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Animation of the steps listed below:
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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The Electronic Universe Project
e-mail: nuts@moo.uoregon.edu