It won't happen tomorrow, but inevitably the Sun will die. Yes, our star will cease to exist. During that event, Earth—and our descendants—will disappear with it. What will that moment be like? What will happen to the Sun? We explain the process by which a dying star becomes a monster capable of devastating a large part of the planetary system that surrounds it.
Like it or not, our destiny as inhabitants of Earth has only one end: everyone dies. It sounds ugly, we don't normally think about it, but it is absolutely real. There is a 100% probability that our planet will turn to ashes within a few billion years. It is very possible that if by then we haven't succeeded in annihilating ourselves, we will have already colonized a good part of the Galaxy. But Earth and everything on it will disappear with the Sun. Just as it happens—and will happen—with all the other stars in the universe, the Sun will die someday. But, how does the process that ends the life of a star occur?
The End of a Star
To know how the end of any star comes, we must first be clear about some basic concepts about its internal functioning. We can think of a star—and our Sun is one of them—as a large ball of gas. At that scale, the gravitational forces between the atoms that compose it prevent it from dissipating into space. As you may have noticed, all stars emit energy—much of it in the form of visible light—toward the space around them.
That enormous amount of energy originates from the thermonuclear fusion reactions that develop in its interior. Simply put (assuming that word can be used in this context), the weight of the gaseous mass itself is enough to force the atoms to “squeeze” together so much that those reactions take place.
As with any terrestrial fission or fusion nuclear reactor, some kind of fuel is needed for the “magic” to happen. During most of a star's life, that fuel is hydrogen, which fuses—four atoms “join”—to form a helium atom. Physicists explain that when this fusion occurs, “excess” mass from the original atoms is expressed as energy that is emitted during the process. That is the origin of the heat, light, and all the radiation that the Sun emits across the electromagnetic spectrum.
But this is not only important because of the energy emitted; all this thermonuclear activity provides a strong outward pressure that maintains the structural integrity of the star. If these reactions were to stop suddenly, the gravitational force between the gas atoms forming the star would collapse it. In some way, the Sun has an exact equilibrium between the expansive force generated by the processes in its core and the gravity provided by its enormous mass. The size of a star at any given moment is due precisely to the relationship between these parameters.
Helium Poisoning
As long as the amount of available hydrogen is large, the star's life proceeds peacefully. The Sun is currently in that phase of its existence. But nothing lasts forever, and stellar fuel eventually runs out. When a star begins to deplete its hydrogen reserve, the helium formed over millions of years begins to interfere with the process. It may even happen that thermonuclear reactions stop. Physicists call this process “helium poisoning.” This causes the amount of energy produced inside to drop abruptly, so the original pressure that kept it “standing” decreases to—comparatively speaking—almost ridiculous levels.
The star contracts due to gravitational effects, and its temperature rises. At that moment the Sun will stop being a gentle beast that provides us light and warmth and become something much more dangerous. Around the hot, dense helium core, the remaining hydrogen begins to burn, but in increasingly outer “layers.” As a result, the star begins a new expansion process. Although its core remains very hot, the outer layers cool down and its color begins to shift toward red. Within about five billion years—you better get your affairs in order—the Sun will go through this stage and become a red giant.
Unfortunately, it's not just a small aesthetic change or a color change. The Sun will grow so large that it will surpass the orbits of Mercury (for sure) and Venus (very likely). Those two planets of the Solar System will be history as they end up inside the Sun. Of course, no one will be on Earth to “enjoy” the spectacle because long before, about 800 million years from now, the progressive increase in the Sun's temperature will have made our planet's average temperatures about 150 degrees Celsius. Finally, in about 7 billion years, the Sun will engulf Earth and the Moon. Within billions of years more. Completely oblivious to our problems, the transformations inside the Sun will continue their inexorable course.
The size of the star is very important when predicting how its end will be. If it is small enough, the compression of the core begins to decrease due to the effect of degenerate free electron gas. Despite its name, it is a quantum effect that occurs because the electrons surrounding the plasma, formed by atomic nuclei, cannot occupy the same quantum states. At some point—and because of this effect—the temperature of the stellar core rises to the ignition point of helium (about 100 million degrees). The Sun is the right size for this to happen, and as a result there will be a brief explosion—which physicists call “the helium flash”—that marks the beginning of a new stage in the already complicated life of the star.
From that moment on, thermonuclear combustion of helium begins, whose fusion results in heavier elements such as carbon and oxygen. When helium begins to run low, the temperatures and pressures in the interior of the dying Sun will allow carbon atoms to fuse to produce neon, gaining a little more time. Anyway, a few hundred million years after entering the red giant phase, almost all the available fuel inside the Sun will have been exhausted, and there will be no way to sustain any kind of nuclear reaction. Gravity will compress the star again, and the final catastrophe will be very near.
The Sun as a White Dwarf
All the mass of the Sun will be crammed into a much smaller volume than it currently occupies, becoming what is called a white dwarf. This white dwarf will be composed of the remains of the original core, but compressed to occupy a volume roughly that of the Earth. All the material that formed the outer layers of the red giant heats up and ionizes due to the radiation emitted by the new core. This gives rise to an impressive spectacle made up of complex and curious filaments like those we can see today in the “cat's eye nebula.” Astrophysicists call this gas a “planetary nebula.”
The Sun will remain for a very long time in its new white dwarf state. In fact, this type of star can maintain its integrity thanks to the pressure of its degenerate electrons (again!), and their lifetime exceeds the current age of the universe. It is a very slow cooling process, since in that state they do not create their own energy as they have exhausted their nuclear fuel.
The death of a star is determined by the size with which it was born and also by how it managed its energy throughout its life. A star that has about sixty times the mass of the Sun “lives” only three million years, much less than the billions of years our star will still live. The end of a very massive star occurs through a very rapid collapse, which we know as a “supernova.” The Sun is too small to have such an end. However, it will still manage to take Earth and everything on it with it at that moment. You have been warned.
