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From the <a href="http://www.astro.cornell.edu/courses/astro201/evol_sun.htm">Astro 201</a> course at Cornell:

<h1> The Evolution of the Sun</h1>

<ul>
<li><p> A cloud of gas and dust begins to contract under the force of gravity.
In regions of <a href="star_birth.htm">star birth</a>, we find gaseous nebulae
and molecular clouds.
These sites of pre-birth are dark patches called globules.</p>

<li><p> The protosun collapsed. As it did, its temperature rose to about
150,000 degrees and the sun appeared very red. Its radius was about 50 present
solar radii.</p>

<li><p> When the central temperature reaches 10 million degrees, nuclear
burning of hydrogen into helium commences.</p>

<li><p> The star settles into a stable existence on the Main Sequence, generating
energy via <a href="hydrogen_burn.htm">hydrogen burning</a>.
This is the longest single stage in the evolutionary
history of a star, typically lasting 90% of its lifetime.
<a href="fusion_control.htm"> Thermonuclear fusion</a>
within the Sun is a stable process, controlled by its
internal structure. </p>

</ul>
<table width=800>
<tr>
<td width=500>
<ul>
<li><p> The hydrogen in the core is completed burned into helium nuclei.
Initially, the temperature in the core is not hot enough to ignite helium
burning. With no additional fuel in the core, fusion dies out. The core cannot
support itself and contracts; as it shrinks, it heats up. The rising
temperature in the core heats up a thin shell around the core until the
temperature reaches the point where hydrogen burning ignites in this
shell around the core. With the additional energy generation in the H-burning
shell, the outer layers of the star expand but their temperature decreases
as they get further away from the center of energy generation.
This large but cool star is now a <b> red giant</b>, with a surface temperature
of 3500 degrees and a radius of about 100 solar radii.</p>

<li><p> The helium core contracts until its temperature reaches about
100 million degrees. At this point, <a href="helium_burn.htm">helium burning</a>
ignites, as helium is
converted into carbon (C) and oxygen (O). However, the core cannot
expand as much as required to compensate for the increased energy
generation caused by the helium burning. Because the expanion does not
compensate, the temperature stays very high, and the helium burning proceeds
furiously. With no safety valve, the helium fusion is uncontrolled and
a large amount of energy is suddenly produced. This <b> helium flash </b>
occurs within a few hours after helium fusion begins.</p.

<li><p> The core explodes, the core temperature falls and the core contracts
again, thereby heating up. When the helium burns now, however, the reactions
are more controlled because the explosion has lowered the density enough.
Helium nuclei fuse to form carbon, oxygen, etc..</p>

<li><p> The star wanders around the red giant region, developing its
distinct layers, eventually forming a carbon-oxygen core.</p>

<li><p> When the helium in the core is entirely converted into C, O, etc.,
the core again contracts, and thus heats up again. In a star like the
Sun, its temperature never reaches the 600 million degrees required for
carbon burning. Instead, the outer layers of the star eventually become
so cool that nuclei capture electrons to form neutral atoms (rather than
nuclei and free electrons). When atoms are forming by capturing photons
in this way, they cause photons to be emitted; these photons then are
readily available for absorption by neighboring atoms and eventually
this causes the outer layers of the star to heat up. When they heat up,
the outer layers expand further and cool, forming more atoms, and releasing
more photons, leading to more expansion. In other words, this process
feeds itself.</p>

<li><p> The outer envelope of the star blows off into space, exposing the
hot, compressed remnant core. This is a <a href="nebulae_planetary.htm">
planetary nebula </a>.</p>

</ul>
</td>
<td width=0>
<img src = "images/sun_life.gif">
</td>
</tr>
</table>

<ul>
<li><p> The core contacts but carbon burning never ignites in a
one solar mass star. Contraction is halted when the electrons become
<b> degenerate</b>, that is when they can no longer be compressed
further. The core remnant as a surface temperature of a hot
10,000 degrees and is now a <b> white dwarf </b>.</p>

<li><p> With neither nuclear fusion nor further gravitational collapse
possible, energy generation ceases. The star steadily radiates is energy,
cools and eventually fades from view, becoming a <b> black dwarf</b>.</p>
</ul>

<p> With this understanding of how the Sun will evolve, we can
<a href="sun_hrtrack.htm">follow its evolution</a>
on the <a href="hr_diagram.htm">HR diagram</a>.</p>
<hr>
<p><b> Suggested readings: </b> </p>


<ul>
<li><p>
"Giants in the Sky: The Fate of the Sun", Kaler, James, B.,
1993, <i> Mercury</i>, <b>Mar-Apr</b>, pp. 34-41.</p>
</ul>