Stellar Evolution: From Red Giants to Black Holes

outward radiation necessary to counteract the force of gravity. Consequently, the nucleus contracts until the pressure given by the electrons is sufficient to oppose gravity or until the nucleus becomes hot enough to begin helium fusion at $10^8$ K. The Red Giant Branch. As the star proceeds in the transmutation of hydrogen into helium through nuclear fusion, helium accumulates in the core, increasing its pressure and thus temperature and luminosity. If the stellar mass is between $0.7M_☉$ and $10M_☉$, when the star exhausts hydrogen fusion in its core, it contracts, raising its temperature. With the higher core temperature, other inner layers can trigger hydrogen fusion, resulting in the expansion of the ionized gas composing the star. Overall, the star notably increases its dimensions, becoming a giant, and its outermost layers cool down, becoming increasingly red. The star thus enters the Red Giant phase. In the Hertzsprung-Russell diagram, the star exits the diagonal line indicating the Main Sequence, following a track toward the upper right corner (Fig. 14). Then, once the temperature in the core has reached about $10^8$ K, helium combustion begins in the core, which stops the cooling of the star and the increase in luminosity. The star thus begins to move downward and leftward in the diagram along the so-called horizontal branch. Similar processes repeat every time the star must transition from one evolutionary phase to another. When the star exhausts one form of energy, the star's core contracts and, simultaneously, according to the third principle of organic evolution we previously described, the outer shell must expand. The first heats up to the point of triggering the new reaction that is the source of energy, while the second consequently cools and is eventually dispersed. Planetary Nebulae and White Dwarfs. When a star is exhausting helium to fuse, it begins a series of thermal pulsations that lead to instabilities and the emission of parts of the outermost layers. If its mass is not sufficient to trigger carbon fusion, then the instabilities become stronger, the star's core collapses, forming what is called a White Dwarf, while its outer gas part is emitted, forming what is called a Planetary Nebula. In the White Dwarf, the mass is not sufficient to trigger carbon fusion, so the only force preventing the core from imploding is the repulsive force of electrons dictated by Pauli's exclusion principle. What are now the physical remains of the star, essentially composed of carbon, cool slowly unless external intervention arises to revive their life. If, in fact, the White Dwarf acquires mass from external elements up to exceeding the Chandrasekhar limit, which equals $1.44M_☉$, carbon fusion can be triggered and the White Dwarf can become a Type Ia Supernova. During the previous phase of instability, the star has already expelled most of its mass, forming the Planetary Nebula. During the Red Giant phase, the star has engulfed any planets in its own Planetary System, which continued to orbit within the star in some form and continued to interact with it, giving rise to the peculiar structures and aesthetic forms of Planetary Nebulae that, like small flowers blooming, gracefully adorn Galaxies. Planetary Nebulae not only have an aesthetic purpose but also play an important role in enriching the elemental composition of the Galaxy. Following their evolution, stars form, through their self-transmuting action, increasingly heavier elements, thus transforming the hydrogen and helium present in the Universe. These elements are then redistributed in the Galaxy through the action of Supernovae as well as Planetary Nebulae. These usually contain larger proportions of elements like carbon, nitrogen, and oxygen, which are recycled in the interstellar medium through the powerful stellar winds generated in this phase of stellar evolution. Thus, successive generations of stars formed from such nebulae will tend to have higher metallicity with marked effects on stellar evolution and fusion reactions. Supernovae, Neutron Stars, and Black Holes. In the case of very massive stars, the process of contraction and expansion occurs repeatedly, passing through all six phases of exoenergetic nuclear combustion, namely hydrogen, helium, carbon, neon, oxygen, and silicon. Each of these phases