Stellar evolution problem

In summary, the theory of stellar evolution explains how stars are formed and evolve over time. It begins with the collapse of gas clouds due to various mechanisms, forming clumps that eventually become protostars. As the protostar becomes denser and hotter, it begins to fuse hydrogen in its core, becoming a main sequence star. As the hydrogen is used up, the star expands and cools, becoming a red giant. For lower mass stars, this leads to the formation of a planetary nebula and a white dwarf, while for higher mass stars, it can lead to the formation of a neutron star or black hole. However, there are still some unanswered questions and uncertainties in this theory, such as the exact mechanisms of gas cloud collapse
  • #1
I am new to astronomy, though really interested in it
Just read up on stellar evolution from multiple sources, but have some questions about it

Let me summarize the theory of stellar evolution
1. gas clouds collapse due to multiple mechanisms, e.g. shock wave, collision of clouds...
2. the cloud fragments into clumps of 10-50 solar mass, each clump continue to collapse due to gravity, and become denser, hotter, more massive due to the infalling mass
3. the clump becomes a disc with a dense core due to conservation of angular momentum, a protostar forms
4. the protostar becomes opaque and cannot emit heat through radiation, temperature increases quickly, and the core starts to fuse hydrogen, generating more heat
5. when the internal pressure due to heat in balanced with gravity, the star stop collapsing, a main sequence star forms
6. hydrogen in core is used up, and core collapse, increase in temp and fuse helium, heat transferred to outer layer and outer layer expand and cools, and become red giant
7. if mass is less than 5 solar mass, the star will eject enough material to form a planetary nebula, so that it's mass is less than 1.4 solar mass, and white dwarf forms due to electron degenerancy pressure
8. for higher mass, fusion continue up the periodic table until iron forms, and the may not eject enough material and a neutron star or black hole forms

Some questions in my mind, please could anyone answer
1. Please correct if there is anything wrong in my summary
2. in my summary point 1, what are actually the mechanisms, in greater detail?
3. in point 6, it seems it doesn't make sense for the outer layer to expand and cools and become is actually gaining heat from the inner layer, and the expansion is due to heating, why does the outer layer "over-expand" that it actually cools despite receiving heat? In analogy, to me it's like kicking a ball and the friction/air resistance is so great that the ball moves backwards

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  • #2
The following regards radiation pressure which hopefully sheds some light on the red giant phase.

Wolf-Rayet stars are massive stars of over 20 solar masses with strong stellar winds of up to 2000 km/s that blow off the outer layers of the star, exposing the helium (or in some cases, the carbon) shell. The strong winds are due to the fact the radiation pressure (the force of light pushing on the gases) in the outer layers is quite strong. This causes them to lose there mass rapidly, a WR star loses between 10^-6 and 10^-5 of a solar mass per year whereas the sun loses 10^-14 of its own mass on a yearly basis. This is equal to about 1 solar mass every million years for the WR star.

In a star the size of our sun, the radiation pressure caused by the fusion of hydrogen into helium is enough to stop the star from collapsing but once the supply of hydrogen in the core has been exhausted, the hydrogen to helium process switches to a shell outside the core and the helium core begins to produce carbon. This combined fusion process of the hydrogen/helium and helium/carbon increases the radiation pressure considerably and pushes the outer hydrogen shells out, this reduces their density and the temperature drops, the plasma in these outer shells becoming red hot as appose to yellow/white hot.

In addition to your summary, it is hypothesised that stars of between 20-40 solar masses upon reaching the supernova stage, collapse into a black hole. Stars of between 40-50 solar masses collapse directly into black holes without a supernova.


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