Language
Country/Area
No result found
Did you know why the AGB star is so special in the Hertz-Plan-Russell diagram?
In the evolution of interstellar space, AGB stars, or the asymptotic giant branch, are part of the evolutionary stage of stars, specifically giving an important position to low- to medium-mass stars (about 0.5 to 8 times the mass of the sun). The evolution of these stars at the end of their lives makes them stand out in the Hertz-Plan–Russell diagram.
AGB stars usually appear as bright red giant stars, with luminosities that can reach thousands of times that of the Sun. Its internal structure consists of an inert core composed mainly of carbon and oxygen, a helium shell undergoing helium fusion, and a hydrogen shell undergoing hydrogen fusion.
When a star's core runs out of hydrogen fuel, the core begins to shrink and heat up, causing the outer layers to rapidly expand and cool, forming a red giant star. This process causes the AGB star to move upward to the right on the Hertz-Plann–Russell diagram, ultimately forming its unique astronomical path.
As the star's core temperature reaches about 3×108 K, the process of helium fusion begins. At this point the star's cooling and increase in luminosity will stall, and it will move downward and to the left, forming what is called a horizontal branch or blue ring. When the helium is completely burned, the star's path will move to the right and upward again, toward the AGB star stage.
The AGB stage is further divided into early AGB (E-AGB) and thermal pulse AGB (TP-AGB). In the E-AGB stage, the main source of energy is the fusion of helium around a carbon and oxygen core. During this stage, the star expands to giant size, with a radius of about one astronomical unit.
Once the helium shell is depleted of fuel, the TP-AGB phase begins. The energy required by the star at this time comes from the fusion of hydrogen in the thin shell. This process causes an instantaneous explosion of helium shells, called a helium shell flash, causing the star's luminosity to spike momentarily and then decrease sharply over several years.
This helium flash causes the star to expand and cool, triggering strong convection currents that pave the way for later interstellar chemical reactions. First, the mixing of material from the core region to the outer layer is called "stirring". This process will generally affect the spectral characteristics of AGB stars.
In addition, the mass loss of AGB stars causes them to be surrounded by a rich layer of interstellar surroundings, which may form gas clouds up to 30 light-years high. This process becomes an important source of dust generation in the universe during different stages of a star's life.
In the periphery of the AGB star, low-density chemical reactions cause the chemical properties of the region to change with distance. As the material expands and cools, initial reactions tend toward thermodynamic equilibrium. After the density drops to a certain level, kinetic effects dominate and many reactions become impossible.
The ultimate fate of AGB stars usually ends in a planetary nebula (PNe), which forms in the core of a burned-out star. Astonishingly, physical samples such as presolar pearl grains have been analyzed in the laboratory, demonstrating the influence of these stars in the universe and their chemical peculiarities.
Have you ever thought about how the evolution of AGB stars will affect the chemical composition and structure of the entire universe?
