After the red giant phase, low mass stars monitor a various evolutionary course than an ext massive stars. For this reason, we are going to very first consider what happens to short mass (less than 8 times the mass of the Sun) stars as they progress past the red huge phase. To really study and also understand stellar development in detail, you would desire to subdivide stars much more finely. That is, you would want to separately consider the evolution of stars the 0.1, 0.5, 1.0, 1.5, 2.0, 3.0, 5.0, and 8.0 solar masses, for example, and you would uncover differences between each. We space going to continue using a solar fixed star as our instance for short mass stellar evolution, but you have to realize that the details the the development of stars that 0.5 solar masses or 5.0 solar masses deviate from the general description presented below.

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During the red gigantic phase the a star"s lifetime, the core is no in equilibrium. All of the fusion is arising in a shell outside of the helium core, so over there is no energy generation or external radiation press to assistance the helium core. For this reason, the core of the star continues to collapse throughout the red huge phase. Collapse way an rise in temperature and also density in the core. In numerous low mass stars (from around 0.5 - 3.0 solar masses), the core can be compressed to the suggest that it becomes a degenerate gas. This has actually important after-effects in stellar evolution, so I will certainly briefly explain what this means.

The gas within stars is a soup of atomic nuclei and cost-free electrons. If friend compress a gas that this kind to a high sufficient density, you need to use 2 of the legislations of quantum mechanics to explain its behavior. This say:

Like electron bound in an atom, the cost-free electrons have the right to only have certain energies the you can represent as energy levels similar to the power level diagrams we used in our research of the Bohr version of the atom.No two the same electrons have the right to be found in the same energy level (the Pauli exemption Principle). Electrons deserve to have two different spins, i m sorry each have actually a slightly different energy, therefore you deserve to have two and also only two electrons per energy level, one v spin up, the various other with spin down.

The net result of these two quantum mechanical results is that once the gas has actually been compressed come the suggest where countless of the lower energy levels have actually been filled, it begins to resist compression. Even though the physical state is still the of a gas, the is harder to compress a degenerate gas 보다 solid steel!


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For an ext detail on degenerate gases, see Stellar Remnants at www.astronomynotes.com.


At some suggest after the main point has become degenerate, the core temperature reaches approximately 100 million kelvin, creating the appropriate conditions for 3 helium nuclei to fuse with each other to type one carbon cell core (these carbon nuclei can also fuse with secondary helium cell core to kind one oxygen nucleus). This is described as the triple-alpha process, and it is an alternative blend process to the proton-proton chain friend learned about previously. In stars with degenerate cores, when this triple-alpha process begins, the whole core ignites at once in what is well-known as the helium flash. The star is currently in the main point helium blend phase the its lifecycle. Contradictory to what your intuition might tell you, in this phase, the external layers the the star actually gain smaller and also hotter (the helium flash occurs in the core of the star in a really short time duration and is not able come be observed directly). Together you can see in the HR diagram below (Fig. 6.4), the evolutionary track of a Sun-like star now moves the star ago towards the key Sequence. This an ar of the HR diagram is dubbed the horizontal branch, due to the fact that stars in this phase of their evolution populate a narrow, practically horizontal box that extends come hotter temperatures from the red giant an ar of the diagram.


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The horizontal branch step of a star"s life is much much shorter than the key Sequence step of that lifetime. The star will convert every one of its core helium into carbon and also oxygen, and also then combination will end once again. The core will certainly again start to collapse inward v no radiation pressure to support it. Since there is still so much helium and also hydrogen exterior of the core of the star, after main point helium fusion ends, the raised temperature can once again ignite shell helium combination just external of the carbon/oxygen core, and shell hydrogen fusion can continue outside that the helium shell. Throughout this second phase of shell fusion, the outer layers of the star will broaden again, however this time through an even larger amount. In this phase, the star can be called an asymptotic large branch star, or sometimes a red supergiant star. Because that example, the star Antares is an M type supergiant. It has a luminosity 13,000 times the of the Sun.

For short mass stars, this is the last stage that their life time in which castle generate power via fusion. As soon as the helium and hydrogen shell blend uses up every one of the accessible fuel, the star"s life is successfully over. However, the star will certainly still leave behind two visible remnants. In the complying with table (6.1) us summarize few of the nature of a usual Sun-like star throughout its energy-generating lifetime:

Table 6.1: an overview of the temperature and luminosity transforms in a Sun-like star throughout its evolutionEvolutionary StageDurationTemperatureSpectral TypeLuminosityRadius
Main Sequence1010 years~6000 KG1 LSun1 RSun
Red Giant109 years~3000 KK - M~2000 LSun~150 RSun
Horizontal branch108 years~4500 KG - K~100 LSun~20 RSun
Asymptotic huge branch107 years~3000 KK - M~10,000 LSun~200 RSun

There is one last building of stars come consider, due to the fact that it theatre a more far-reaching role as stars age; this is the stellar wind.

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In this movie, we see an artist"s impression the the solar wind, which is the present of high power particles gift emitted by the Sun"s corona. Throughout the key Sequence life time of Sun-like stars, this wind is not very strong—that is, the complete amount of mass being lost by the sunlight is small. However, during the later phases that a star"s evolution, the massive loss rate connected with the stellar wind deserve to increase significantly. By the moment of the helium flash, a Sun-like star the initial mass 1.0 MSun may have actually only 0.7 MSun remaining. By the time the star has completed its atom fusion, there will be a significant amount of expelled gas indigenous its envelope in its immediate vicinity.