[AR] Re: stellar fusion (was Re: Freeman Dyson, RIP)
- From: "Anthony Cesaroni" <anthony@xxxxxxxxxxx>
- To: <arocket@xxxxxxxxxxxxx>
- Date: Wed, 11 Mar 2020 19:29:30 -0400
https://www.scientificamerican.com/article/the-elusive-thoery-of-everything/
Ignore the Stephen Hawkings story. Look at the one below;
A few years ago the city council of Monza, Italy, barred pet owners from
keeping goldfish in curved fishbowls. The sponsors of the measure explained
that it is cruel to keep a fish in a bowl because the curved sides give the
fish a distorted view of reality."
I jest.
Anthony J. Cesaroni
President/CEO
Cesaroni Technology/Cesaroni Aerospace
http://www.cesaronitech.com/
(941) 360-3100 x101 Sarasota
(905) 887-2370 x222 Toronto
-----Original Message-----
From: arocket-bounce@xxxxxxxxxxxxx <arocket-bounce@xxxxxxxxxxxxx> On Behalf
Of Henry Spencer
Sent: Wednesday, March 11, 2020 2:17 AM
To: Arocket List <arocket@xxxxxxxxxxxxx>
Subject: [AR] stellar fusion (was Re: Freeman Dyson, RIP)
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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