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Ted,<br>
<br>
As it happens, I've just started learning about stellar evolution and
main sequence stars for a project I am working on at home. Here's a
quick rundown of what I've found out so far (it's complicated - I still
have a lot to learn):<br>
<br>
As I understand it in general terms, the Sun is in the hydrogen-fusion
portion of it's life which is called the "main sequence". Stars spend
most of their lives in this stage in a general equilibrium. The fusion
from the core is in effect pushing back against the gravity of all that
hydrogen and other atoms. The hydrogen is being fused into helium at
the core, thus the makeup of the stellar core is changing. This will
continue more-or-less at an equilibrium (compared to other stars and
other phases of development) for about 10 - 12 billion years. The Sun
is thought to be about 5 billion years old at the moment. The age at
which a star uses up it's hydrogen fuel is based mostly on it's mass.
At some point in the future, the hydrogen that is hot enough to fuse in
the core will start to run out, and the core will have built up a large
amount of hot helium "ash" that is too cold to fuse (helium requires a
much higher temperature to start fusion). Thus, the force pushing
against the gravity of the rest of the Sun will be less and the Sun
will contract a bit which will raise the temperature and ignite some of
the hydrogen closer to the core. This will cause the Sun to expand a
bit after the hydrogen shell around the inner helium core has started
fusion. This is a "last gasp" before the fuel in the core runs out.
Once the hydrogen in the core is gone, there is no force pushing back
against the weight of the outer layers of the star and the core
collapses. The pressure internally skyrockets and gets quickly hot
enough to ignite some more of the layers surrounding the core. Since
there is more fuel in a larger volume surrounding the core, it expands
in luminosity and thus radius. Even though the luminosity is
increasing, the temperature at the surface will decrease because the
radius is increasing which is why the star turns red. It takes
millions of years to get through this phase, and then the hydrogen runs
out, the star collapses, and the temperature gets high enough to ignite
the helium at the core. The helium will burn out too, and then some
other element (carbon?) will ignite, etc. <br>
<br>
The spike that the article was talking about before the red giant stage
must be the "last gasp" I talked about above. I wasn't aware that
there would be a gradual increase in luminosity to that stage, but
maybe there is one.<br>
<br>
However, it's my understanding that the forces in the Sun, while
understood at a generic level of abstraction, are still quite chaotic
and unstable. Since the Sun is the major impactor on temperature here
on Earth (the second major impact being the internal heat from the
Earth's core), any small change in the Sun's output could have drastic
effects on us since we are extremely fragile. The Sun is hot enough to
keep us warm from a distance of about 11 light-minutes. I also
understand that the outer layer of the Sun is much, much hotter than
the core. The core is at about 5700K, the outer surface is at about 10
million K. Why the outer surface is so much hotter is still an
unsolved problem in astrophysics. This lack of understanding frightens
me. Solar outputs varies by about 0.04% on an eleven year cycle, and
solar flares can vary output by 5%. How stable is this phase of
stellar evolution, really? We've been around for a mere few million
years, only the last few thousand of which we've had the ability to
write things down. This is a small fraction of time for the Sun. Our
observations of the Sun has occurred over such a small timescale that
we can basically say that we've only seen it as a static observation.<br>
<br>
I think the 0.8 billion year number you quoted is the time it will take
for the slow process of hydrogen fusion at the core to raise the
Earth's temperature 5%. This seems to imply that the solar output will
increase slowly from now up until that value is reached. This is at
odds with other things I have read, which state that the Sun will cool
as the hydrogen in the core starts to run out before it increases again
right before becoming a red giant. When the Sun expands to be a red
giant, it will happen in a few million years and go from twice it's
output to some much larger number (which I haven't found, yet) that
will be enough to boil off the seas and the atmosphere both. However,
the variability of the Sun at this stage may affect us more than the
slow increase or decreases in temperature that we will see in this
current phase.<br>
<br>
I am not, however, making the claim that humans aren't impacting the
Earth by a large amount. Nor am I making any other claims having to do
with global warming. I am just letting you know what I have so far
assimilated, much of which may (and probably will be) found out to be
wrong. I am not staking out a position here, and will not feel
obligated to defend any of what I have said in the future. I'm not an
expert, and expect to continue to learn and unlearn things as we go.<br>
<br>
Paul<br>
<br>
<br>
Ted Moffett wrote:
<blockquote
cite="midd03f69e0704231323w6c982dffh331e1366bbf68cff@mail.gmail.com"
type="cite">
<div> </div>
<div>All-</div>
<div> </div>
<div>Our sun will eventually cause fatal increases in Earth
temperature in its inevitable evolution into a red giant star. When
will these temperature impacts become significant? The article
below estimates that in 800 million years the sun's impact will raise
Earth temperatures by 5 degrees, the same amount predicted by some
global warming models for the human impact on global temperatures in
the next century:
</div>
<div> </div>
<div><a href="http://www.sussex.ac.uk/press_office/media/media191.pdf">http://www.sussex.ac.uk/press_office/media/media191.pdf</a></div>
<div> </div>
<div> </div>
<div><a
href="http://72.14.253.104/search?q=cache:hAqjwfgBKT4J:www.sussex.ac.uk/press_office/media/media191.pdf+time+remaining+Earth+biosphere+sun+expansion&hl=en&ct=clnk&cd=6&gl=us">http://72.14.253.104/search?q=cache:hAqjwfgBKT4J:www.sussex.ac.uk/press_office/media/media191.pdf+time+remaining+Earth+biosphere+sun+expansion&hl=en&ct=clnk&cd=6&gl=us
</a></div>
<div> </div>
<div>"As a first application, let us ask how long it will take for
the temperature of the Earth to rise by 5 degrees (the rise anticipated
in the next century or so if the current human-induced greenhouse
effect continues unchecked). The equation predicts it will take
the evolving sun about about 0.8 billion years to produce this rise- so
human activity may be accelerating astronomical effects by a factor of
about 10 million."</div>
<div>------</div>
<div>This puts the highly doubtful claim that the current warming
trend on Earth is mostly due to increases in solar activity into
perspective, it seems.</div>
<div> </div>
<div>Ted Moffett</div>
<div> </div>
<div> </div>
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