Nuclear detonation inside the sun

In article ,
Keith Harwood wrote:
...The other reactions in the proton-proton chain are not exactly
speed demons either...

What happened to the old carbon cycle? Last I heard it only mattered in
stars smaller than the sun, but I never found out why?

Contrariwise, if I'm remembering correctly -- it is significant only in
big stars, because normal-sized stars don't get hot enough for it to
proceed at a significant rate.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Henry Spencer wrote:
In article ,
Keith Harwood wrote:
...The other reactions in the proton-proton chain are not exactly
speed demons either...

What happened to the old carbon cycle? Last I heard it only mattered in
stars smaller than the sun, but I never found out why?

Contrariwise, if I'm remembering correctly -- it is significant only in
big stars, because normal-sized stars don't get hot enough for it to
proceed at a significant rate.

Bigger than the sun, but not much bigger. The CNO cycle becomes dominant for
core temperatures above 30 million degrees or so, and due to its extremely high
temperature dependence (about T^20, as opposed to T^6 for the p-p chain), it
very rapidly comes to entirely dominate stellar energy production once you pass
that temperature threshold. For the sun, the CNO cycle contributes no more than
a percent or two of the total energy output, but by 1.5 solar masses it gives
half the luminosity and by 1.7 solar masses essentially the entire luminosity.

Physically, the CNO cycle is a much harder reaction to get going than the p-p chain,
due to the tremendous energy needed to overcome the Coulomb barrier between
a nitrogen nucleus and a proton. It's not just 7 times harder (as one might naiively
expect) but dozens or hundreds of times harder, because nuclear fusion proceeds by
quantum tunnelling, which depends exponentially on the potential barrier.
Conditions which would allow the CNO cycle to proceed for fusion power are
thus almost certainly orders of magnitude harder to achieve than proton-proton reactions.

- Marshall

Henry Spencer wrote:

Contrariwise, if I'm remembering correctly -- it is significant only in
big stars, because normal-sized stars don't get hot enough for it to
proceed at a significant rate.

However, because the reaction on C has the lowest barrier, there's
almost no carbon in the center of the Sun (it's all been turned into
nitrogen.)

Paul

Marshall Perrin wrote:

Conditions which would allow the CNO cycle to proceed for fusion power are
thus almost certainly orders of magnitude harder to achieve than proton-proton reactions.

That is not at all clear. Manmade CNO would probably be *pycno*nuclear, not
thermonuclear. Compress hydrogen + heavy elements to very high density, but
keep it cold so electron screening is more effective. Under those conditions,
fusion on C/N/O would be faster than p+p.

There have been projections that H + D fusion can be made to go in cold
( 1000 K) hydrogen at 20 g/cm^3. It may be feasible to reach these conditions
used precompressed solid hydrogen that is further compressed by dozens
of weak shocks.

Paul

(Iain McClatchie) wrote in message . com...
I suppose we don't know much about the rate vs (temperature, density)
curve for the H+H-D reaction if it's so slow.

The temperature dependence should be quite well known. Mostly it's
the Coulomb barrier. There's also a mild dependence that can be
calculated from quantum mechanics.

Is there any information on the temperature or pressure sensitivity
of the H+H-D reaction?

Yes. See above for temperature dependence. The rate also depends on
density squared, as you would expect for a two-body reaction. This
information is used in solar models. The models match both neutrino
production and helioseismology results, so they seem to be pretty
accurate.

In answer to another post, according to Bahcall et al. (2003 Phys.
Rev. Lett. 90, 131301), the standard solar model says that 1.5% of the
solar luminosity should come from the CNO cycle. Present neutrino
measurements place an upper limit on the actual value of less than
7.8%, i.e., consistent with the model and getting close to providing
yet another strong test.