💡DI What is a red giant?
• GIGANTE ROJA 🔴 | ¿Qué es una GIGANTE ROJA? 💡
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A red giant is a low- or intermediate-mass giant star (less than 8-9 solar masses) that, after consuming the hydrogen in its core during the main sequence phase, converting it into helium through nuclear fusion, begins to burn hydrogen in a shell around the inert helium core. This initially results in an increase in the star's volume and a cooling of its surface, giving it a reddish color. In this phase prior to the red giant phase, the star is called a subgiant. At a certain point, the star's atmosphere reaches a critical minimum temperature below which it can no longer descend, forcing the star to increase its luminosity and volume at a virtually constant surface temperature (i.e., color). The star swells to a typical radius of about 100 million km: the star has thus become a red giant. Throughout this process, the energy emitted by the giant comes from the aforementioned shell and from the conversion of gravitational energy into heat by the virial theorem.
During the main sequence phase, as thermonuclear reactions produce helium, it accumulates in the center of a star due to its greater density (it is heavier than hydrogen). As hydrogen is consumed by fusing into helium, when a critical amount of helium is reached, the internal pressure decreases, and the star reacts by compressing and heating up a little more until it becomes impossible to fuse the little hydrogen remaining in its center. The star is then said to have suffered helium poisoning. Once the hydrogen is exhausted, the helium core cannot support the weight of the star and begins to compress, triggering the star's transformation into a red giant.
If the star is sufficiently low in mass, the degenerate free electron gas partially stops the compression. The temperature rises to the ignition point of helium, around 100 million degrees. In stars more massive than this limit, this transition occurs smoothly, as the gas has barely degenerated when the core ignites. In stars with masses between 0.5 and 2.5 solar masses, however, the core is partially degenerate and intensifies its reactions as its temperature increases. This continues until, suddenly, it returns to the ideal gas regime, producing a thermal avalanche with a powerful explosion in which energies comparable to those of a supernova are released, but which does not endanger the integrity of the star because most of this energy is used to eliminate the electron degeneracy: it is the helium flash. Finally, in stars, the central temperature is never high enough for helium fusion to occur. However, the evolution of these stars is so slow that there has not yet been time since the formation of the universe for an isolated star of that mass to evolve into a red giant.
The ignition of the helium ends the red giant phase. Although this process is somewhat violent, it does not affect the integrity of the star, which will continue for a few more million years in a new, stable phase of red clump (if its metallicity is high), or horizontal branch (if its metallicity is low), fusing the new fuel. The star descends again in the Hertzprung-Russell diagram, but is always more luminous and cooler than during the main sequence phase.
The outer layers of red giants are only weakly gravitationally bound, so mass loss is significant at this stage. Furthermore, the convective zone of red giants is very deep, so shock waves contribute to further accelerating the stellar wind. On the other hand, these stars emit a lot in the infrared part of the spectrum, which is strongly absorbed by stardust, which receives more momentum and transmits it to the gas. Finally, greater metallicity (which leads to greater opacity) also causes greater ejections of matter. The cumulative mass loss between the red giant and asymptotic giant phases is estimated at between 40 and 60% of the star's total initial mass.