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<div>Paul et. al.</div>
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<div>Wonderful accessible (no incredible math required!) educational short course in stellar evolution at this web site, from "down under:"</div>
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<div><a href="http://www.physics.usyd.edu.au/~helenj/Galaxies/SG3.pdf">http://www.physics.usyd.edu.au/~helenj/Galaxies/SG3.pdf</a></div>
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<div>Ted Moffett</div>
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<div><span class="gmail_quote">On 4/23/07, <b class="gmail_sendername">Paul Rumelhart</b> <<a href="mailto:godshatter@yahoo.com">godshatter@yahoo.com</a>> wrote:</span></div>
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<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">
<div bgcolor="#ffffff" text="#000000">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="http://midd03f69e0704231323w6c982dffh331e1366bbf68cff@mail.gmail.com" type="cite">
<div><span class="e" id="q_11221edbe19be194_1">
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<div>All-</div>
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<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:
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<div><a onclick="return top.js.OpenExtLink(window,event,this)" href="http://www.sussex.ac.uk/press_office/media/media191.pdf" target="_blank">http://www.sussex.ac.uk/press_office/media/media191.pdf</a></div>
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<div><a onclick="return top.js.OpenExtLink(window,event,this)" 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" target="_blank">
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>
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<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>
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<div>Ted Moffett</div>
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