When the sun becomes a white dwarf why will it take SO long to cool off?

(Richard Tobin) wrote in
:

In article , Brian Tung
wrote:

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.

Why does smaller surface area mean less radiation? If each particle is
radiating as much, what difference does it make how small a volume they
are contained in?

The radiation is only from the surface as the bulk of the white dwarf is
essentially opaque. It is an interesting point though with respect to
neutron stars. When they are initially formed, their temperature is
expected be of the order of 10^11 K degrees. However their initial rate of
cooling is much faster than you would expect from the Stefan-Boltzmann law
applied to their surface area. The reason is that much of their initial
heat energy is lost via neutrino emission rather than photons. A large
percentage of the neutrinos can escape from the bulk of the neutron star
unimpeded as the neutron star is largely transparent to neutrinos.

http://arxiv.org/PS_cache/astro-ph/pdf/0409/0409751.pdf

Klazmon



-- Richard

"SW" == Sam Wormley writes:

SW jacob navia wrote:
Excuse me but what is "degenerate" in a white dwarf? I mean they
are made of normal matter, albeit very concentrated, not neutron
stars, nor quark stars, just balls of iron and heavy elements at
very high densities. Nothing degenerate at all.

SW Besides being degenerate, white dwarf are mostly carbon and
SW oxygen.

I think that's only true of some WDs. The composition of a WD depends
upon what its progenitor mass was. Stars less massive than the Sun I
believe are expected to become helium WDs because they don't have the
mass to initiate the fusion of He to C.


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SW ...white dwarf are mostly carbon and oxygen.

In article ,
Joseph Lazio writes:
I think that's only true of some WDs. The composition of a WD depends
upon what its progenitor mass was. Stars less massive than the Sun I
believe are expected to become helium WDs because they don't have the
mass to initiate the fusion of He to C.

Joe is correct, of course. I don't know what the dividing line is;
those courses were a long time ago! I vaguely recall it's quite a
bit less than a solar mass but could easily be wrong. I don't
suppose it's too much lower than a solar mass, though, or no helium
white dwarfs would yet have formed. White dwarfs with helium and
even hydrogen _atmospheres_ exist, but that doesn't necessarily
reflect the interior composition.

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