Stellar Evolution and Nuclear Burning: From Main Sequence to Stellar Death

The process occurs increasingly faster, so that while helium takes 1 million years to be exhausted in the core of a star of about $20M_{\odot}$, carbon takes less than a thousand years, neon 1 year, oxygen in the stellar core is exhausted in a few months, and silicon in less than a day. At this stage, the star is composed of different layers, the outermost of which is composed of hydrogen, then one of helium and nitrogen, then helium carbon and neon, a successive one of oxygen and carbon, another of oxygen neon and magnesium, a final layer composed of silicon and sulfur, and finally, in the central core we have inert nickel and iron. When iron begins to be synthesized, its production no longer increases the star's energy but requires it, and the central core collapses increasingly rapidly. In a fraction of a second, the star emits as much energy as hundreds of Suns throughout their entire existence and produces a shock wave so powerful as to expel all the material that composed it and spread it into the interstellar medium, enriching it with all the heavy elements that the star had produced during its evolution along with those elements. To the elements produced by the star during its evolutionary path must be added immense quantities of elements produced by the explosive nucleosynthesis of the Supernova which, together with the previous ones, spread into space enriching the interstellar medium. If the stellar mass is less than $20M_{\odot}$, in addition to the rest of a Supernova, we have that the collapse of the stellar core stops when the star reaches the dimensions of a few tens of kilometers: thus a Neutron Star is born. Conversely, if the stellar mass is even greater, we have no known force capable of hindering the star's collapse. The star's core reaches dimensions smaller than its own Schwarzschild radius, i.e. $r_s = \frac{2GM}{c^2}$; and the core becomes so compact that not even light can escape from it, thus generating a so-called Black Hole. Summary. Summarizing the typical evolution of a star of mass similar to the solar one, we can observe the following evolutionary phases: hydrogen shell burning. This occurs when the star consumes its initial supply of hydrogen in the core and begins to collapse again. The temperature and density in a shell around the core reach a temperature high enough to produce hydrogen fusion. This causes the star to expand, leaving the main sequence and ascending the Red Giant branch. During this time, the helium in the core is inert. helium core burning. Once the star exhausts its hydrogen reserve in the shell, the star collapses again and the temperature rises rapidly. The dense helium core suddenly reaches the ignition temperature through Helium flashes and evolution changes discontinuously toward lower luminosity and the star shrinks again in size. Now it is stable again, on a Helium Main Sequence whose burning duration will however be much shorter than before. helium shell burning. When helium is exhausted in the core, the star begins helium fusion in a shell around the core which is now essentially carbon. Again the core contracts and the star expands. In the Hertzsprung-Russell diagram, the star moves again toward a Red Giant stage along the so-called Asymptotic Giant Branch. Due to the temperature dependence of helium fusion, the star is unstable and during this stage thermal pulses begin that cause the expulsion of the Planetary Nebula. carbon burning. More massive stars can repeat this scenario with collapse to a stable carbon burning stage, with increasingly shorter duration. End of life. Stars of various masses behave differently in detail, but in the end they all undergo the same fate of collapse due to the exhaustion of available nuclear fuel. Depending on the stellar mass, and thus the pressure exerted by the core, oxygen, neon and silicon burning can occur. Once the final collapse takes place, its violence and the forces involved determine the subsequent stellar evolution. The White Dwarf is the destiny of stars of mass comparable to the Sun, after the Planetary Nebula stage removes all the outer layers.