"Radium" wrote in
oups.com:


Llanzlan Klazmon wrote:
"Radium" wrote in news:1156103700.335569.172140
@i3g2000cwc.googlegroups.com:


Brian Tung wrote:
Radium wrote:
I've read about the sun's life cycle. Apparently, when the sun
becomes a white dwarf, it will take at least a trillion years to
completely cool off. Why such a long time?

Because at that point, the Sun will still have a lot of heat left,
but it will be radiating it much slower than it does now.

A white dwarf is the hot exposed core of the progenitor star. As
such, it contains most of the heat that was in the star at the time
that it died. But the white dwarf radiates heat much slower than it
did when the star was alive, simply because its surface area is so
much smaller.

The Sun as a white dwarf will be, let's say, 100 times smaller (by
diameter) than it is now, meaning it will be 10,000 times smaller by
area. To be sure, it will initially be quite hot, perhaps four
times hotter (in kelvins) than it is now, so it'll radiate tens of
times more energy per unit area than it does now. Still, that means
that its overall rate of radiation (and therefore rate of cooling)
will be several hundreds of times slower than it is now.

That factor will only increase as the Sun cools down, and the rate
at which it radiates off into space slows down. It will approach
the cold of interstellar space only very slowly at the end.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
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http://astro.isi.edu/reference/faq.html

When the sun become a black dwarf, will it ever get a chance to cool
to around 70 Fahrenheit? Or will it likely form another star before?


You can do the calculation yourself. Knowing the surface area of the
dwarf, the specific heat of degenerate matter made of carbon and oxygen
combined with the Stefan-Boltzmann law. i.e radiated power goes as the
fourth power of temperature. That is good enough as a first
approximation anyway. BTW, if a black dwarf manages to accumulate
additional mass through accretion then it runs into a small problem
once it reaches about 1.4 Solar masses. At that point, the pressure
caused by the object's own gravity, can no longer be resisted by
electron degeneracy pressure.

This means that the
dwarf starts to collapse but now all that Carbon and Oxygen is then
available as fuel. Kablooooie. Type 1a supernova.

Correct me if I am wrong, but don't scientists beleive that the sun is
not massive enough to become a supernova?

You are correct but I specifically said the dwarf accumulated additional
matter till the point it reached 1.4 solar masses. Reread what I wrote.

Klazmon.




Klazmon.