In 2011, the Kepler space telescope recorded the outburst of a red supergiant. At the moment of maximum brightness it became a billion times more luminous, that is, brighter than all the stars in the Galaxy put together, about 100 billion stars.
What is it that leads to dying in such a catastrophic way? To understand it, we must bear in mind that the life of a star is a continuous struggle against its own gravity, which tends to concentrate all the mass in the center. The only way to prevent it is to use the energy released by the central nuclear furnace. But what happens when you run out of fuel? Then gravity kicks in again, and gravitational collapse threatens their future. In the case of the Sun, this will happen when the hydrogen in the core runs out, and that is where the nuclear life of our star will end. But in those that are much more massive, the situation is totally different: when the hydrogen is finished, the nucleus contracts enough to increase the temperature and start the combustion of helium. Fusion of helium to carbon releases less energy than fusion of hydrogen to helium, so the reaction must go faster to generate enough energy to maintain the star’s structure: in a million years, the helium in the core runs out. As the helium fuel tank nears reserve, the core contracts again to increase its temperature until fusion of carbon and helium into oxygen occurs.
Once all the helium is exhausted, the star must increase the temperature of the core to 600 million degrees: then, in addition to oxygen, a panoply of other elements such as sodium, magnesium, etc. is formed. This reaction releases half of the energy of helium, then the star can only sustain it for 100,000 years. Then it is the turn of oxygen, which produces silicon and sulfur, in a series of reactions that last only 10,000 years. This will be a constant in the rest of the star’s life: the next nuclear reaction will produce less energy, around a tenth of the previous one. As one pathway closes and the next opens, the star acquires the characteristic onion-shell structure: a dense core made up of heavy atoms covered by successive layers of oxygen, carbon, helium and hydrogen.
At the moment when the fusion of silicon begins to produce iron, the fate of the star is cast. This one-day trip ends with a spectacular explosion that our galaxy witnesses every 50 years: the supernova.
As the combustion of silicon ends, the core begins to contract and its temperature rises above 5,000 million degrees. The energy released is so intense that the star begins to undo the work it has been doing throughout its life, since the photons generated are so energetic that they break the iron nuclei into helium, a process that also robs the star of energy. In order to maintain its structure and not collapse, the core begins to compress, faster and faster, increasing its density. When it reaches 10,000 tons per cubic centimeter, the electrons reach enough energy to convert protons into neutrons, stealing more energy from the star. As if that were not enough, this process releases neutrinos, which escape torrentially from the star. The loss of energy is fast and inexorable and gravity is doing its job, collapsing the core of the star more and more quickly, which increases the density and triggers the creation of neutrons. To get an idea of this forward flight, imagine the Earth shrinking to the size of Madrid or Barcelona in less than a second.
Is there nothing to stop this contraction? When the density reaches an unimaginable 100 million tons per cubic centimeter, all the atomic nuclei break apart and what remains is a soup of soup of neutrons and other subatomic particles that have names as unique as pions. The core of the star, which would collapse under the action of gravity, is supported by degeneracy pressure. The weight of the star, which tends to concentrate all the mass in the center, does not win because two particles of matter cannot be in the same place at the same time. And the collapse stops.
But this does not occur throughout the interior of the star. The densest part of the core, the central part, sinks in less than a second. Meanwhile, the outer zone, which still contains a good amount of iron, is left with nothing to support it and is collapsing at a speed of 60,000 km/s, 20% the speed of light. But suddenly the crash stops; matter in free fall hits a wall a hundred billion times harder than brick, the degenerate neutron core, and a rebound occurs: the matter is shot out while generating a shock wave that travels through 10,000 km/s and passes through matter in its fall. Under normal conditions at that speed the wave would reach the surface of the star in 30 minutes but the collapsing core matter practically stops it. The neutrinos that are created at a temperature of 5,000 million degrees accumulate without being able to escape due to the very high density that matter reaches, interacting with neutrons and turning them back into protons. The situation is such that these neutrinos, which during the life of the star move outward as if there were no matter in their path, take 10 seconds to leave, which means a million times longer.
While this is happening in the core, the rest of the star’s envelope is unaware of what is going on below and that it should collapse. His situation is like the Coyote in the Road Runner cartoon: he doesn’t know that he has ended up on a cliff and must fall. Though in truth it never will. As it travels outwards, the shock wave increases its speed because the matter it encounters has a lower density. In a matter of minutes it reaches the surface, launching matter at thousands of kilometers per second and causing such a brightness that the star emits, in a few weeks, as much energy as the Sun in its last 4,500 million years of existence. And the star explodes.
But this process involves more than just destruction. In the region located behind the shock wave, rich in heavy elements and neutrons, all the atoms above iron in the periodic table, such as gold, silver, platinum, uranium or carbon, are created in a few seconds. americium from our smoke detectors.
But not everything ends here. Matter from the star collides with the surrounding interstellar medium, heating it to a temperature of a million degrees, while continuing an expansion that will last for thousands of years, until 100,000 years later the remnants of what was once a giant star will dissolve. entirely in the interstellar medium. Now, the material from the star expelled into the cold of space will form gas clouds in which, with luck, millions of years later new stars will appear: this was the origin of our Solar System.
