[Vision2020] [Bulk] Human Activity Accelerating Astronomical Effects By Factor Of 10 Million

[Paul Rumelhart]

Ted,

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):

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. 

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.

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.

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.

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.

Paul


Ted Moffett wrote:
>  
> All-
>  
> 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:
>  
> http://www.sussex.ac.uk/press_office/media/media191.pdf
>  
>  
> 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>
>  
> "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."
> ------
> 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.
>  
> Ted Moffett
>  
>  
> ------------------------------------------------------------------------
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