We are made of stars. Tracing human origins back to the Big Bang

The phrase "We are made of stars" is commonly used as a poetic metaphor to emphasize the radiant beauty of the human spirit. It resonates with our artistic sensibilities and inspires positive emotions. We never expected it to hold any scientific value, let alone accurately describe the human body's chemical composition. This view changed in the 1980s when American astrophysicist Carl Sagan famously declared that ancient stars are indeed our ancestors. Furthermore, he even traced our family tree back to the Big Bang to convince skeptics. If we could move beyond our exclusive tendencies and embrace a sense of unity with the Universe, we would recognize that Carl Sagan's claim is a natural outcome of physical laws.

Fig. 1. The atoms of our body were once inside the stars, which died to let us live.

Let's start with the beginning of everything, the birth of our Universe. The Big Bang was a violent event, packed with action. The initial temperature was unbelievably high, and the density was unimaginably great. Space was saturated with pure energy, poised to convert into matter. Within moments, the Universe's rapid expansion caused the temperature to drop enough for the Higgs field to activate. And the first protons, neutrons, and electrons gained mass to serve as building blocks in creating the material world we inhabit now.

Protons united with neutrons to create nuclei, which attracted electrons to form atoms. Hydrogen (H) was the first chemical element to emerge due to its simple atomic structure, consisting of one proton and one electron (Fig. 2). The second element, helium (He), was formed through a more complex process involving the fusion of two protons. The third element in the periodic table, lithium (Li), encountered even more challenges, requiring three protons in its nucleus. As a result, this chapter of Genesis, known as Big Bang nucleosynthesis, ended with the Universe composed of 75% hydrogen, 25% helium, and traces of lithium.

Fig. 2 Atomic structure of the first four elements in the periodic table.

This stage concluded within a few minutes. The formation of heavier elements with more protons in their nuclei took significantly longer, spanning millions of years. The number of protons in a nucleus determines the identity of a chemical element. Protons, being positively charged, repel each other. To bind them together, prolonged periods of extremely high density and temperature are necessary, conditions that the expanding and cooling Universe could not provide. These conditions could only be met within the massive stars that had yet to form.

Before the stars existed, the Universe was a dark place filled with thick clouds of hydrogen and helium. Over time, clouds separated into clumps, which grew in size with their cores becoming denser and hotter. When their temperature reached 10 million Kelvin, hydrogen fusion commenced, and the first stars lit up. These primordial stars were massive, substantially larger than our Sun. They acted as giant crucibles, forging new elements through a process known as stellar nucleosynthesis. You can get an idea of the hydrogen fusion that ignited the primordial stars from the video taken during the hydrogen bomb test conducted by the United States in 1954 (Fig. 3).

Fig. 3  The hydrogen bomb test conducted in the US, March 1954. Source: Wikipedia

The extreme temperatures and pressures inside these stars enabled complex fusion reactions, leading to the creation of the following line of the chemical elements in the periodic table, like oxygen (8 protons in its nucleus), silicon (14 protons), and calcium (20 protons). However, the stars kept new elements to themselves. Only when they started fusing iron (26 protons in its nucleus) did they mark their end and explode as supernovae, releasing these elements into the surrounding interstellar space. During this cataclysmic event, temperatures soared close to the highest known, fusing the elements as heavy as silver (47 protons in its nucleus) and gold (79 protons) through the process known as supernova nucleosynthesis.

The remnants of these stars, now enriched with new elements, served as the building material for the next generation of stars known as Population II. These stars underwent similar nucleosynthesis processes, converting more hydrogen and helium into heavier elements, thereby increasing their presence in the Universe. Still, even today, the Universe is predominantly composed of the most basic elements formed during the Big Bang, hydrogen and helium, with only 2% left for the elements with more complex nuclear structures. This observation remains the most compelling evidence in favor of the Big Bang theory.

Our Sun belongs to the most recent group of Population I stars. Due to its relatively small size, it can't maintain the conditions necessary to make heavy elements, so it mainly fuses hydrogen into helium. The chemical elements in our body, like oxygen (65% of the body mass), carbon (18%), and others, came from a nebula, a gas and dust cloud left behind by a dying star, whose remnants were reused during the formation of our solar system. That star, in turn, contained the elements it inherited from its predecessor.

Fig. 4  Carl Sagan was a great scientist and a profound thinker.

So, yes, the chemical elements that compose our bodies were once inside the stars, which died to let us live. On early Earth, high-energy events like lightning and volcanic activity enabled the formation of organic matter from simple molecules like water (H2O) and carbon dioxide (CO2), eventually leading to the emergence of DNA. The genetic code marked the beginning of a new era, an evolution of the life forms from the most primitive to as complete as humans, capable of searching for the answers about our origins and the origins of our Universe.