From: Kent Budge <"kgbudge,"@,sandia,.gov>
Subject: Re: luminance of thermo-nukes, was that whole asteroid gunk
Date: 24 Feb 1997
Newsgroups: alt.folklore.science,rec.arts.sf.science,sci.astro,sci.physics,
	rec.arts.sf.tv

Hugh Gibbons wrote:

> In article <5elas0$lr8@news.sandia.gov>, Kent Budge
> <"kgbudge,"@,sandia,.gov> wrote:
>
> > It's been suggested, in all seriousness, by astronomers trying
> > to explain the neutrino anomaly.  Not a permanent shutdown, mind
> > you -- a temporary fluctuation.
> >
> > The solar neutrino flux problem was generating enough wild ideas
> > that one astronomer finally composed the "One Toothfairy Rule."
> > The OTF states that one is permitted to introduce no more than
> > one wild, speculative new physical concept per solar model
> > attempting to explain the solar neutrino flux problem.
> >
> I know this is very basic, but if it takes that long to transport
> energy up from a stellar core, what makes variable stars do what
> they do?

The pulsations of a variable star are an acoustic, rather than thermal,
phenomena.  A sound wave takes about half an hour to travel across the
Sun, versus thousands of years for a thermal "wave."  So the pulsations
can take place on a much shorter time scale.  The effect of the
pulsations is to modulate the existing energy flow, rather than to
change the total energy radiated over long periods of time.

Some more factoids:

Pulsating variables are oscillating in one of their normal acoustic
modes.  The oscillation is much greater in amplitude at the surface
of the star than near its center, where the amplitude is very small.
So the oscillation is essentially a surface phenomenon.

The oscillation is also very nearly adiabatic -- that is, very
little of the total energy of the oscillation is radiated away in
the course of a single pulsation.  Interestingly, it is this tiny
amount of radiated energy that drives the pulsation.

It works something like this.  Just under the surface of the star
there is a zone where helium is partially ionized.  Partially ionized
helium has a whopping heat capacity (any added energy goes mostly into
ionizing more helium, rather than raising the temperature) and becomes
more opaque as it is heated.

The result is:  At the start of an oscillation, as the star begins
to expand, a lot of heat is stored in the helium ionization zone.
As the star expands, this stored heat is released, driving the
expansion.  At the point of maximum expansion, the helium zone has
become transparent (it becomes less opaque as it cools) and this
drives the subsequent contraction of the star.

So the helium zone acts like a piston, modulating the flow of energy
from the core of the star (which is practically unaware of the
oscillations) and converting part of the thermal energy flow to
mechanical energy.

It *is* theoretically possible for a star's pulsations to be driven by
changes in the core energy generation rate, but I'm not aware of any
actual examples, except possibly white dwarf stars that are generating
small amounts of energy at the base of their hydrogen atmospheres.

--

-Kent

(usual disclaimer)
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