[AR] stellar fusion (was Re: Freeman Dyson, RIP)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Wed, 11 Mar 2020 02:17:07 -0400 (EDT)
On Tue, 10 Mar 2020, roxanna Mason wrote:
Is the sun self regulation, i.e. exhibits positive stability in a thermal
feedback loop or the opposite, runaway fusion?
Normal stars are self-regulating, because if they get hotter, the thermal
pressure supporting the mass of the star increases, and it expands and
cools.
Runaway fusion *is* seen in white dwarf stars, because the "degeneracy"
pressure which supports them is temperature-independent. When such a star
accretes mass from a companion star, two things can happen:
First, when a white dwarf steadily accumulates hydrogen-rich gas from a
companion, the hydrogen layer on the surface gets hotter and hotter.
When it hits about 20MK, CNO fusion starts, and with its horrendous
temperature dependence, in most cases immediately runs away into an
explosion, fusing maybe 5% of the hydrogen layer and blowing the whole
layer off. That's a nova, or at least the most common type of nova. It
doesn't disrupt the star itself, and some (most?) novas repeat eventually.
Second, when the usual sort of white dwarf persistently accretes matter
from a companion (perhaps with occasional setbacks due to nova outbursts),
when it's around 1.4x the mass of the Sun, just before it gets massive
enough to collapse into a neutron star, its interior can get hot enough to
start carbon burning. (Details are unclear.) This then (probably)
undergoes a deflagration-to-detonation transition, and the result is a
nuclear detonation wave ripping through fusion fuel packed to tremendous
density (white-dwarf density is about a million tons per cubic meter) --
the runaway to end all runaways, a thermonuclear bomb with the mass of a
star. A sizable fraction of the total mass of the star burns in a few
seconds, the internal temperature reaches billions of degrees, the entire
star is blasted outward at perhaps 5% of the speed of light, and the
result is a type Ia supernova.
(You can also get carbon ignition, fusion runaway, and a type Ia supernova
when two white dwarfs merge; there is debate about how common this is,
relative to the accretion case.)
(Supernovas of types Ib, Ic, and II are completely different animals that
involve core collapse rather than runaway fusion.)
Henry
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