From: Amara Graps (Amara.Graps@mpi-hd.mpg.de)
Date: Fri Jul 21 2000 - 07:47:54 MDT
From: "Robert J. Bradbury" <bradbury@aeiveos.com>, Mon, 17 Jul 2000
>I have read Prof. Lorry's (:-)) comment on this and I can only mildly
>disagree with what he says. I've seen no peer reviewed paper that
>says you cannot disassemble stars or that disassembling them produces
>supernovas. Criswell's stellar husbandry (aka star lifting) approach
>(which I have online copies of if someone wants it), to my knowledge has
>not been disproven.
David R. Criswell, yes?
sidetracking --
I just put the pieces together that he is the same D.R. Criswell that
did so much work on the electrostatic transport of lunar dust in the
70s. He's kind of famous for that work.
Criswell, D.R., chapter "Horizon-glow and the motion of lunar dust",
in _Photon and Particle Interactions with Surfaces in Space_,
ed. R.Grard, Reidel, 1973. and many more papers....
Electrostatic dust transport on the Moon was first envisioned by
T. Gold in 1955 and then later, after the Surveyor 5,6,7 spacecraft
missions, the topic was embraced again by D.R. Criswell and co-workers
to explain some features seen in the lunar images. In those images, a
western horizon glow was observed following the local sunset. The most
likely explanation for the glow in the images is forward-scattered
sunlight by a cloud of dust particles, less than about 10 microns in
radii, migrating away from the sunlit hemisphere of the moon. The
cloud is created from tiny particles, levitated above (a few to a few
tens of centimeters) the lunar surface, the levitation caused by
repulsive forces between the charged particles and the extremely low-
electrically- conductive lunar surface.
back to starlifting --
The paper that you (Robert) have by Criswell, is this the same as the
chapter: "Solar System Industrialization Implications for Interstellar
Migrations" in the book: _Interstellar Migration and Human
Experience_? I've skimmed the chapter, but I need to read it in more
detail.
>I have seen suggestions (hints?) that as Michael
>says, this may age the star prematurely. I think it depends on
>whether or not the star has ignited its helium burning cycle
>(which is what turns it into a red giant). If all the star is
>doing is burning H into He, then, removing the outer mass should
>decompress the core and allow the star to shift down the
>luminosity curve (G-->K-->M-->L). You may have to do it *very*
>slowly to allow the star to adjust. I suspect there may be
>some additional complexity depending on the degree to which the
>star is losing heat primarily through radiation or convection
>(which may depend on its mass and perhaps metalicity).
>Perhaps if Amara reads this at some point she could comment.
email is not such a reliable method to reach me these months
(PhD endpoint near), phone and in person is better.
What I see missed in this discussion, after main sequence, is that
nuclear burning is also (possibly/likely) occurring *IN THE SHELL OF
THE STAR* too, depending on the star's mass.
Starting from stellar evolution just after main sequence:
(stating references: here I use info from a helioseismologist from
Aarhus, DK named Jorgen Christensen_Dalsgaard, "Lecture notes:
Stellar Structure and Evolution", an 1995 edition of his notes which
he gave me in 1997, while I was still working for the Stanford solar
oscillations group)
Hydrogen is exhausted near the center, and the star is left with a
core consisting of helium and a small amount of heavy elements.
Initially the temperature of the core is far below the 10^8K required
for helium ignition, and there is no nuclear energy generation in the
core -- the temperature is almost isothermal.
Surrounding the core is a region containing hydrogen where the
temperature is still high enough for hydrogen burning to proceed.
This region is the "hydrogen shell source", and it provides the energy
from which the luminosity of the star is derived. As the hydrogen is
converted to helium in the shell source, the mass of the inert helium
core increases. This leads to contraction of the core, which releases
gravitational potential energy, part of which goes towards increasing
the thermal core energy. And so the process continues, until the
temperature of the core is sufficiently high for helium burning to
begin.
So then very roughly, the process is repeated: the star burns helium
in the core (while still maintaining a hydrogen shell source) until
helium is exhausted; the star then has a contracting core consisting
of 12C and 16O, surrounded by a helium shell source and a hydrogen
shell source; the contraction of the core may proceed up to the point
where the temperature is high enough for carbon ignition; and so on.
Christensen_Dalsgaard says that this sketch leaves out a lot of
"fascinating detail". In particular, the response of the observable
properties of the star to the changes in the core. He said that the
response of the star can be understood in terms of a simple principle:
THE SHELL-BURNING LAW: When the region within a burning shell
contracts, the region outside the shell expands; and when the region
inside the shell expands, the region outside the shell contracts.
This rule appears to apply also in some cases where there are two
shell-burning regions present.
How can one have two shell sources?
For a moderate mass star, say 5*M_sol, the picture proceeds as
above. Helium burning is established in the core, and the contraction
of the surface stops, and the star enters an extended period of core
helium burning. However even during this phase, the hydrogen shell
source contributes a substantial part of the energy. Helium is
converted into carbon and oxygen The mean molecular weight of material
in the core increases. The shell-burning law operates, causing the
envelope to expand. When helium is exhausted in the core, helium
burning is established in a shell, while hydrogen shell burning
continues; hence the star has two shell sources.
The more massive the star, the more shell-burning sources exist.
Kippenhahn and Weigert, 1990 have a nice schematic illustration of the
"onion-shell" structure in the interior of a highly evolved massive
star, showing the burning processes proceeding in the various shells,
and the resulting composition. Also note that the late stages of
stellar evolution are very rapid, compared to the hydrogen and helium
phases. Oxygen burning only lasts of order 6 months. "Silicon-burning"
is finished in about a day. So therefore the probability of observing
a star while in these later evolutionary phases is very small.
Amara
--
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Amara Graps | Max-Planck-Institut fuer Kernphysik
Interplanetary Dust Group | Saupfercheckweg 1
+49-6221-516-543 | 69117 Heidelberg, GERMANY
Amara.Graps@mpi-hd.mpg.de * http://galileo.mpi-hd.mpg.de/~graps
***************************************************************
"Never fight an inanimate object." - P. J. O'Rourke
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