"The Cosmos is within us; we're made of star stuff. We are a way for the cosmos to know itself."
Famous for stating, “We are star stuff,” Carl Sagan wasn’t merely using a metaphor. In fact, scientific understanding confirms that the early universe was mainly comprised of hydrogen with a little bit of helium. As these gaseous elements coalesced into larger formations, they increased in density. Eventually, these pockets of gas grew large and hot enough at their core to initiate fusion, hence giving birth to a star. Fusion has, directly or indirectly, created most elements in the periodic table – including those that comprise our bodies.
At its most basic level, fusion involves the combination of two hydrogen atoms to form one Deuterium atom. If you add another hydrogen atom, you get Helium. The process results in a loss of protons, which in turn produces an energy release – the very mechanism behind star creation.
A typical star will continue to smash atoms together, creating various elements. For example, the fusion of two helium atoms results in Helium-3, two Helium-3 atoms produce Helium-4, and two Helium-4 atoms create Beryllium. This process continues up the periodic table until iron is formed.
At the core of every star, including our sun, the process of fusion is ceaselessly occurring. The simplest way to understand fusion is as a process where lighter elements combine under extreme conditions of temperature and pressure to form heavier elements, with a massive release of energy.
In the stellar context, the fusion process begins with hydrogen, the lightest and most abundant element in the universe. Under the intense pressures and temperatures found in the cores of stars, hydrogen atoms come together, overcoming their natural repulsive forces, to form helium, the second lightest element.
The primary fusion process in stars like our sun is the proton-proton chain reaction. This reaction begins with two protons (hydrogen nuclei) coming together. Because of their high energies and velocities, they overcome their natural repulsion (due to both being positively charged) and collide. When they do, one of the protons transforms into a neutron through a process called beta-plus decay. This involves the emission of a positron and a neutrino, changing the proton into a neutron.
The result is deuterium, an isotope of hydrogen that has one proton and one neutron. The deuterium then fuses with another proton, forming a light isotope of helium (Helium-3). Two Helium-3 atoms can then come together to form a Helium-4 atom (two protons and two neutrons), with two protons being emitted in the process. The net result of this complex chain reaction is the conversion of four hydrogen atoms into one helium atom, releasing a tremendous amount of energy in the process.
As stars exhaust their hydrogen fuel, they begin to fuse helium in a process known as the triple-alpha process. This occurs at much higher temperatures and involves the fusion of three helium-4 atoms (alpha particles) to form carbon-12. The name ‘triple-alpha’ refers to the fact that the helium-4 nucleus is also known as an alpha particle. This process plays a crucial role in larger, older stars and contributes significantly to the formation of heavier elements up to iron.
The fusion process doesn’t stop at helium or carbon. Depending on the size of the star and the conditions within its core, the fusion process can continue, creating even heavier elements. Oxygen, neon, and silicon can all be produced via fusion, ultimately culminating in the creation of iron. However, once a star begins to produce iron, it signals the end of its lifecycle. Iron cannot be fused into heavier elements with a net release of energy, and without energy production to counteract gravitational forces, the star begins to collapse, leading to a supernova if the star is large enough.
Elements heavier than iron, such as gold, uranium, and platinum, are created in these final, explosive moments of a star’s life, during the supernova event. This is also when these elements are scattered into the cosmos, contributing to the interstellar medium from which new stars, planets, and even life can form.
When stars start producing iron, no extra protons remain to generate energy, marking the star’s impending demise. With no outward pressure from the energy release, the star slowly collapses in on itself. If massive enough, it can go supernova, with giants potentially transforming into neutron stars. Collisions between neutron stars can yield even heavier elements. These neutron stars, on the verge of collapsing into black holes, are short-lived, leading to a constant creation of new elements.
Nebulae, often captured in spectacular images, are essentially stellar factories. They form stars and accompanying celestial bodies such as planets, moons, and asteroids. The necessary elements for creating a star and its system are present in these gas fields. A simple push, potentially from a nearby supernova, can initiate the process, after which gravity takes control. If the nebula is located within a galaxy, surrounding stars can provide the necessary momentum.
Our bodies are primarily composed of six elements: oxygen, hydrogen, nitrogen, carbon, calcium, and phosphorus. Interestingly, these elements are common in typical stars like our sun. Just imagine; the elements constituting your body today were not present at the dawn of time. They were instead created through the process of stellar fusion. Thus, every atom in your body was forged within a star, reinforcing the truth behind Carl Sagan’s statement – we are indeed star stuff.