What are the Detailed Steps and Mechanisms of Stellar Evolution?

[Valium]

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 red...it 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

Thanks

 

[stevebd1]

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.

http://en.wikipedia.org/wiki/Wolf-Rayet_Star

regards
Steve

 

[sir-pinski]

for your interest in astronomy and for taking the time to read up on stellar evolution! It's a fascinating topic and there's always more to learn and understand.

1. Your summary is overall correct, but there are a few minor details that could be clarified. For example, step 2 should mention that the clumps form due to the collapse of the gas cloud, not just continue to collapse. And step 7 should mention that the white dwarf forms due to electron degeneracy pressure, not just mention that it exists.

2. The mechanisms for gas cloud collapse can vary, but some common ones include the shock wave from a supernova explosion, the gravitational pull of a nearby massive object, or the collision of multiple gas clouds.

3. In step 6, the outer layers do not necessarily gain heat from the inner layers. As the core fuses heavier elements, the outer layers expand due to the increased pressure and temperature, causing them to cool and appear red. It's not quite like kicking a ball, as the expansion is not due to friction or resistance, but rather the result of increased energy and pressure within the star's core. Think of it more like blowing up a balloon - the air inside expands and cools, causing the balloon to grow larger.