We Are Made of Stars. Tracing Human Origins Back to the Big Bang

The phrase "We are made of stars" was commonly used as a poetic metaphor to highlight the beauty of the human spirit. It was never anticipated to hold any scientific value, let alone accurately describe our physical makeup. However, this view changed in the 1980s when American astrophysicist Carl Sagan famously stated that the atoms composing our bodies were created in the cores of ancient stars and dispersed across the Universe when the stars exploded as supernovae. These stars originated from primordial clouds formed after the Big Bang. Consequently, the entire history of the Universe, from its inception to the present, is encoded in the chemical composition of our bodies, implying that the Universe is within us, just as we are within the Universe.

Fig. 1. The Universe is within us, just as we are within it. Image: Shutterstock

The history of humanity begins with the origin of everything: the Big Bang. This was a cataclysmic event marked by extremely high temperatures, densities, and fast transformations. The newly born space was saturated with energy, ready to convert into matter. Within a second, the rapidly expanding universe cooled to a temperature of 10^15 Kelvin, triggering the activation of the Higgs field and allowing elementary particles, such as quarks and electrons, to acquire mass. At this stage, the universe resembled a soup of free particles, which would serve as the fundamental building blocks in the construction of the material world we inhabit today.

As the universe continued to expand and cool, the elementary particles became less chaotic and began forming bonds. Quarks combined to form protons and neutrons, and the protons and neutrons combined to form nuclei. The positive nuclei attracted negative electrons, leading to the formation of 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), with two protons in its nucleus, required a far more complex process of fusion. The third element, lithium (Li), with three protons in its nucleus, faced even greater challenges. Consequently, this phase, known as Big Bang Nucleosynthesis, concluded with the universe being composed of 75% hydrogen, 25% helium, and traces of lithium.

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

While the Big Bang Nucleosynthesis lasted only minutes, the next stage, the formation of heavier elements with more protons in their nuclei, took about a billion years to complete. The number of protons in the atom determines the identity and chemical properties of the element. Each element in the Periodic Table, Fig. 3, differs from the previous element by one extra proton in its nucleus. Since the protons are positively charged and repel each other, binding them together becomes increasingly problematic with every proton added. This explains why the heavier elements with more protons in their atoms require prolonged periods of extremely high pressure and temperature, conditions that the expanding and cooling Universe could not provide. These conditions could only be met inside the massive stars that had yet to emerge.

Fig. 3  Periodic table of the chemical elements. Source: Type Calendar.

Before the emergence of the stars, the Universe was a dark place filled with thick clouds of hydrogen and helium. This period is known as the Cosmic Dark Ages. Over time, clouds separated into clumps, which grew in size with their cores becoming denser and hotter. When the temperature inside them reached 10 million Kelvin, hydrogen fusion commenced, and the first stars lit up the early Universe. These primordial stars were massive, substantially larger than our Sun. They served as giant cosmic crucibles, fusing new, heavier elements in their cores through a process known as Stellar Nucleosynthesis. The video in Fig. 4, showcasing the 1954 hydrogen bomb test by the United States, provides insight into how the first primordial stars were ignited through the hydrogen fusion process.

Fig. 4  The hydrogen bomb test conducted in the US, 1954. Source: Wikipedia

The extreme temperature maintained inside these stars enabled complex fusion reactions, resulting in the formation of heavier chemical elements, such as oxygen (8 protons) and calcium (20 protons), which are found in our bodies. However, the stars kept these elements to themselves, locked in their cores. It was only when they began fusing iron (26 protons in its nucleus) that they reached the end of their life cycle and exploded as supernovae, dispersing new elements into the surrounding interstellar space. During these explosions, the temperatures reached the highest levels known in the Universe since the Big Bang, allowing the final elements, as heavy as silver (47 protons) and gold (79 protons) to emerge through a process known as Supernova Nucleosynthesis.

The remnants of these stars, 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 category of Population I stars. Due to its relatively small size, it can't sustain the conditions needed to create heavy elements and primarily fuses hydrogen into helium. The heavy elements that constitute our bodies, such as oxygen and carbon, came from a nebula, the gas and dust cloud left behind after the explosion of a Population II star, which our solar system used during its formation. That star had itself recycled chemical elements created by its predecessor, a primordial Population III star, which formed from the hydrogen and helium clouds of the Big Bang. The human body consists of about 60% water (H2O), which includes hydrogen atoms in its molecules. This hydrogen links us to the Big Bang, while other atoms connect us to the stars that enriched the Universe with additional elements.

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

Therefore, based on observations with powerful telescopes and laboratory experiments, we can conclude that we are composed of stardust, except for hydrogen, which was formed during the Big Bang. Every atom in our bodies was once part of a primordial cloud or a star that died to let us live. On early Earth, high-energy events like lightning and volcanic activity facilitated the formation of organic matter, beginning with simple molecules and ultimately leading to the emergence of DNA. The establishment of DNA marked the start of a new era, an evolution of life forms from primitive, single-celled organisms to intellectually advanced humans, capable of inquiring about their origins and discovering the answers.