Re: twinkle, twinkle

Hal Finney (
Mon, 23 Dec 1996 08:13:53 -0800

[Warning: I am no physicist, and what I am about to write is based on
some general reading many years ago.]

One thing to realize about the sun is that it generates energy based
on hydrogen fusion, unlike our own fusion bombs which use deuterium
and tritium. Hydrogen is much harder to make fuse, which is why we
don't use it.

The sun is very large, of course, and so its volume to surface ratio
is huge. All the energy generated in the fusion region in the interior
ultimately has to escape through the surface. But really, considering
how big the sun is, the surface is not all that hot. The sun's energy
generation is quite inefficient by our standards.

The interior of the sun is very hot and very dense, which is what allows
hydrogen fusion to occur. Each small region receives a flux of photons
from its neighbors, and adds to it with its own energy generation. As
we move outward into cooler areas, the photons continually carry heat
from the center, which will eventually be radiated to the surface. But
within the interior they are largely trapped, being absorbed and re-
radiated, with more photons added all the time. This keeps everything
hot even though the actual rate of energy production is rather modest
in some sense.

I have read that the power generated by a human-sized region of the sun
is approximately equal to that radiated by a bathtub full of warm water.
This refers to the excess energy generated by that region due to hydrogen
fusion, over the already-huge amounts of energy being received due to
radiation from adjoining areas. This is a tiny, tiny excess, and it is
only because the sun is so huge that it adds up to a considerable amount
of energy by our standards.

So the only reason the sun is able to fuse hydrogen at all is because it
is so hot and so dense, and the only reason it stays so hot is because it
is relatively opaque to radiation so it traps most heat in the interior
and allows it to leak out relatively slowly. Presumably it became hot in
the first place due to compressive heating when it was formed. Bodies too
small to form the high temperatures and densities needed never initiate
hydrogen fusion, and then when they cool their chance is lost for good.

This should make it clear how difficult it would be to turn Jupiter into
a star. Even if we set off a small fusion explosion in its interior,
that would only heat up a limited local area. While this might initiate
some local hydrogen fusion, the heat production from that reaction is
so modest that it could not heat up adjoining regions to the temperatures
needed. Hydrogen fusion is just too inefficient and slow.