Stars

“…For the compressed core that’s getting ever hotter, nature intervenes with a new physical process: nuclear fusion. When a collection of atoms gets sufficiently hot and dense, they slam together with such force that they can meld more deeply than they do in chemical processes like the burning of natural gas. Whereas chemical burning is a reaction that involves the electrons that surround atoms, nuclear fusion is a reaction that joins nuclei at the center of atoms. Through such deep melding, nuclear fusion generates copious quantities of energy manifested as rapidly moving particles. And it is such rapid thermal motion that generates outward pressure capable of balancing the inward force of gravity. Nuclear fusion in the core thus halts the contraction. The result is a concentrated, stable, and sustained source of heat and light. A star is born…” (63).

“For almost five billion years, [our] sun has supported it tremendous mass against the crushing force of gravity through the energy produced by the fusion of hydrogen nuclei in its core. That energy powers a frenzied environment of fast-moving particles that exert a strong outward pressure. And much like the pressure produced by an air pump that props up a child’s inflatable bounce house, the pressure produced by fusion in the sun’s core props up the sun, keeping it from collapsing under its own enormous weight. This standoff between gravity pulling inward and particles pushing outward will hold firm for about another five billion years. But then the balance will be upended. Even though the sun will still be chock-filled of hydrogen nuclei, hardly any will be in the core. Hydrogen fusion produces helium, nuclei that are heavier and denser than hydrogen, and so just as sand poured into a pond displaces water as it fills the pond’s bottom, helium displaces hydrogen as it fills the sun’s center.

“That’s a big deal. The center of the sun is where you find its hottest temperatures, currently about fifteen million degrees, well in excess of the ten million degrees required to fuse hydrogen into helium. But to fuse helium nuclei requires a temperature of about one hundred million degrees. Because the sun’s temperature is nowhere near that threshold, as helium displaces hydrogen in the core, fusion’s fuel supply will dwindle. The outward pressure from fusion’s production of energy in the core will subside, and consequently the inward pull of gravity will gain the upper hand. The sun will begin to implode. As its spectacular heft collapses inward, the sun’s temperature will skyrocket. The intense heat and pressure, still shy of the conditions necessary for helium to start burning, will spark a new round of fusion within a thin shell of hydrogen nuclei surrounding the helium core. And with such extreme conditions, hydrogen fusion will proceed at an extraordinary pace, producing a more intense outward push than the sun had ever experienced, not only halting the implosion but thrusting the sun to swell tremendously.

“Earth’s surface temperature will soar into thousands of degrees, hot enough to dry out the oceans, eject the atmosphere, and flood the surface with molten lava…” (250-51).

Greene, Brian. Until the End of Time: Mind, Matter and Our Search for Meaning in an Evolving Universe. New York: Alfred A. Knoff, 2020.