| Topic: |
Science > Physics |
| User: |
"Yousuf Khan" |
| Date: |
02 Aug 2005 08:33:32 PM |
| Object: |
Fusion: what's the diff? |
What's the difference between core fusion that's supposed to power a
brown dwarf star, and full thermonuclear fusion that's supposed to
power a full star like our Sun? Here's an excerpt from an article
that's got me asking this question:
"Brown dwarfs are the ill-defined middle ground between planets and
stars. A star is a star because it shines on its own, generating light
through thermonuclear reactions in which hydrogen is converted to
helium. Brown dwarfs, though they can burn deuterium in another type of
reaction called "core fusion," fall short of full-blown stellar
thermonuclear fusion."
http://www.space.com/scienceastronomy/solarsystem/planet_confusion_001101-2.html
Yousuf Khan
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| User: "Yousuf Khan" |
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| Title: Re: Fusion: what's the diff? |
04 Aug 2005 02:37:48 PM |
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Bruce Scott TOK wrote:
Yes, with about equal probability (tritium and proton, or He3 and a
neutron). These reactions do not involve neutrinos because they are
("conventional") strong nuclear reactions. Look at the details of the
above reactions and note that no proton gets converted into a neutron or
vice versa.
Okay, so the emission of a neutrino is a sure sign that it's a Weak
Force reaction?
I don't remember all the PPI chain numbers. Go to a library and look
them up in:
Clayton, D, Principles of Stellar Evolution and Nucleosynthesis,
McGraw-Hill (1968)
Found this instead:
Solar Fusion & Neutrinos
http://www.tim-thompson.com/fusion.html#ppcycle
What's the ratio of proton-proton reactions that result in a deuterium
being produced vs. those which just fly apart again? I assume the slow
rate of deuterium production is what slows it down?
Basically, if they fly apart and do nothing then it is not a reaction,
just a scattering event. The cross section for pp fusion is many orders
of magnitude smaller than the one for scattering.
Alright understood.
The probability of a reaction per unit volume depends on the density
(amount of stuff) and temperature (how fast it is flying about and
potentially running into each other). Higher density --> more
encounters within a given distance. Higher temperature --> more kinetic
energy per encounter on average.
And that's why supermassive stars use up their reactants faster,
because they're denser and hotter, I guess?
A couple of more questions.
(1) When they write that fusion bombs are harnessing the power of the
Sun, are they really? I mean journalists tend to get very imprecise
sometimes. Is the fusion that takes place in a fusion bomb simulate the
Sun, or is it more like the inside of a brown dwarf?
(2) Will our Sun go through all of the fusion cycles and finish off at
iron? Or is that only for supernova class stars? Where will our Sun
finish off?
Yousuf Khan
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| User: "Llanzlan Klazmon" |
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| Title: Re: Fusion: what's the diff? |
07 Aug 2005 07:19:47 PM |
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"Yousuf Khan" <yjkhan@gmail.com> wrote in
news:1123184267.918420.181480@o13g2000cwo.googlegroups.com:
Bruce Scott TOK wrote:
<SNIP>
(2) Will our Sun go through all of the fusion cycles and finish off
at
iron? Or is that only for supernova class stars? Where will our Sun
finish off?
For stars like the Sun the answer is no and yes. Why? Because stars
like the sun don't really get past producing Carbon, Nitrogen and
Oxygen. However the white dwarf remnant they leave behind, isn't
necessarily a dead end. In the case of a white dwarf with a close
binary companion. The dwarf can accumulate material from the companion
star. This material rich in hydrogen can undergo a spontaneous
detonation and create a nova. This can happen multiple times for a
white dwarf. It is also possible that the dwarf accumulates material
until it's mass exceeds about 1.4 solar masses. At this point a
different type of detonation occurs called a Type 1A supernova. The
reaction is so energetic that the dwarf is completely destroyed. A
large percentage of the dwarf's mass gets converted to heavier
elements such as Co 56 and Ni 56. The optical afterglow of the
supernova is caused mainly from the radioactive decay of some of these
heavier elements which energises that expanding debris of the star.
Note that the really heavy elements > Fe are not produced by this sort
of explosion as an endothermic reaction is required for that. This can
only be powered by gravitational collapse such as the formation of a
neutron star or black hole. (Type II supernova).
Klazmon.
Yousuf Khan
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| User: "Yousuf Khan" |
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| Title: Re: Fusion: what's the diff? |
09 Aug 2005 04:24:43 PM |
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Llanzlan Klazmon wrote:
For stars like the Sun the answer is no and yes. Why? Because stars
like the sun don't really get past producing Carbon, Nitrogen and
Oxygen. However the white dwarf remnant they leave behind, isn't
necessarily a dead end. In the case of a white dwarf with a close
binary companion. The dwarf can accumulate material from the companion
star. This material rich in hydrogen can undergo a spontaneous
detonation and create a nova. This can happen multiple times for a
white dwarf. It is also possible that the dwarf accumulates material
until it's mass exceeds about 1.4 solar masses. At this point a
different type of detonation occurs called a Type 1A supernova. The
reaction is so energetic that the dwarf is completely destroyed. A
large percentage of the dwarf's mass gets converted to heavier
elements such as Co 56 and Ni 56. The optical afterglow of the
supernova is caused mainly from the radioactive decay of some of these
heavier elements which energises that expanding debris of the star.
The white dwarf is completely destroyed? What happens to the solid
carbon core from before? Completely scattered out as carbon atoms, or
carbon rocks or something? Or is the carbon core now big enough to
squeeze even further down into a neutron star core?
Note that the really heavy elements > Fe are not produced by this sort
of explosion as an endothermic reaction is required for that. This can
only be powered by gravitational collapse such as the formation of a
neutron star or black hole. (Type II supernova).
When we talk about 1.4 solar mass limit to produce supernovas is that
just the mass of the remnant we're talking about or the whole star
before it went supernova?
Yousuf khan
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| User: "Llanzlan Klazmon" |
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| Title: Re: Fusion: what's the diff? |
09 Aug 2005 06:54:11 PM |
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"Yousuf Khan" <yjkhan@gmail.com> wrote in
news:1123622683.104972.104020@z14g2000cwz.googlegroups.com:
Llanzlan Klazmon wrote:
For stars like the Sun the answer is no and yes. Why? Because stars
like the sun don't really get past producing Carbon, Nitrogen and
Oxygen. However the white dwarf remnant they leave behind, isn't
necessarily a dead end. In the case of a white dwarf with a close
binary companion. The dwarf can accumulate material from the
companion
star. This material rich in hydrogen can undergo a spontaneous
detonation and create a nova. This can happen multiple times for a
white dwarf. It is also possible that the dwarf accumulates
material
until it's mass exceeds about 1.4 solar masses. At this point a
different type of detonation occurs called a Type 1A supernova. The
reaction is so energetic that the dwarf is completely destroyed. A
large percentage of the dwarf's mass gets converted to heavier
elements such as Co 56 and Ni 56. The optical afterglow of the
supernova is caused mainly from the radioactive decay of some of
these
heavier elements which energises that expanding debris of the star.
The white dwarf is completely destroyed? What happens to the solid
carbon core from before? Completely scattered out as carbon atoms,
or
carbon rocks or something? Or is the carbon core now big enough to
squeeze even further down into a neutron star core?
No. The white dwarf is totally and utterly destroyed in the case of a
Type Ia supernova. The process is thought to start with thermonuclear
deflagration at the core of the star which propagates outwards - some
models have the deflagration change to a detonation (shock wave) as
the burning approaches the surface of the star. In any case the entire
star is destroyed in very short order. Interestingly the actual burst
of light from the actual explosion is very short lived because of the
rapid expansion. What actually makes the supernova aftermath so bright
is the radioactive decay of the fusion products. At peak brightness,
the SN Ia can outshine the entire host galaxy. Because of this they
can be detected at vast distances.
Neutron stars only form as the result of gravitational collapse of a
more massive star. This process is called a type II supernova. Very
different to the case above.
Here is an example of a simulation of a white dwarf blowing up
(deflagration model only):
http://www.mpa-garching.mpg.de/mpa/research/current_research/hl2004-
10/hl2004-10-en.html
Note that the really heavy elements > Fe are not produced by this
sort
of explosion as an endothermic reaction is required for that. This
can
only be powered by gravitational collapse such as the formation of
a
neutron star or black hole. (Type II supernova).
When we talk about 1.4 solar mass limit to produce supernovas is
that
just the mass of the remnant we're talking about or the whole star
before it went supernova?
In the case of a white dwarf it is talking about the mass of the whole
white dwarf but remember that when a white dwarf forms, it throws of a
considerable portion of the original mass of the star in the form of a
planetary nebula. With a white dwarf you have all this thermonuclear
fuel under extreme pressure. The situation is unstable and if enough
extra matter is accummulated (to total mass around 1.4 solar masses),
you get a spontaneous explosion as above.
Klazmon.
Yousuf khan
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| User: "Yousuf Khan" |
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| Title: Re: Fusion: what's the diff? |
10 Aug 2005 12:23:32 PM |
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Llanzlan Klazmon wrote:
No. The white dwarf is totally and utterly destroyed in the case of a
Type Ia supernova. The process is thought to start with thermonuclear
deflagration at the core of the star which propagates outwards - some
models have the deflagration change to a detonation (shock wave) as
the burning approaches the surface of the star. In any case the entire
star is destroyed in very short order. Interestingly the actual burst
of light from the actual explosion is very short lived because of the
rapid expansion. What actually makes the supernova aftermath so bright
is the radioactive decay of the fusion products. At peak brightness,
the SN Ia can outshine the entire host galaxy. Because of this they
can be detected at vast distances.
Okay, I understand, my previous assumption was that all of the
thermonuclear fusion was happening on layers outside the old white
dwarf hulk, i.e. at the surface of the hulk where all of the falling
hydrogen material from the companion star was falling onto. My
assumption was that the old carbon core was essentially an inert
material not capable of any further fusion. But the site you directed
me to below, mentions that the old carbon core is undergoing final
fusion towards iron. So that explains how the old core can be totally
dispersed in the explosion.
Here is an example of a simulation of a white dwarf blowing up
(deflagration model only):
http://www.mpa-garching.mpg.de/mpa/research/current_research/hl2004-
10/hl2004-10-en.html
Does the whole of the old carbon core reignite as the material falls
onto it, or just the outer surface layers of carbon? I guess, even if
the surface layers of carbon reignited, they'd eventually reignite the
layers of carbon just below it, those below that, etc. until the whole
core is ignited. Basically is it a slow layer-by-layer reignition or a
very quick reignition?
In the case of a white dwarf it is talking about the mass of the whole
white dwarf but remember that when a white dwarf forms, it throws of a
considerable portion of the original mass of the star in the form of a
planetary nebula. With a white dwarf you have all this thermonuclear
fuel under extreme pressure. The situation is unstable and if enough
extra matter is accummulated (to total mass around 1.4 solar masses),
you get a spontaneous explosion as above.
I assume the explosion isn't just as a result of reaching the 1.4
critical mass, but because there is no more carbon left to consume, and
there's only iron left in the core?
Yousuf Khan
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| User: "Llanzlan Klazmon" |
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| Title: Re: Fusion: what's the diff? |
10 Aug 2005 09:23:01 PM |
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"Yousuf Khan" <yjkhan@gmail.com> wrote in
news:1123694612.559696.136720@g47g2000cwa.googlegroups.com:
Llanzlan Klazmon wrote:
No. The white dwarf is totally and utterly destroyed in the case of
a
Type Ia supernova. The process is thought to start with
thermonuclear
deflagration at the core of the star which propagates outwards -
some
models have the deflagration change to a detonation (shock wave) as
the burning approaches the surface of the star. In any case the
entire
star is destroyed in very short order. Interestingly the actual
burst
of light from the actual explosion is very short lived because of
the
rapid expansion. What actually makes the supernova aftermath so
bright
is the radioactive decay of the fusion products. At peak
brightness,
the SN Ia can outshine the entire host galaxy. Because of this they
can be detected at vast distances.
Okay, I understand, my previous assumption was that all of the
thermonuclear fusion was happening on layers outside the old white
dwarf hulk, i.e. at the surface of the hulk where all of the falling
hydrogen material from the companion star was falling onto. My
assumption was that the old carbon core was essentially an inert
material not capable of any further fusion.
That is in fact what happens in the case of an ordinary nova. You get
a thermonuclear reaction fueled by new material falling onto the white
dwarf's surface. Going Nova doesn't do any harm to the white dwarf
star - it only involves the star's surface. It isn't powerful enough
do anything much to the dwarfs core degenerate material (mostly Carbon
and Oxygen). The fusion of the Carbon and Oxygen core can only be
initiated when the pressure in the core goes high enough to overcome
the activation energy barrier. It is a similar concept to an ordinary
chemical explosive like TNT, which doesn't do anything until you give
it some sort of kick to set it off. It happens that the pressure due
to gravity overcomes the activation energy barrier for fusion of this
core material when the dwarf's total mass goes over the critical value
of about 1.4 solar masses. The initial explosion starts in the core as
a deflagration, which propagates at less than the speed of sound in
the degenerate core material (much higher than the speed of sound we
are used to in earthly materials). Deflagration is a very efficient
process and nearly all the star's material gets fused. From
observation of actual type 1a supernova we can deduce that the
deflagration undergoes a phase transition to a different kind of
explosion as it nears the stars outer layers. It changes over to a
detonation (supersonic shock wave). Detonations are not so efficient,
so a good percentage of the outer material doesn't get completely
burned to the Fe/Ni/Co group like the core material does. The whole
process once it is activated in the core to the demise of the star
takes roughly two seconds - quite a bit different to the millions or
billions of years for the slow burning that goes on in the cores of
main sequence stars.
But the site you directed
me to below, mentions that the old carbon core is undergoing final
fusion towards iron. So that explains how the old core can be
totally
dispersed in the explosion.
Yes. Note that the bulk of the star undergoes this fusion in around
two seconds - that is why the supernova tyoe 1a is so energetic. It is
explosive unlike the normal slow burning of the main sequence stars.
Here is an example of a simulation of a white dwarf blowing up
(deflagration model only):
http://www.mpa-
garching.mpg.de/mpa/research/current_research/hl2004-
10/hl2004-10-en.html
Does the whole of the old carbon core reignite as the material falls
onto it, or just the outer surface layers of carbon? I guess, even
if
the surface layers of carbon reignited, they'd eventually reignite
the
layers of carbon just below it, those below that, etc. until the
whole
core is ignited. Basically is it a slow layer-by-layer reignition or
a
very quick reignition?
As I mentioned above. Fusion of new hydrogen & deuterium accumulating
at the stars surface results in only a Nova. It doesn't do anything
the core. White dwarfs can Nova multiple times. I suppose that if the
dwarf was very close to the 1.4 SM limit, an ordinary Nova on the
surface could perhaps kick the core into a supernova - I don't know -
you could try contacting one of the groups that do these simulations
about what they think about that.
In the case of a white dwarf it is talking about the mass of the
whole
white dwarf but remember that when a white dwarf forms, it throws
of a
considerable portion of the original mass of the star in the form
of a
planetary nebula. With a white dwarf you have all this
thermonuclear
fuel under extreme pressure. The situation is unstable and if
enough
extra matter is accummulated (to total mass around 1.4 solar
masses),
you get a spontaneous explosion as above.
I assume the explosion isn't just as a result of reaching the 1.4
critical mass, but because there is no more carbon left to consume,
and
there's only iron left in the core?
No the core is composed of Carbon and Oxygen in the form of
degenerate matter.
http://en.wikipedia.org/wiki/Degenerate_matter
The explosion of a Type 1a supernova is powered by the burning of this
Carbon and Oxygen into mainly Fe/NI/Co isotopes. As said the whole
star effectively undergoes this fusion in two seconds flat. The
expanding debris is then further energised by the radioactive decay of
many of the fusion products, particulary Ni 56.
Klazmon
Yousuf Khan
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| User: "Yousuf Khan" |
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| Title: Re: Fusion: what's the diff? |
11 Aug 2005 12:42:22 AM |
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Llanzlan Klazmon wrote:
Okay, I understand, my previous assumption was that all of the
thermonuclear fusion was happening on layers outside the old white
dwarf hulk, i.e. at the surface of the hulk where all of the falling
hydrogen material from the companion star was falling onto. My
assumption was that the old carbon core was essentially an inert
material not capable of any further fusion.
That is in fact what happens in the case of an ordinary nova. You get
a thermonuclear reaction fueled by new material falling onto the white
dwarf's surface. Going Nova doesn't do any harm to the white dwarf
star - it only involves the star's surface.
Oh good, then I did understand it correctly, except it was just a
special case that I had envisioned.
It isn't powerful enough
do anything much to the dwarfs core degenerate material (mostly Carbon
and Oxygen).
Which is what surprised me initially when I heard that the whole core
is destroyed in the supernova explosion. This stuff is probably the
hardest solid in the universe right after neutronium.
The initial explosion starts in the core as
a deflagration, which propagates at less than the speed of sound in
the degenerate core material (much higher than the speed of sound we
are used to in earthly materials).
I assume that the speed of sound in this material is a significant
proportion of the speed of light in vaccuum. Probably the speed of
light and sound are equivalent within this material?
Deflagration is a very efficient
process and nearly all the star's material gets fused. From
observation of actual type 1a supernova we can deduce that the
deflagration undergoes a phase transition to a different kind of
explosion as it nears the stars outer layers. It changes over to a
detonation (supersonic shock wave). Detonations are not so efficient,
so a good percentage of the outer material doesn't get completely
burned to the Fe/Ni/Co group like the core material does. The whole
process once it is activated in the core to the demise of the star
takes roughly two seconds - quite a bit different to the millions or
billions of years for the slow burning that goes on in the cores of
main sequence stars.
Why is the final Fe/Ni/Co fusion process so fast compared to ordinary
fusion processes? Is it because it's happening inside this superdense
degenerate material?
Also does the transformation to Fe/Ni/Co happen this fast in Type II
Supernovas? I assume that the density of the core isn't big enough to
go so fast.
The explosion of a Type 1a supernova is powered by the burning of this
Carbon and Oxygen into mainly Fe/NI/Co isotopes. As said the whole
star effectively undergoes this fusion in two seconds flat. The
expanding debris is then further energised by the radioactive decay of
many of the fusion products, particulary Ni 56.
In a Type II supernova, you're left behind with a neutron star or
blackhole. I assume that you're saying that there is not a single thing
left behind in a Type Ia, every last thing is atomized as if the star
never existed?
Yousuf Khan
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| User: "Llanzlan Klazmon" |
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| Title: Re: Fusion: what's the diff? |
11 Aug 2005 08:35:57 PM |
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"Yousuf Khan" <yjkhan@gmail.com> wrote in
news:1123738942.636389.247850@g47g2000cwa.googlegroups.com:
<SNIP>
Why is the final Fe/Ni/Co fusion process so fast compared to
ordinary
fusion processes? Is it because it's happening inside this
superdense
degenerate material?
I don't know the details but I believe it is because we are dealing
with "strong" interactions, which have become accessable because the
electron degeneracy pressure has been overwhelmed. If you do a google
search on 'Type Ia supernova simulations' you will probably find
references to their publishings which should give the details of all
the actual fusion reactions are included in their simulations.
Also does the transformation to Fe/Ni/Co happen this fast in Type II
Supernovas? I assume that the density of the core isn't big enough
to
go so fast.
SN Type II's are fast too ;-) but I understand that the reactions are
different. SN type II's also power endothermic reactions and thus
produce elements of greater atomic number than Fe. From what I recall,
what happens in a Type II supernova is a lot more complicated than
with a Type Ia. I gather the type II is believed to start with the
photo-dissociation of all the nucleii in the central core of a massive
star, an endothermic reaction ending up leaving a "neutron gas", which
then promptly collapes resulting in "all hell breaking loose" ;-).
This is because the electron degeneracy pressure gets taken completely
out of the picture once all the protons get converted to neutrons (a
process which removes all the electrons present). The heavy elements I
mentioned above are produced by a combination of the so called r-
process & s-process nuclear reactions in the layers above the
collapsing core. Surprisingly the core is not destroyed but survives
in the form of a neutron star (or possibly becomes a black hole if
massive enough). Neutronium is pretty tough stuff ;-)
The explosion of a Type 1a supernova is powered by the burning of
this
Carbon and Oxygen into mainly Fe/NI/Co isotopes. As said the whole
star effectively undergoes this fusion in two seconds flat. The
expanding debris is then further energised by the radioactive decay
of
many of the fusion products, particulary Ni 56.
In a Type II supernova, you're left behind with a neutron star or
blackhole. I assume that you're saying that there is not a single
thing
left behind in a Type Ia, every last thing is atomized as if the
star
never existed?
Correct. Just the expanding debris cloud remains. Note that Type Ia's
also involves a companion star - usually a red giant from which the
white dwarf got the extra matter that pushed it over the limit. The
companion star does end up rather worse for wear as the supenova is
powerful enough to blow away the companion star's outer layers but
doesn't completely destroy the companion.
Klazmon
Yousuf Khan
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| User: "Yousuf Khan" |
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| Title: Re: Fusion: what's the diff? |
11 Aug 2005 11:39:25 PM |
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Another question, if let's say we have a neutron star and a companion
having its material pulled off and falling onto the neutron star, would
we get nova or supernova explosions from the neutron star like with the
white dwarf?
Yousuf Khan
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| User: "Yousuf Khan" |
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| Title: Re: Fusion: what's the diff? |
11 Aug 2005 11:36:01 PM |
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Llanzlan Klazmon wrote:
collapsing core. Surprisingly the core is not destroyed but survives
in the form of a neutron star (or possibly becomes a black hole if
massive enough). Neutronium is pretty tough stuff ;-)
Well, I would assume that if the neutron star or blackhole didn't
survive, then we wouldn't have anything to produce the gravity to power
the endothermic reactions that produce those heavier elements?
Correct. Just the expanding debris cloud remains. Note that Type Ia's
also involves a companion star - usually a red giant from which the
white dwarf got the extra matter that pushed it over the limit. The
companion star does end up rather worse for wear as the supenova is
powerful enough to blow away the companion star's outer layers but
doesn't completely destroy the companion.
Does it need to get its falling matter from a red giant, because the
red giant's outer layers are looser?
Also if it's getting its matter from a red giant, aren't they made of
mostly helium, so wouldn't that mean that what's falling onto the white
dwarf is mostly helium?
Yousuf Khan
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| User: "Llanzlan Klazmon" |
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| Title: Re: Fusion: what's the diff? |
12 Aug 2005 12:45:28 AM |
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"Yousuf Khan" <yjkhan@gmail.com> wrote in
news:1123821361.098233.27230@g14g2000cwa.googlegroups.com:
Llanzlan Klazmon wrote:
collapsing core. Surprisingly the core is not destroyed but
survives
in the form of a neutron star (or possibly becomes a black hole if
massive enough). Neutronium is pretty tough stuff ;-)
Well, I would assume that if the neutron star or blackhole didn't
survive, then we wouldn't have anything to produce the gravity to
power
the endothermic reactions that produce those heavier elements?
Correct. Just the expanding debris cloud remains. Note that Type
Ia's
also involves a companion star - usually a red giant from which the
white dwarf got the extra matter that pushed it over the limit. The
companion star does end up rather worse for wear as the supenova is
powerful enough to blow away the companion star's outer layers but
doesn't completely destroy the companion.
Does it need to get its falling matter from a red giant, because the
red giant's outer layers are looser?
Any source of material will do. But a red giant companion is the most
likely way suffient material can be grabbed in a reasonable period of
time.
Also if it's getting its matter from a red giant, aren't they made
of
mostly helium, so wouldn't that mean that what's falling onto the
white
dwarf is mostly helium?
No. There is still plenty of hydrogen in the outer layers of a red
giant. The core would be depleted of hydrogen.
Klazmon.
Yousuf Khan
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| User: "Yousuf Khan" |
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| Title: Re: Fusion: what's the diff? |
12 Aug 2005 05:13:26 PM |
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Llanzlan Klazmon wrote:
Also if it's getting its matter from a red giant, aren't they made
of
mostly helium, so wouldn't that mean that what's falling onto the
white
dwarf is mostly helium?
No. There is still plenty of hydrogen in the outer layers of a red
giant. The core would be depleted of hydrogen.
So during the hydrogen-burning life of a star all of its power is
produced by the core, and the core never replenishes its hydrogen fuel
supply from its own outer layers? So whatever hydrogen became part of
the star's core at formation, is the only hydrogen that will be used to
power the star for the rest its days?
Yousuf Khan
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| User: "Llanzlan Klazmon" |
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| Title: Re: Fusion: what's the diff? |
14 Aug 2005 06:23:58 PM |
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"Yousuf Khan" <yjkhan@gmail.com> wrote in news:1123884806.596317.291210
@g44g2000cwa.googlegroups.com:
Llanzlan Klazmon wrote:
Also if it's getting its matter from a red giant, aren't they made
of
mostly helium, so wouldn't that mean that what's falling onto the
white
dwarf is mostly helium?
No. There is still plenty of hydrogen in the outer layers of a red
giant. The core would be depleted of hydrogen.
So during the hydrogen-burning life of a star all of its power is
produced by the core, and the core never replenishes its hydrogen fuel
supply from its own outer layers? So whatever hydrogen became part of
the star's core at formation, is the only hydrogen that will be used to
power the star for the rest its days?
Yousuf Khan
This obviously depends on how efficiently the stellar atmosphere mixes. It
appears that convection would mix the outer parts fairly well but it looks
like convection doesn't get all the way to the core. There is a lot of
material on the web on this sort of thing. I would say that it is far from
totally understood.
Klazmon.
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| User: "Bruce Scott TOK" |
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| Title: Re: Fusion: what's the diff? |
15 Aug 2005 07:07:22 AM |
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Klazmon wrote:
"Yousuf Khan" <yjkhan@gmail.com> wrote in news:1123884806.596317.291210
So during the hydrogen-burning life of a star all of its power is
produced by the core, and the core never replenishes its hydrogen fuel
supply from its own outer layers? So whatever hydrogen became part of
the star's core at formation, is the only hydrogen that will be used to
power the star for the rest its days?
This obviously depends on how efficiently the stellar atmosphere mixes. It
appears that convection would mix the outer parts fairly well but it looks
like convection doesn't get all the way to the core. There is a lot of
material on the web on this sort of thing. I would say that it is far from
totally understood.
There is a form of motion called "meridional circulation" that basically
results from the fact that a rotating star cannot be in both hyrdostatic
and thermal diffusive equilibrium --- the surfaces of constant pressure
and temperature do not coincide, due to the Coriolis forces. Since the
hydro time scale is much shorter than the thermal, the Sun is in
hydrostatic equilibrium while horizontal temperature differences drive
flow cells in planes of constant longitude. This was once thought to be
able to mix convectively stable regions, but it is believed on the
strength of solar oscillation data that there are indeed strong
compositional gradients in the redial direction. The meridional
circulation exists but is too slow to effectively mix the interior.
The position and extent of the convection zone is now very well
understood; the position of the boundary to the radiative core is known
to three significant digits (there is a clear break in the gradient of
the sound speed, exactly the signature of the onset of convection).
Look this up via the SOHO website, and the personal sites of John
Bahcall and Jorgen Christensen-Dalsgaard. A good start might be here:
http://astro.phys.au.dk/helio_outreach/english/engHA0.html
A technical introduction is here:
http://solarphysics.livingreviews.org/open?pubNo=lrsp-2005-1&page=title.html
The current figure on the base of the convection zone is
r_b = 0.713 +/- 0.003 R_0
where R_0 is the solar radius.
--
ciao,
Bruce
drift wave turbulence: http://www.rzg.mpg.de/~bds/
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| User: "Llanzlan Klazmon" |
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| Title: Re: Fusion: what's the diff? |
15 Aug 2005 06:56:49 PM |
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Bruce Scott TOK <Use-Author-Supplied-Address-Header@[127.1]> wrote in
news:200508151207.j7FC7M04017806@ipp.mpg.de:
Klazmon wrote:
"Yousuf Khan" <yjkhan@gmail.com> wrote in news:1123884806.596317.291210
So during the hydrogen-burning life of a star all of its power is
produced by the core, and the core never replenishes its hydrogen fuel
supply from its own outer layers? So whatever hydrogen became part of
the star's core at formation, is the only hydrogen that will be used
to power the star for the rest its days?
This obviously depends on how efficiently the stellar atmosphere mixes.
It appears that convection would mix the outer parts fairly well but it
looks like convection doesn't get all the way to the core. There is a
lot of material on the web on this sort of thing. I would say that it is
far from totally understood.
There is a form of motion called "meridional circulation" that basically
results from the fact that a rotating star cannot be in both hyrdostatic
and thermal diffusive equilibrium --- the surfaces of constant pressure
and temperature do not coincide, due to the Coriolis forces. Since the
hydro time scale is much shorter than the thermal, the Sun is in
hydrostatic equilibrium while horizontal temperature differences drive
flow cells in planes of constant longitude. This was once thought to be
able to mix convectively stable regions, but it is believed on the
strength of solar oscillation data that there are indeed strong
compositional gradients in the redial direction. The meridional
circulation exists but is too slow to effectively mix the interior.
The position and extent of the convection zone is now very well
understood; the position of the boundary to the radiative core is known
to three significant digits (there is a clear break in the gradient of
the sound speed, exactly the signature of the onset of convection).
Look this up via the SOHO website, and the personal sites of John
Bahcall and Jorgen Christensen-Dalsgaard. A good start might be here:
http://astro.phys.au.dk/helio_outreach/english/engHA0.html
A technical introduction is here:
http://solarphysics.livingreviews.org/open?pubNo=lrsp-2005-1&page=tit
le.html The current figure on the base of the convection zone is
r_b = 0.713 +/- 0.003 R_0
where R_0 is the solar radius.
Thanks Bruce. Very interesting references. I saw Steve's caveat about the
differences with stars of varying mass too.
Klazmon.
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| User: "Bruce Scott TOK" |
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| Title: Re: Fusion: what's the diff? |
16 Aug 2005 10:36:34 AM |
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Klazmon wrote:
Bruce Scott TOK <Use-Author-Supplied-Address-Header@[127.1]> wrote in
news:200508151207.j7FC7M04017806@ipp.mpg.de:
[...]
The position and extent of the convection zone is now very well
understood; the position of the boundary to the radiative core is known
to three significant digits (there is a clear break in the gradient of
the sound speed, exactly the signature of the onset of convection).
Look this up via the SOHO website, and the personal sites of John
Bahcall and Jorgen Christensen-Dalsgaard. A good start might be here:
http://astro.phys.au.dk/helio_outreach/english/engHA0.html
A technical introduction is here:
http://solarphysics.livingreviews.org/open?pubNo=lrsp-2005-1&page=tit
le.html The current figure on the base of the convection zone is
r_b = 0.713 +/- 0.003 R_0
where R_0 is the solar radius.
Thanks Bruce. Very interesting references. I saw Steve's caveat about the
differences with stars of varying mass too.
Same here... I did write for the Sun, but the bot about the other stars
also the main motivation people are now investing effort in _astro_
seismology. If we can get good oscillation data on other stars we'll be
in a position to pin their models down the way we've done for the Sun,
though the latter will always be best known.
--
ciao,
Bruce
drift wave turbulence: http://www.rzg.mpg.de/~bds/
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| User: "Steve Willner" |
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| Title: Re: Fusion: what's the diff? |
15 Aug 2005 02:14:42 PM |
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In article <200508151207.j7FC7M04017806@ipp.mpg.de>,
Bruce Scott TOK <Use-Author-Supplied-Address-Header@[127.1]> writes:
The position and extent of the convection zone is now very well
understood; the position of the boundary to the radiative core is known
to three significant digits (there is a clear break in the gradient of
the sound speed, exactly the signature of the onset of convection).
http://astro.phys.au.dk/helio_outreach/english/engHA0.html
http://solarphysics.livingreviews.org/open?pubNo=lrsp-2005-1&page=title.html
Just a quick note that while this is all true for the Sun, the start
of the convection zone is strongly dependent on stellar mass. Stars
much more massive than the Sun are convective at the cores (because
of the temperature dependence of the CNO cycle) but not at the
surfaces (because hydrogen and helium are ionized all the way to the
surface). In stars much less massive than the Sun, the "surface"
convective zone may extend all the way to the core, but none of these
stars has ever become a red giant because they haven't lived long
enough.
This gives just a small flavor of how complicated the full range of
stellar properties can be.
--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)
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| User: "Yousuf Khan" |
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| Title: Re: Fusion: what's the diff? |
16 Aug 2005 12:27:49 PM |
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Steve Willner wrote:
Just a quick note that while this is all true for the Sun, the start
of the convection zone is strongly dependent on stellar mass. Stars
much more massive than the Sun are convective at the cores (because
of the temperature dependence of the CNO cycle) but not at the
surfaces (because hydrogen and helium are ionized all the way to the
surface).
Are you saying that the cores of more massive stars are more fluid than
in the Sun? I was under the impression that stellar cores were
basically solid due to the extreme pressure.
Also in solid cores wouldn't the heat transmission be carried out by
conduction rather than convection or radiation?
In stars much less massive than the Sun, the "surface"
convective zone may extend all the way to the core, but none of these
stars has ever become a red giant because they haven't lived long
enough.
So you're saying that it's not necessarily true that the less massive a
star is the longer it lives? Some small stars live for less time than
the Sun?
Yousuf Khan
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| User: "Odysseus" |
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| Title: Re: Fusion: what's the diff? |
17 Aug 2005 12:19:52 AM |
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Yousuf Khan wrote:
Steve Willner wrote:
<snip>
In stars much less massive than the Sun, the "surface"
convective zone may extend all the way to the core, but none of these
stars has ever become a red giant because they haven't lived long
enough.
So you're saying that it's not necessarily true that the less massive a
star is the longer it lives? Some small stars live for less time than
the Sun?
I don't think so: since the smallest stars have lifetimes at least an
order of magnitude longer than the present age of the universe, none
of them has even reached middle age, let alone entered the final
red-giant (red-dwarf-giant?) stage.
--
Odysseus
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| User: "Yousuf Khan" |
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| Title: Re: Fusion: what's the diff? |
17 Aug 2005 12:51:47 PM |
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Odysseus wrote:
Yousuf Khan wrote:
Steve Willner wrote:
In stars much less massive than the Sun, the "surface"
convective zone may extend all the way to the core, but none of these
stars has ever become a red giant because they haven't lived long
enough.
So you're saying that it's not necessarily true that the less massive a
star is the longer it lives? Some small stars live for less time than
the Sun?
I don't think so: since the smallest stars have lifetimes at least an
order of magnitude longer than the present age of the universe, none
of them has even reached middle age, let alone entered the final
red-giant (red-dwarf-giant?) stage.
Oh, I see, misinterpretation of the wording. He was saying these stars
haven't lived long enough to reach their red-giant stage. Whereas I was
thinking he meant that somehow these stars have already fizzled out
long ago.
Yousuf Khan
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| User: "Bruce Scott TOK" |
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| Title: Re: Fusion: what's the diff? |
17 Aug 2005 06:40:48 AM |
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Yousuf Khan wrote:
Are you saying that the cores of more massive stars are more fluid than
in the Sun? I was under the impression that stellar cores were
basically solid due to the extreme pressure.
Nothing is solid at these conditions. The phase of stellar matter is
gas. The only exception is neutron star cores which appear to be
something like superfluid, with quantum effects important for the whole
structure.
In the Earth, though core temperatures are high by ordinary standards
(some thousands of degrees K), they are very very low compared to
stellar conditions, for which 5000K is a cool surface and 1 million K is
a low-temperature core. The solar core temperature is about 15 million K.
Also in solid cores wouldn't the heat transmission be carried out by
conduction rather than convection or radiation?
In solid cores, yes, neglecting "plastic" flow. But conduction in stars
is only important when electron degeneracy makes the mean free path
really long. Conduction << radiative transfer under most stellar
conditions.
In stars much less massive than the Sun, the "surface"
convective zone may extend all the way to the core, but none of these
stars has ever become a red giant because they haven't lived long
enough.
So you're saying that it's not necessarily true that the less massive a
star is the longer it lives? Some small stars live for less time than
the Sun?
No, lifetime scales strongly inversely with mass. Convective refers to
the way heat is transported out, but the motions are slow, and the
amount of energy to be transported depends on gravitational contraction
of a body under hydrostatic equilibrium... the more mass the faster it
goes and the higher the temperature. Gravitational contraction is
stopped when fusion reactions start, and these also scale strongly with
temperature. This is why stellar lifetime gets short when the mass gets
larger: more stuff to burn but it burns much faster.
BTW to Steve W: I am almost ready for that PPI scaling calculation...
--
ciao,
Bruce
drift wave turbulence: http://www.rzg.mpg.de/~bds/
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| User: "Autymn D. C." |
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| Title: Re: Fusion: what's the diff? |
17 Aug 2005 07:49:07 AM |
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Kelvins are not degrees.
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| User: "Yousuf Khan" |
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| Title: Re: Fusion: what's the diff? |
17 Aug 2005 12:56:07 PM |
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Autymn D. C. wrote:
Kelvins are not degrees.
I'm sure everyone understood what he meant.
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| User: "Y.Porat" |
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| Title: Re: Fusion: what's the diff? |
11 Aug 2005 11:32:59 PM |
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i agree with most of your post
under huge pressure smaller nuclei are combined to bigger ones
and you will be surprised that the 'electron collapse' is somewhat
easier than people imagine because .......
THERE ARE NOT SO MANY ELECTRONS TO COLLAPSE AS THE PARADIGM STATES!!
just have a look at my site
and you will get a tangible idea about what is a small nucleus
and what is a big one like say Lead
or the Iron one !!
and most important:
what are the active electrons
and what are electrons that exist only in the imagination of the
paradigm people!!
(that model is a new era in physics though presented in a very
'primitive look')
ATB
Y.Porat
------------------------------
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| User: "Bruce Scott TOK" |
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| Title: Re: Fusion: what's the diff? |
05 Aug 2005 07:46:15 AM |
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Yusuf Khan asked:
Bruce Scott TOK wrote:
Yes, with about equal probability (tritium and proton, or He3 and a
neutron). These reactions do not involve neutrinos because they are
("conventional") strong nuclear reactions. Look at the details of the
above reactions and note that no proton gets converted into a neutron or
vice versa.
Okay, so the emission of a neutrino is a sure sign that it's a Weak
Force reaction?
Yes, exactly. And the demand is made by (1) charge conservation, so in
conversion of a p to an n you need to make a positron (e+) or in
conversion of an n to a p you need to make an electron (e-). And now,
(2) you must also conserve lepton number. So if you made an e+ you have
to make a neutrino (nu) or if you made an e- you have to make an
anti-neutrino (nu-bar). Hence, beta decay, which changes the atomic
number of a nucleus (its charge, i.e., its number of protons), is a weak
interaction which creates an e- and hence also a nu-bar. Inverse beta
decay creates an e+ and a nu. So for the pp reaction to occur you must
have them close enough together for long enough (basically an unstable
He2 non-bound state) in order for the inverse beta decay to occur.
In any of these reactions an up quark is converted into a down quark or
vice versa.
I don't remember all the PPI chain numbers. Go to a library and look
them up in:
Clayton, D, Principles of Stellar Evolution and Nucleosynthesis,
McGraw-Hill (1968)
Found this instead:
Solar Fusion & Neutrinos
http://www.tim-thompson.com/fusion.html#ppcycle
OK. Hope it helped. Don't forget Bahcall's site, especially for latest
info on anything involving solar neutrinos (he also has non-technical
versions).
The probability of a reaction per unit volume depends on the density
(amount of stuff) and temperature (how fast it is flying about and
potentially running into each other). Higher density --> more
encounters within a given distance. Higher temperature --> more kinetic
energy per encounter on average.
And that's why supermassive stars use up their reactants faster,
because they're denser and hotter, I guess?
Yes. THey use a different sort of reaction... look up CNO cycle on
your site above, and then one of Bahcall's notes concerning the fact
that we know enough about solar neutrinos to know the PPI chain and not
the CNO cycle makes most of the Sun's power.
A couple of more questions.
(1) When they write that fusion bombs are harnessing the power of the
Sun, are they really? I mean journalists tend to get very imprecise
sometimes. Is the fusion that takes place in a fusion bomb simulate the
Sun, or is it more like the inside of a brown dwarf?
It's lazy. At the most superficial level, H bombs and the Sun both do
fusion. But the pp reaction is so low-probability we're never going to
make anything practical with it in the lab. In fact, we're probably
never going to detect the reaction (recall the indirect observation
using the solar pp neutrino didn't occur until 1992). The information
we have on the cross section comes from calculations.
(2) Will our Sun go through all of the fusion cycles and finish off at
iron? Or is that only for supernova class stars? Where will our Sun
finish off?
The Sun is not massive enough. It will get as far as the
3 alpha --> C12
reaction but that's about it. It will send off some outer layers and
form a planetary nebulae, possibly also undergo shell flashes, but the
core will settle down, held up by "electron degeneracy" well above the
density at which "neutron drip" occurs (google those terms or look them
up in [1] below), and finish as a slowly cooling white dwarf.
[1] Shapiro, S, and Teukolsky, S, Black Holes, White Dwarfs, and Neutron
Stars: the physics of compact objects, Wiley (1983)
--
ciao,
Bruce
drift wave turbulence: http://www.rzg.mpg.de/~bds/
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| User: "Yousuf Khan" |
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| Title: Re: Fusion: what's the diff? |
05 Aug 2005 07:02:52 PM |
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Bruce Scott TOK wrote:
And that's why supermassive stars use up their reactants faster,
because they're denser and hotter, I guess?
Yes. THey use a different sort of reaction... look up CNO cycle on
your site above, and then one of Bahcall's notes concerning the fact
that we know enough about solar neutrinos to know the PPI chain and not
the CNO cycle makes most of the Sun's power.
Yeah, I just barely glanced over the CNO stuff earlier when I visited
the site. Really the only thing I really remember about that cycle is
that it's supposedly producing upto 2% of the heat of the Sun. So I
assume you're saying that the CNO cycle is producing more of the heat
inside supergiant stars?
Also I gotta wonder about the earliest stars, the ones that came right
after the Big Bang? Were they completely pp cycle stars even if they
were supergiants, since there was no carbon, nitrogen or oxygen yet? Of
course that's assuming that the Big Bang only left us with a bunch of
hydrogen, and there was nothing higher-order produced by the BB.
It's lazy. At the most superficial level, H bombs and the Sun both do
fusion. But the pp reaction is so low-probability we're never going to
make anything practical with it in the lab. In fact, we're probably
never going to detect the reaction (recall the indirect observation
using the solar pp neutrino didn't occur until 1992). The information
we have on the cross section comes from calculations.
But have we produced pp reactions inside accelerators, practical or
not?
The Sun is not massive enough. It will get as far as the
3 alpha --> C12
reaction but that's about it. It will send off some outer layers and
form a planetary nebulae, possibly also undergo shell flashes, but the
core will settle down, held up by "electron degeneracy" well above the
density at which "neutron drip" occurs (google those terms or look them
up in [1] below), and finish as a slowly cooling white dwarf.
[1] Shapiro, S, and Teukolsky, S, Black Holes, White Dwarfs, and Neutron
Stars: the physics of compact objects, Wiley (1983)
Okay, having Googled a bunch of sites, it looks like this has the best,
non-technical description of neutron drip, good old Wikipedia:
Neutron star - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Neutron_star
So it would seem that the Sun is only just below the limit to go
Supernova, at the 1.44 Solar masses Chandrekar limit. It's 44% below, I
guess.
Yousuf Khan
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| User: "Bruce Scott TOK" |
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| Title: Re: Fusion: what's the diff? |
08 Aug 2005 12:36:22 PM |
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Yousuf Khan wrote:
|> Bruce Scott TOK wrote:
|> > >And that's why supermassive stars use up their reactants faster,
|> > >because they're denser and hotter, I guess?
|> >
|> > Yes. THey use a different sort of reaction... look up CNO cycle on
|> > your site above, and then one of Bahcall's notes concerning the fact
|> > that we know enough about solar neutrinos to know the PPI chain and not
|> > the CNO cycle makes most of the Sun's power.
|>
|> Yeah, I just barely glanced over the CNO stuff earlier when I visited
|> the site. Really the only thing I really remember about that cycle is
|> that it's supposedly producing upto 2% of the heat of the Sun. So I
|> assume you're saying that the CNO cycle is producing more of the heat
|> inside supergiant stars?
CNO has a much steeper temperature dependence (cf: Clayton) which is why
it takes over for more massive stars. The transition is rather abrupt
and the Sun is just barely not massive enough to be CNO dominant.
|> Also I gotta wonder about the earliest stars, the ones that came right
|> after the Big Bang? Were they completely pp cycle stars even if they
|> were supergiants, since there was no carbon, nitrogen or oxygen yet? Of
|> course that's assuming that the Big Bang only left us with a bunch of
|> hydrogen, and there was nothing higher-order produced by the BB.
Google "stellar populations".
There was a news piece on this not 10 days ago.
_Really_ primordial stars would have no CNO elements, but only about
100 My after galaxy formation you already have lots of them around.
|> But have we produced pp reactions inside accelerators, practical or
|> not?
Not p + p --> D, but p + p --> many p's and lots of other stuff, which
you get when the energy is high enough to create particles (1 GeV is the
proton/neutron mass scale). Those are all strong interactions.
|> Okay, having Googled a bunch of sites, it looks like this has the best,
|> non-technical description of neutron drip, good old Wikipedia:
|>
|> Neutron star - Wikipedia, the free encyclopedia
|> http://en.wikipedia.org/wiki/Neutron_star
|>
|> So it would seem that the Sun is only just below the limit to go
|> Supernova, at the 1.44 Solar masses Chandrekar limit. It's 44% below, I
|> guess.
It turns out these effects scale very steeply with stellar mass, so you
don't get all those exotic effects unless you're really close to that
limit.
--
ciao,
Bruce
drift wave turbulence: http://www.rzg.mpg.de/~bds/
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