I've always heard/read that it takes millions of years for core energy to
'wiggle' its way to the surface.
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On Mon, Mar 9, 2020 at 8:35 PM Henry Spencer <hspencer@xxxxxxxxxxxxx> wrote:
On Fri, 6 Mar 2020, Keith Henson wrote:
If you want <100nm (skip the Mylar and keep the aluminum), now *that*
probably needs an in-space factory.
Before he got into nanotechnology, Eric Drexler made 200 nm thick
solar sail material...
I think it was down around 50nm, in fact -- that's where it starts to be
no longer 100% opaque. He did think that full-scale use would require
sail manufacturing in space, because the stuff is too fragile for being
scrunched down into a compact package and then unfurled in space.
But then, again, we're talking about sending most of the light, and the
radiated heat, *inward*. Which is fine for individual craft, and
totally unacceptable for a Dyson sphere because of what it would
eventually do to the Sun (hard to say just what that would be, but the
possibilities all look unappealing).
This is something the stellar modeling people could answer, but I
really doubt heating up the outer layers of a star would do much to
the burn rate deep in the center.
Depends on how patient you are (note my word "eventually"). :-) The time
scale for energy propagation out is long, thousands of years if I recall
correctly, and effects going the other way are likely to take similar
amounts of time, but the message will get through eventually. If the
outer layers get hotter and hotter, eventually the outward heat flow will
start to slow, and the core temperature will start to rise.
That rise will start out slow, but it won't stay that way :-), because the
rates of fusion processes are outlandish functions of temperature. The
rate of the proton-proton chain, which gives most of the Sun's output,
scales merely :-) with the sixth power of temperature, although other
effects intervene and it would level off at around 10x its current output.
However, there's also the carbon cycle (more precisely, the "CNO" cycle),
which only contributes 1-2% of the Sun's energy output now, but scales
with the *twentieth* power of temperature -- in stars not a lot hotter
than the Sun, it completely dominates the burn rate.
(See <https://www.nobelprize.org/uploads/2018/06/bethe-lecture.pdf> --
Hans Bethe's Nobel-prize lecture -- for a bit more detail. Note in
particular his Fig. 4, where the temperature scale along the X-axis is
linear and the power/mass scale on the Y-axis is *logarithmic*.)
About the time Drexler was working on this, we proposed hanging a
hemisphere of sails light enough to float in sunlight... A more likely
use would be heating up the surface of an M-type to look more like a G.
The light would come out along a slit to illuminate the planets...
Actually probably unnecessary! As I understand it, a type-M star is
called "red" but that's astronomer-speak for a star that's a *little bit*
red-ish compared to our Sun. It's not red like a stoplight -- the light
would be fairly similar to an incandescent lamp, i.e. unlikely to be
problematic enough to be worth stellar-scale engineering to change it.
Henry