nucleus. By converting some of their mass into energy when doing this, these fusion-transformed nuclei generate the power that keeps stars bright and hot. In the case of our sun, fusion releases 400 trillion trillion watts of energy every single second— equivalent to what would be produced by 90 billion megatons of exploding TNT.
Our middle-aged sun, around 4.6 billion years old, is currently fusing hydrogen as its primary fuel, as all stars do at the start of their lives. Once this hydrogen is burned up, the sun will burn helium until it dies. Yet this isn’t the case for larger stars. Once all the available hydrogen and helium are used up, giant stars are able to burn through multiple elements as fuel, hurtling them towards a simultaneously violent and bountiful ending.
In large stars, nuclear fusion runs in a relay race, each element passing on the baton of fusion to the next available runner as they expire. The reaction that converts stellar hydrogen into helium takes only a few million years in the largest stars, a quick lap around the track in the lifespan of the universe. Exhausted of hydrogen, the star must begin fusing the next option, helium, a much less efficient fuel—all of it will be used up in only 100,000 years. The fusion reactions begin to happen faster and faster now: once all the helium is exhausted, the star turns next to carbon, which burns up in 10,000 years, and then to oxygen, which only lasts 1,000, until all that is left is a core of burning silicon, so massive and so hot that every atom fuses into iron in a single day.
That day is the star’s last. The nucleus of an iron atom is so tightly locked together that it takes more energy to fuse these atoms that the star has left. In an instant, spent of fuel, the star’s inner core collapses.
The impact of this collapse creates a pressure wave that blows everything the star has created out into the universe. This explosion, called a supernova, creates such high temperatures that atoms further fuse within it to instantly form the heaviest elements in our universe, like plutonium, uranium, silver and gold.
When the universe was young, stars were the only places that most elements could be found; there were no planets yet circling these stars, or even chunks of rock floating mindlessly through space. Today, the trillions of supernovae that have occurred since the Big Bang have spread these elements far and wide, from the distant reaches of space to the tips of your fingers. This process, from beginning to end, is called stellar nucleosynthesis—and it is the reason that you, and me, and everything we know, owe our existence to the stars that burned out in the universe and donated their stardust to assemble our planetary kingdom.