Astronomy Demonstrates How the Human Story Began in a Supernova
The universe overflows with mind-blowing facts, from the entanglement of particles across vast expanses of space and time to the evolution of human consciousness. However, only a handful of facts can challenge our perception of reality as successfully as the realization that flowing in our blood are atoms of iron that were once responsible for the collapse of stars.
Everything in the physical universe should have an origin and cause. Each atom, composing each molecule in our bodies, has an origin and has existed for billions of years before ending up where it now is. But to find out more about this, we must look into the life cycle of stars, the birthplace of most elements in the universe.
The nebular hypothesis is the most widely accepted model for the lifecycle of stars and the formation of planetary systems. This hypothesis addresses the formation process of the Solar system and, by extrapolation, of planetary systems in general. The philosopher Immanuel Kant first published the hypothesis in 1755, and in 1796 the polymath Pierre Laplace modified it to its current general form.
Artist representation of the birth of a star within an accretion disc
The idea is that stars and planets are born from clouds of gas and dust drifting in space.
These enormous clouds engage in a tug-of-war between the natural tendency of gases to dissipate and the gravitational forces created by the mass of the cloud, pulling the matter towards the center. Gravity inevitably wins the fight, generating a swirling vortex that concentrates the matter at its center and progressively accelerates, becoming flat like a disc. With increasing velocity, the temperature and the density at the core of the vortex rise continuously until the point where there is so much energy that hydrogen atoms undergo nuclear fusion, in which hydrogen nuclei are fused together forming atoms of helium, and liberating neutrons and large quantities of energy in the process.
At that moment, a star is born.
As explained by Dr. Megan Bedell in her presentation at the 2019 Sagan Summer Workshop: Astrobiology for Astronomers, as a star is being formed, clumps of matter accumulate around it and eventually form new planets. Thus, since both come from the same primordial cloud, host stars reflect the starting conditions for planets, and their compositions are inevitably related.
A planetary nebula, formed by the expelled gaseous outer layers of a Sun-like star as it reaches the end of its life
The starting mass of a star is the principal determinant of which of the limited number of possible paths it will follow during its lifecycle.
The energy generated by the nuclear fusion reactions inside a star counteracts the gravitational forces crushing it inward. Because the star’s mass determines how much fuel for fusion it contains, it consequently also determines what the star’s fate will be.
As long as there is plenty of hydrogen in the core of a star, it will fuel the nuclear fusion of hydrogen into helium for millions or billions of years. When the hydrogen begins to run out, the star’s core starts to shrink and get hotter, and the rate of fusion accelerates. At this point, average and low mass stars such as our sun will expand into red giants. When all the hydrogen in the core is gone, the star becomes increasingly hotter as it continues to shrink. At one point, the core becomes so hot that the helium undergoes nuclear fusion reactions creating heavier elements, like carbon and oxygen.
Stages of development in the core of low-mass stars as they run out of hydrogen. Helium flash is the critical moment when they reach temperatures hot enough to trigger fusion of helium atoms
During this new stage, the star becomes smaller and hotter as it consumes the helium in its core until it is predominantly composed of carbon and oxygen. Then, there is so little fuel left in the star that the pressure of its energy can no longer resist the force of gravity, so it collapses onto itself. The outer layers of the star disperse, leaving behind the bare core. The dispersed outer layers of that star form a new nebula, free to start the star formation process all over again.
The Helix Nebula. The death of an average or low mass star like the Sun leaves a planetary nebula with dispersed outer layers and a white-dwarf star at its core
High-mass stars follow a different path.
Greater mass means more fuel to burn but also more powerful gravitational force. Because of that, high-mass stars will burn through their hydrogen faster and reach much higher temperatures in their cores. When their cores begin to shrink as they run out of hydrogen, they get much hotter than the cores of a low-mass star – so much so that the nuclear fusion reactions inside them create elements heavier than those produced in smaller stars.
With increasing temperatures towards the center of the core, lighter atoms sequentially fuse into heavier ones, progressively forming heavier elements until the creation of iron in the innermost layer. Iron is composed of 26 protons, 30 neutrons and 26 electrons in its neutral form, and is the heaviest element that can be fused within a star. Once the star is left with an iron core, it is so stable that nuclear fusion reactions cease to occur.
At this moment, the star collapses within a single second due to gravity, triggering a colossal explosion that ejects the heavy nuclei created by it into space. This event is called a supernova.
The explosion releases such an incredible burst of energy that it synthesizes dozens of elements heavier than iron, like copper and gold, as it happens. The collapse of high-mass star is so powerful that it may create a neutron star or even a black hole.
Artist impression of the supernova 1993J, from the galaxy M81
After an undefined number of stars were born and died, a nebula containing the collection of atoms that would eventually form the Solar system came to be.
From that apparently random assemblage of matter, Earth was formed and, within it, life emerged. Then after countless generations of organisms through which matter continually cycled, the history of life on Earth eventually led to the clumps of matter that constitute us humans.
From the first atoms created by the Big Bang, all the way down to what makes up our bodies, we’re all but clumps of recycled star dust. We carry the ancestry of unknown nebulae and collapsed stars within ourselves. So, next time you contemplate the magnificence of a sunset or the beauty of a starry night sky, remember that in your blood flow atoms that once marked the collapse of the ancestors of the stars you now admire.
Everything which is in the cosmic is in our bodies also,