Astronomy HyperText Book: Stellar Evolution

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The HR Diagram

Stellar Evolution is driven entirely by the never ending battle between Pressure and Gravity . As imbalances are reached, the star is driven to find a new Energy source. Each new stage in stellar evolution is hence marked by a different energy generation mechansism. These stages are discussed below:

Structure of a Main Sequence Star Here see that a main sequence star has a simple structure. Pressure and gravitational forces are equal, the star is stable and its core is sufficiently hot to fuse Hydrogen into Helium . Eventually the core of the main sequence star will become pure Helium and that will mark a new evolutionary phase for the star. --> More about main sequence stars
Structure of a Post Main Sequence Star Now the star has two chemically distinct zones, a core of Helium surrounded by an envelope of Hydrogen. Since Helium has 2 protons in its nucleus then the fusion of Helium requires a higher temperature, to overcome this electrostatic barrier, than the fusion of Hydrogen. Hence the core of the star is not sufficiently hot for Helium fusion to occur initially. Without a source of core energy generation, the core cannot support itself against gravitational collpase and so it begins to collapse. As it collapses it heats. This heat is then transferred to a thin shell of hydrogen around the core which suddenly finds itself at a temperature in which hydrogen fusion can occur. As the core continues to collpase, the temperature in the hydrogen fusing shell continues to rise and thus the luminosity in the shell increases as does the pressure. The entire star is somewhat out of balance. The core collpase occurs because gravity exceeds pressure - the collapse is also aided by the pressure that the burning shell exerts on it. The burning shell, with its ever increasing temperature (due to transfer of heat from the collapsing core) supplies sufficient pressure to the outer layers of the star to cause them to expand. As they expand these outer layers cool. Thus, post main sequence evolution is defined by two external attributes which are manifestation of internal changes:
- Expansion and cooling of the envelope; the star is getting redder
- The luminosity of the star is increasing because the temperature of the hydrogen burning shell keeps increasing
This imbalance will continue until the star again finds a source of core energy generation.
Structure of a Red Giant Star Now the core of the star has heated to a temperature of 100 Million Degrees which is the threshold temperature for the fusion of Helium into Carbon . The star is again stable as pressure and gravity are equal. The lifetime of a star as a Red Giant is about 10% of its main sequence lifetime. The luminosity generated by the core fusion of Helium into Carbon is far greater than the shell luminosity associated with the fusion of Hydrogen into Helium.
MORE LATER!

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