Stars and Stellar Nucleosynthesis
In a scene of the animated film The Lion King, Simba the young lion, Pumbaa the flatulent warthog and Timon the wisecracking meerkat are looking at the night sky. Pumbaa asks Timon if he ever wonders “what those sparkling dots are up there” that break the monotony of darkness. The little guy confidently offers: “Pumbaa, I don’t wonder. I know. They’re fireflies, who got stuck in that big bluish black thing.” The big fellow sheepishly replies with “Oh, geez, I always thought they were balls of gas burning, billions of miles away.” Timon retorts: “Pumbaa, with you, everything is gas.”
Of course, stars are not fireflies but rather they basically are, like Pumbaa speculated, “balls of gas” (rigorously, not gas, but plasma); and most of them are indeed “burning billions of miles away.” (Our nearest star, the Sun, is an average of 93 million miles away from the Earth). Stars are mostly hydrogen gas, made up of trillions upon trillions of hydrogen nuclei, which coalesced and formed a cloud massive enough to exert a huge gravitational attraction that acts on itself to compact the gas and thus to increase its density.
Stars originate from “molecular clouds,” made mostly of hydrogen and helium. As the density of the cloud increases, meaning that the hydrogen nuclei get closer together, the temperature and density of the system also increases. Without going into other details, what happens, if there are enough hydrogen nuclei, is that the pressure and temperature at the core of the collapsing, ever denser cloud of hydrogen gas increase enough to eventually cause the hydrogen nuclei to undergo a process called nuclear fusion. Hydrogen nuclei fuse to yield Helium nuclei.
Once the nuclear fusion gets started, we have a Protostar, which would begin to shine and officially become a star as soon as it emits light and heat out to interstellar space. That emission may begin at least one million years after the nuclear fusion gets started, which corresponds to the time that it takes for the photons, the particles of light, to escape to outer space after a long ping-ball game in the dense core where the fusion is taking place. Once the “ball of gas” starts emitting light, we have a new star.
It is fascinating to realize that much of the raw materials that make up our human bodies were manufactured inside stars through a process known as Stellar Nucleosynthesis; that is, through the production of heavier atoms from lighter ones. The lightest elements -hydrogen and helium– were manufactured in the first instants of the Universe. But the rest were produced in stars. The production of those elements heavier than hydrogen and helium (the first two elements in the Periodic Table) is one of the consequences of the interplay of gravity and outer pressure that keeps a star in equilibrium.
A typical star is made mainly of gazillions of hydrogen nuclei, themselves manufactured in the first instants of our Universe. The high temperatures and pressures reached at the core of a star make possible the “burning” of the star’s “fuel”, hydrogen nuclei that are turned into helium nuclei through a process known as nuclear fusion (basically the same process that makes a hydrogen bomb so devastating). Since each helium nucleus is slightly lighter than the sum of the mass of the two hydrogen nuclei that yielded it, the lost mass is accounted for with the emission of energy (mainly electromagnetic radiation, which is massless but certainly energetic), in step with the equivalence between energy and mass as first postulated by Einstein in 1905.
As long as the star’s nuclear fuel is not exhausted, the star will shine and not collapse under its own weight; that is, it will not shrink as a consequence of its strong gravitational field. The energy produced by the process of nuclear fusion, in the form of heat and electromagnetic radiation (“light” of different frequencies), creates an outward pressure that counters the gravitational field and keeps the star in a state of equilibrium.
Once the star uses up all its hydrogen, the pull of gravitation creates the conditions for the fusion of helium into heavier elements and of those new, heavier elements into still heavier ones. The production through nuclear fusion of heavier elements keeps the star from collapsing under its own weight, until it reaches a point where it can fuse no more nuclei. Depending on the mass that the agonizing star has at that point, it could “die” in different ways. The “more productive” stellar death would be in the form of a supernova.
After the exhaustion of hydrogen, a star will continue to engage in nuclear fusion, until it yields iron as the heaviest element. Iron is very stable, meaning that the production of elements heavier than iron (like silver, gold and uranium) requires an enormous amount of energy, which the star cannot provide.
If the star is massive enough at this point, its gravitational collapse does the trick by producing a huge amount of gravitational energy, which is radiated outward and bursts away the outer layers of the star, in an explosion so powerful that it provides the energy for the production by fusion of nuclei heavier than iron. That exploding star is what we call a supernova. At the same time all those nuclei are scattered into interstellar space, providing the heavier elements that will be incorporated into the raw materials that in turn will end up in new stars and solar systems.
Our Sun is one of those second or third generation stars. Observations with spectroscopy techniques show that our Sun is made up mostly of hydrogen and helium, but that it contains traces of the rest of the elements of the Periodic Table, including carbon, oxygen, nitrogen, iron, gold and uranium. All those elements ended up in the Sun and its surrounding planets and also provide most of the raw materials of which our bodies are made, particularly carbon, oxygen, nitrogen, iron, phosphorus, calcium, sodium and so forth. We are literally made of stardust.