The Stuff of Stars - How We're Made of Stardust

The Crab Nebula, a supernova remnant. Within this cloud are all the atoms forged during the life of a star, which may eventually end up as the building blocks of a planet, and perhaps even life. Image Credit: NASA, ESA, J. Hester and A. Loll (Arizona State University)

"The cosmos is within us. We are made of star-stuff. We are a way for the universe to know itself."

-Carl Sagan

When Carl Sagan delivered his famous quote on his TV series “Cosmos,” he wasn’t just being poetic; he was making a literal statement. We are literally made of material that comes from stars. But how? Let’s dive into the meaning of this famous quote.

What We're Made Of

We’ve probably all heard before that the human body is made up mostly of water, as well as protein, fat, and minerals. All of that has a chemical composition. Water is H₂O, meaning one molecule of water is made up of two hydrogen atoms and one oxygen atom. The atomic composition of the human body by mass is about 61% oxygen, 23% carbon, 10% hydrogen, 2.6% nitrogen, 1.4% calcium, and about 2% of a list of other elements. These atoms all come from the food we eat, which gets integrated into our bodies as we grow and replace our cells. The food we eat either gets them from the food it ate or from the Earth and air. But where did the Earth get them?

The Source of Everything

Let’s rewind to the very beginning of the universe. In the very first moments following the Big Bang, the entire universe would have been one hot, dense sea of energy soup with no matter of any kind. But, over the course of the first 20 minutes of the universe, it cooled enough for this energy to condense into matter. Hydrogen, the most basic element of all, with just a single proton, was the most common, but the heat and energy of the time also allowed for additional protons and neutrons to come together in the process of nuclear fusion and create helium, as well as some trace amounts of lithium and beryllium. But that was it. All the atoms in the universe at that time were only the first four elements on the periodic table. Where did the remaining 90% of mass in our bodies originate?

Stellar Nucleosynthesis

Stars. It all comes from stars. Nebulae, clouds of hydrogen and helium, leftover from the formation of the universe, are the birthplace of stars. When a star first forms, it is essentially a ball of hydrogen and helium that’s been super-heated into a plasma. In the core of the star, something special happens. The temperature and pressures of the core—27 million°F and 260 billion times the pressure of the Earth’s atmosphere, respectively—are enough to cause the atoms inside to undergo nuclear fusion.

The Carina nebula, a star-forming cloud of hydrogen and helium left over from the Big Bang. This hydrogen and helium will be the ingredients to create heavier elements. Image Credit: NASA, ESA, CSA, STScI, Megan Reiter (Rice University), with image processing by Joseph DePasquale (STScI), Anton M. Koekemoer (STScI)

In nuclear fusion, multiple atoms are smashed together to form heavier atoms. For example, four nuclei of hydrogen can combine to form one nucleus of helium, plus some energy. This energy released is the primary energy source of a star. You can also combine three helium atoms to form a carbon atom, and so on. This creation of new, heavier elements in the core of a star is known as stellar nucleosynthesis. Through this process, we get the most abundant heavy elements in the universe, all the way up the periodic table through iron. Iron is like a poison for a star, as it takes more energy to fuse than the process produces. Once iron production begins, the star’s life is over, as it can’t produce enough energy to fight against its inwards pull of gravity and collapses. At this point, nearly every atom in the human body has been accounted for.

Even heavier elements, like copper, silver, and gold, can’t be formed through regular stellar nucleosynthesis, and instead are formed by the deaths and collisions of stars. For example, when a massive star dies, it will explode in a supernova. This explosion has enough power to cause the fusion of these otherwise missing elements. These explosions also have the benefit of spreading all the elements that star created throughout its life out into space. These heavier elements will mix with the existing clouds of hydrogen and helium.

From Stardust to People

When a new star forms, a disk of leftover material, known as the protoplanetary disk, will form around the star, and will include those heavier elements from other stars. It’s from the material in that disk that planets orbiting the star form. This is how the Earth and all of its elements came to be in the state we see today, and therefore all the atoms that make up not only us, but everything around us, came to be.

Not only are we star-stuff, but the entire universe is. Almost everything from the smallest grain of sand to planets themselves are made out of atoms forged in the hearts of stars. Carl Sagan really wasn’t exaggerating. We are made of star-stuff. It’s more than just a romantic view, it’s the truth of the universe.