18. We Are Star Stuff

Matter is much older than life. Billions of years before the sun and earth even formed, atoms were being synthesized in the insides of hot stars and then returned to space when the stars blew themselves up. Newly formed planets were made of this stellar debris, the earth and every living thing are made of star stuff (Carl Sagan).

About ten million years after the Big Bang, enough cooling and expansion had occurred to fill the universe with a mist of particles, containing mostly hydrogen and some helium, as also some types of elementary particles (including neutrinos), some electromagnetic radiation, and perhaps some other, unknown, particles. The universe was just cold, dark, and formless at that stage.

Then, when enough cooling had occurred, some quantum-mechanical primordial fluctuations in the densities of the particles resulted in a clumping of some of the particles, rather like the nucleation that precedes the growth of a crystal from a fluid. The presence of such clumped particles brought the gravitational forces into prominence, resulting in a cascading effect. Portions of the mist began collapsing into large swirling clouds. Over a period of a few hundred million years, huge galaxies, each containing billions of young stars of various sizes, formed and began to shine. The formless darkness of the initial period was gone.

The large superstars among these were strongly bright spheres, the brightness coming from the nuclear fusion of hydrogen and helium in their interiors, made possible by the prevailing extreme temperatures and pressures. This is how many of the heavier elements got formed in the interiors of these large stars.

The emergence of heavier elements by the process of nuclear fusion continued steadily until the element iron (Fe) started forming. The iron nucleus is the most stable of them all, having the largest binding energy per nucleon. [Protons and neutrons inside the nucleus are jointly called nucleons; and 'binding energy' is defined as the amount of energy required to extract a nucleon from inside the nucleus and take it far away from it]. Therefore iron cannot fuse with one or more nucleons and release radiative energy of the nuclear process; such a process would not lower the potential energy and the free energy. Consequently, the presence of iron acts as a 'poison' for the nuclear fusion process. Thus the appearance of iron marked the beginning of the end of the available nuclear fuel, and therefore the end of the life of the star. In due course, the smaller among such stars simply ceased to shine, shrinking into cold and dead entities.

But a very different fate awaited the larger stars. No longer able to sustain their size because of the progressively decreasing processes of nuclear fusion of elements, they began to collapse under their own immense gravitational pull. A rapid change occurred in their interiors. Under the immense squeezing generated by the collapse, the iron-element core imploded. This resulted in a new state of matter as the electrons and the protons in the atoms were squeezed together. The dominant process of interaction was the electroweak interaction: p⁺ + e^- → n⁰ + ν_e (i.e., protons and electrons combined to produce neutrons and electron-neutrinos).

Thus, this collapse led to a compression of the star to an extremely dense ball of pure neutron matter, with the neutrino cloud bursting outwards, resulting in an explosion of the outer shell of the star (the 'SUPERNOVA EXPLOSION'). This is how the synthesized elements (up to the atomic number (Z) for iron), residing in the outer shell of the star, were scattered into the universe, accompanied by a brilliant flash of light.

A consequence of such supernova explosions (which still occur from time to time, and illuminate the galaxies with brilliant flashes of light) was the emergence of clouds of dust and gas and the debris containing heavy elements. These clouds encircled the galaxies in spiraling arms. THE INTENSITY OF THE SUPERNOVA EXPLOSIONS AND THE TEMPERATURES INVOLVED WERE SO HIGH THAT ELEMENTS HEAVIER THAN IRON WERE ALSO SYNTHESIZED AND SCATTERED INTO SPACE.

The chemical elements in our bodies have come from that star stuff: In the outer portions of the spirals occurred a condensation of the dust, the clouds and the debris, resulting in the formation of the second generation of (smaller) stars (including our Sun), as also planets, moons, comets, asteroids, etc.

Our solar system was formed when the universe was ~9 billion years old. In the initial period, our Earth underwent several violent upheavals (bombardment by comets and meteors, as also huge earthquakes and volcanic eruptions). By the time the Earth was ~2.5 billion years old, its continents had formed. Life appeared in due course, which further influenced the ecosphere in a major way. In particular, free oxygen (as opposed to oxygen chemically bound with other elements) was liberated as a waste product by the algae that consumed carbon dioxide present in the atmosphere and in the oceans.

Two billion years ago, our Earth was extremely radioactive as well. The heavier-than-iron elements produced in the outer shell of the exploding stars during the supernova explosion were/are radioactive, as their binding energy per nucleon was lower than that of iron: Such elements can increase their binding energy per nucleon (and thus attain a more stable state) by undergoing nuclear fission, either spontaneously or with the assistance of free neutrons. Uranium was among the heaviest elements produced during the last few seconds of the supernova explosion. Thus this element was a part of the Earth right from the beginning.

Watch this video for an interesting visualization of our cosmic and terrestrial history: