Star Ignition Question

[Daryl McCullough]

A friend of mine raised the question as to what the
current theory is about how stars "ignite". That is,
how hydrogen fusion inside a star gets started. If
you have an initial huge ball of hydrogen, the hydrogen
will become compressed through gravitational attraction.
But is gravity alone sufficient to compress and heat hydrogen
enough to ignite a self-sustaining nuclear fusion reaction?

According to this web page
http://nuclearplanet.com/stellar%20ignition%20and%20dark%20matter.htm
JM Herdon made the claim in 1996 that heating due to infalling matter is not
sufficient. Is Herndon's claim considered respectable?

--
Daryl McCullough
Ithaca, NY

[mathman]

The proposal by Herdon that nuclear fission is needed to ignite the fusion process in the stars doesn't make much sense. After the big bang most of the baryonic matter in the universe was H1 and He4 with small amounts of other light isotopes up to Li7. Fissionable materials just weren't there. As a result the first generation stars had only H fusion to power them. The main point is that if there is enough stuff (like the sun) fusion will take place.

You need a fission trigger for an H bomb. However, the controlled fusion programs don't use any such trigger.

[Phillip Helbig---remove CLOTHES to reply]

In article <g23qhe0kl0@drn.newsguy.com>, stevendaryl3016@yahoo.com
(Daryl McCullough) writes:
> A friend of mine raised the question as to what the
> current theory is about how stars "ignite".
>
> According to this web page
> http://nuclearplanet.com/stellar%20ignition%20and%20dark%20matter.htm
> JM Herdon made the claim in 1996 that heating due to infalling matter is not
> sufficient. Is Herndon's claim considered respectable?

No. Note that the page is also maintained by the same JM Herndon.

This is something which, as far as I know, is simply not a problem in
conventional astrophysics. Sounds similar to creationists claiming
"evolution proved to be a hoax" and so on in a manner which, to a casual
reader, gives the impression that an outsider (creationist) has solved a
major problem in the field (which, however, is unknown to those working
in the field) with a radical hypothesis. A quote from:

http://nuclearplanet.com/index.html

"Earth, says geophysicist J. Marvin Herndon, is a gigantic natural
nuclear power plant."

Look at http://nuclearplanet.com/Important%20Discoveries.htm and draw
your own conclusions.

[Oh No]

Thus spake Daryl McCullough <stevendaryl3016@yahoo.com>
>A friend of mine raised the question as to what the
>current theory is about how stars "ignite". That is,
>how hydrogen fusion inside a star gets started. If
>you have an initial huge ball of hydrogen, the hydrogen
>will become compressed through gravitational attraction.
>But is gravity alone sufficient to compress and heat hydrogen
>enough to ignite a self-sustaining nuclear fusion reaction?

yes
>
>According to this web page
>http://nuclearplanet.com/stellar%20ignition%20and%20dark%20matter.htm
>JM Herdon made the claim in 1996 that heating due to infalling matter is not
>sufficient. Is Herndon's claim considered respectable?
>
I have not heard of this theory, but the basic theory of stellar
evolution works extremely well. One can calculate the mass required to
generate sufficient heat to ignite fusion, and one finds good agreement
with observed sizes of stars.

Regards

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.teleconnection.info/rqg/MainIndex

[Uncle Al]

Daryl McCullough wrote:
>
> A friend of mine raised the question as to what the
> current theory is about how stars "ignite". That is,
> how hydrogen fusion inside a star gets started. If
> you have an initial huge ball of hydrogen, the hydrogen
> will become compressed through gravitational attraction.
> But is gravity alone sufficient to compress and heat hydrogen
> enough to ignite a self-sustaining nuclear fusion reaction?
>
> According to this web page
> http://nuclearplanet.com/stellar%20ignition%20and%20dark%20matter.htm
> JM Herdon made the claim in 1996 that heating due to infalling matter is not
> sufficient. Is Herndon's claim considered respectable?

Daryl McCullough wrote:
>
> A friend of mine raised the question as to what the
> current theory is about how stars "ignite". That is,
> how hydrogen fusion inside a star gets started. If
> you have an initial huge ball of hydrogen, the hydrogen
> will become compressed through gravitational attraction.
> But is gravity alone sufficient to compress and heat hydrogen
> enough to ignite a self-sustaining nuclear fusion reaction?
>
> According to this web page
> http://nuclearplanet.com/stellar%20ignition%20and%20dark%20matter.htm
> JM Herdon made the claim in 1996 that heating due to infalling matter is not
> sufficient. Is Herndon's claim considered respectable?

"Heating by the in-fall of dust and gas is takes place at the surface
of the forming star." THERE'S your problem.

The star will contract and pseudoadiabatically heat from compression.
Gravitation is relentless. Core conditions will eventually reach
fusion ignition conditions because temperature is the only thing that
inflates a star against collapse. When you crush Fermi exclusion you
cannot help but generate heat well beyond chemistry.

Look up core conditions in the sun. It's heat output/volume is less
than mammalian metabolism. There is a lot of volume and a lot of
time.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2

[Ian Parker]

On 6 Jun, 15:34, stevendaryl3...@yahoo.com (Daryl McCullough) wrote:
> A friend of mine raised the question as to what the
> current theory is about how stars "ignite". That is,
> how hydrogen fusion inside a star gets started. If
> you have an initial huge ball of hydrogen, the hydrogen
> will become compressed through gravitational attraction.
> But is gravity alone sufficient to compress and heat hydrogen
> enough to ignite a self-sustaining nuclear fusion reaction?
>
> According to this web pagehttp://nuclearplanet.com/stellar%20ignition%20and%20dark%20matter.htm
> JM Herdon made the claim in 1996 that heating due to infalling matter is not
> sufficient. Is Herndon's claim considered respectable?
>
Let us put a few numbers in. Let us assume that the nebula is at about
4K. How much does it have to be compressed to get to 100 million K

Now TV(gamma-1) is a constant

http://en.wikipedia.org/wiki/Adiabatic_process

Temperature goes up by a factor of 25 million. For a monatomic gas
gamma = 5/3. Probable average for interstellar gas thoughout its range
is 1.3.

V^-0.3 = 25 million

Hence V contracts by about 4*10^24. If we take the cube root of this
we have a contraction of 1.5*10^8 in radius.

As stars start of from a nebula some light year across this seems an
eminently reasonable answer. The volume contraction is quite a bit
above what would be needed by adiabatic equations. There is a lot of
complexity. The star starts spinning, planets form. There is a great
deal of complexity in what actually happens but there is no a priori
reason for assuming shock waves. In fact angular momentum and planet
formation tends to make the compression slower than it would otherwise
be.


- Ian Parker

[Robert S]

On 6 Jun, 15:34, stevendaryl3...@yahoo.com (Daryl McCullough) wrote:
> A friend of mine raised the question as to what the
> current theory is about how stars "ignite". That is,
> how hydrogen fusion inside a star gets started. If
> you have an initial huge ball of hydrogen, the hydrogen
> will become compressed through gravitational attraction.
> But is gravity alone sufficient to compress and heat hydrogen
> enough to ignite a self-sustaining nuclear fusion reaction?
>
> According to this web page
> http://nuclearplanet.com/stellar%20ignition%20and%20dark%20matter.htm
> JM Herdon made the claim in 1996 that heating due to infalling matter is not
> sufficient. Is Herndon's claim considered respectable?

Fusion is about temperature plus density plus how long matter is held
at that temperature and density.

What temperatures can pycnonuclear fusion occur at?

The idea of all star cores being originally ignited by fission is
ludicrous - where did the heavy nuclei originate?

[Daryl McCullough]

Ian Parker says...

>Let us put a few numbers in. Let us assume that the nebula is at about
>4K. How much does it have to be compressed to get to 100 million K
>
>Now TV(gamma-1) is a constant
>
>http://en.wikipedia.org/wiki/Adiabatic_process
>
>Temperature goes up by a factor of 25 million. For a monatomic gas
>gamma = 5/3. Probable average for interstellar gas thoughout its range
>is 1.3.
>
>V^-0.3 = 25 million
>
>Hence V contracts by about 4*10^24. If we take the cube root of this
>we have a contraction of 1.5*10^8 in radius.
>
>As stars start of from a nebula some light year across this seems an
>eminently reasonable answer.

The point made by Herndon is that stellar contraction is not
adiabatic, since energy at the surface of the star radiates
away into space (the energy lost per second is proportional
to the fourth power of the temperature). The consensus here
seems to be that Herndon is a crackpot, so I assume that this
effect has been taken into account in models of stellar ignition.

--
Daryl McCullough
Ithaca, NY

[Uncle Al]

Daryl McCullough wrote:
>
> Ian Parker says...
>
> >Let us put a few numbers in. Let us assume that the nebula is at about
> >4K. How much does it have to be compressed to get to 100 million K
> >
> >Now TV(gamma-1) is a constant
> >
> >http://en.wikipedia.org/wiki/Adiabatic_process
> >
> >Temperature goes up by a factor of 25 million. For a monatomic gas
> >gamma = 5/3. Probable average for interstellar gas thoughout its range
> >is 1.3.
> >
> >V^-0.3 = 25 million
> >
> >Hence V contracts by about 4*10^24. If we take the cube root of this
> >we have a contraction of 1.5*10^8 in radius.
> >
> >As stars start of from a nebula some light year across this seems an
> >eminently reasonable answer.
>
> The point made by Herndon is that stellar contraction is not
> adiabatic, since energy at the surface of the star radiates
> away into space (the energy lost per second is proportional
> to the fourth power of the temperature). The consensus here
> seems to be that Herndon is a crackpot, so I assume that this
> effect has been taken into account in models of stellar ignition.

Physics Today 61(6) 70 (2008)

"For objects with mass less than about 0.072[solar mass], degeneracy
pressure halts contraction before the critical H fusion temperature is
reached. Hydrostatic equilibrium but not thermal equilibrium is
achieved."

Surface radiation is a minor factor. It requires centuries for a core
photon to diffuse to the surface. Stellar core compression is
pseudoadiabatic as surely as a diesel cylinder works to spec despite
an active cooling system.

You might want to be elsewhere during ignition. It will be a positive
feedback big bump. Things then settle down as equilibrium spreads
outward.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2

[Ian Parker]

On 10 Jun, 17:52, stevendaryl3...@yahoo.com (Daryl McCullough) wrote:
> Ian Parker says...
>
> >Let us put a few numbers in. Let us assume that the nebula is at about
> >4K. How much does it have to be compressed to get to 100 million K
>
> >Now TV(gamma-1) is a constant
>
> >http://en.wikipedia.org/wiki/Adiabatic_process
>
> >Temperature goes up by a factor of 25 million. For a monatomic gas
> >gamma = 5/3. Probable average for interstellar gas thoughout its range
> >is 1.3.
>
> >V^-0.3 = 25 million
>
> >Hence V contracts by about 4*10^24. If we take the cube root of this
> >we have a contraction of 1.5*10^8 in radius.
>
> >As stars start of from a nebula some light year across this seems an
> >eminently reasonable answer.
>
> The point made by Herndon is that stellar contraction is not
> adiabatic, since energy at the surface of the star radiates
> away into space (the energy lost per second is proportional
> to the fourth power of the temperature). The consensus here
> seems to be that Herndon is a crackpot, so I assume that this
> effect has been taken into account in models of stellar ignition.
>
Not to put too fine a point on it I think he is cracked. There is no
reason to suppose that adiabatic compression cannot cause thrmonuclear
ignition. Indeed nebulae are observed to have a temperature of
20,000K. Of course thermonuclear fusion starts in an opaque star and
because of heat loss the outer layers are very much cooler than the
core.

To me the interesting question is about planet formation and not about
adiabatic compression. As stated earlier a gas clould has angular
momentum which stops the contraction. Only after planet formation can
thermonuclear reactions start. Let us look at a star contracting
WITHOUT angular momentum. Thermonuclear reactions start in the core,
but the star is still contracting at 500-1000km/s (the core faster)
this means that a detonation occurs in the core. Planet formation is
therefore integral to stellar ignition and a star contracts slowly as
planetary rings accelerate.

We have a solar system where the gas giants are in the outer reaches
of the solar system and rocky planets are near the Sun. Is this
typical? "Hot Jupiters" - gas giants near a star have been observed.
Their abundance may be due to the fact that a "typical" solar system
cannot be observed with present instruments. Nothing the size of the
Earth can be seen unless it is going round a pulsar. There are in
facts "Earths" going round pulsars.

Questions :- How do "Hot Jupiters" form? Are they typical?

:- How do secondary planets form after supernovae? Now
here there IS a role for shock waves. In a supernova SOME material
stays in the vicinity of gthe star and gets accelerated by magnetic
fields.

To me these are the interesting questions.


- Ian Parker

[Dirk Bruere at NeoPax]

Ian Parker wrote:

>
> We have a solar system where the gas giants are in the outer reaches
> of the solar system and rocky planets are near the Sun. Is this
> typical? "Hot Jupiters" - gas giants near a star have been observed.
> Their abundance may be due to the fact that a "typical" solar system
> cannot be observed with present instruments.

Do we know the abundance of hot Jupiters?

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
Remote Viewing classes in London

[Ian Parker]

On 12 Jun, 04:03, Dirk Bruere at NeoPax <dirk.bru...@gmail.com> wrote:
> Ian Parker wrote:
>
> > We have a solar system where the gas giants are in the outer reaches
> > of the solar system and rocky planets are near the Sun. Is this
> > typical? "Hot Jupiters" - gas giants near a star have been observed.
> > Their abundance may be due to the fact that a "typical" solar system
> > cannot be observed with present instruments.
>
> Do we know the abundance of hot Jupiters?
>
http://exoplanets.org/planet_table.shtml

This is a more up to date list with some discussion on selectivity.

http://en.wikipedia.org/wiki/Extrasolar_planets

Here is a comprehensive extrasolar table. Anything below 100 days is
hot. The number of HJs depends on selection. If you assume that all
HJs have been found (HJs are the easiest type of extrasolar planet to
detect since they produce strong doppler signals) it means that they
are rare. Set the list off against the number of stars.

- Ian Parker

[Thomas Smid]

On 7 Jun, 20:46, Ian Parker <ianpark...@gmail.com> wrote:
> On 6 Jun, 15:34, stevendaryl3...@yahoo.com (Daryl McCullough) wrote:> A friend of mine raised the question as to what the
> > current theory is about how stars "ignite". That is,
> > how hydrogen fusion inside a star gets started. If
> > you have an initial huge ball of hydrogen, the hydrogen
> > will become compressed through gravitational attraction.
> > But is gravity alone sufficient to compress and heat hydrogen
> > enough to ignite a self-sustaining nuclear fusion reaction?
>
> > According to this web pagehttp://nuclearplanet.com/stellar%20ignition%20and%20dark%20matter.htm
> > JM Herdon made the claim in 1996 that heating due to infalling matter is not
> > sufficient. Is Herndon's claim considered respectable?
>
> Let us put a few numbers in. Let us assume that the nebula is at about
> 4K. How much does it have to be compressed to get to 100 million K
>
> Now TV(gamma-1) is a constant
>
> http://en.wikipedia.org/wiki/Adiabatic_process
>
> Temperature goes up by a factor of 25 million. For a monatomic gas
> gamma = 5/3. Probable average for interstellar gas thoughout its range
> is 1.3.
>
> V^-0.3 = 25 million
>
> Hence V contracts by about 4*10^24. If we take the cube root of this
> we have a contraction of 1.5*10^8 in radius.
>
> As stars start of from a nebula some light year across this seems an
> eminently reasonable answer. The volume contraction is quite a bit
> above what would be needed by adiabatic equations. There is a lot of
> complexity. The star starts spinning, planets form. There is a great
> deal of complexity in what actually happens but there is no a priori
> reason for assuming shock waves. In fact angular momentum and planet
> formation tends to make the compression slower than it would otherwise
> be.
>
> - Ian Parker

The initial size of the cloud is actually irrelevant for the final
temperature of the star. The latter depends only on the average
potential energy of an atom in its gravitational field, which for an
atom of mass m and a star with mass M and radius R is of the order of
E_pot=-GmM/R . Because of the virial theorem of classical mechanics,
this must be (on average) equal to -2*kinetic energy i.e. E_kin=GmM/
(2R) which for the sun is about 1 keV i.e. T= 10^7 K (E_kin=kT).

The important point here is that in order for the cloud to collapse,
it must actually permanently lose energy. This requires inelastic
collision processes, which in turn will result in radiative emissions.
So strictly speaking there is in fact no particular point where a star
'ignites'. The emission of radiation (due to electronic processes)
goes hand in hand with the collapse.

See my page http://www.plasmaphysics.org.uk/research/starformation.htm
for more in this respect.

Thomas

[Jonathan Thornburg]

Thomas Smid <thomas.smid@gmail.com> wrote:
[[...]]
> The initial size of the cloud is actually irrelevant for the final
> temperature of the star. The latter depends only on the average
> potential energy of an atom in its gravitational field, which for an
> atom of mass m and a star with mass M and radius R is of the order of
> E_pot=-GmM/R . Because of the virial theorem of classical mechanics,
> this must be (on average) equal to -2*kinetic energy i.e. E_kin=GmM/
> (2R) which for the sun is about 1 keV i.e. T= 10^7 K (E_kin=kT).
>
> The important point here is that in order for the cloud to collapse,
> it must actually permanently lose energy. This requires inelastic
> collision processes, which in turn will result in radiative emissions.
> So strictly speaking there is in fact no particular point where a star
> 'ignites'. The emission of radiation (due to electronic processes)
> goes hand in hand with the collapse.

True, however there *is* a fairly sharp transition when Deuterium
ignites, and the radiated energy from D fusion exceeds that from
gravitational collapse just before then. At that point the collapse
stops and you have a roughly quasistationary protostar.
>
> See my page http://www.plasmaphysics.org.uk/research/starformation.htm
> for more in this respect.
>
> Thomas
>

[Thomas Smid]

On 20 Jun, 22:54, Jonathan Thornburg <jonat...@helium.soton.ac.uk>
wrote:
> Thomas Smid <thomas.s...@gmail.com> wrote:
>
> [[...]]
>
> > The initial size of the cloud is actually irrelevant for the final
> > temperature of the star. The latter depends only on the average
> > potential energy of an atom in its gravitational field, which for an
> > atom of mass m and a star with mass M and radius R is of the order of
> > E_pot=-GmM/R . Because of the virial theorem of classical mechanics,
> > this must be (on average) equal to -2*kinetic energy i.e. E_kin=GmM/
> > (2R) which for the sun is about 1 keV i.e. T= 10^7 K (E_kin=kT).
>
> > The important point here is that in order for the cloud to collapse,
> > it must actually permanently lose energy. This requires inelastic
> > collision processes, which in turn will result in radiative emissions.
> > So strictly speaking there is in fact no particular point where a star
> > 'ignites'. The emission of radiation (due to electronic processes)
> > goes hand in hand with the collapse.
>
> True, however there *is* a fairly sharp transition when Deuterium
> ignites, and the radiated energy from D fusion exceeds that from
> gravitational collapse just before then. At that point the collapse
> stops and you have a roughly quasistationary protostar.

There is in fact no fusion required to stop the collapse. It simply
will stop if the gas density becomes so high that no individual atoms
(and thus no inelastic collisions leading to radiative energy loss)
can exist anymore (which is of the order of 10^23 cm^-3).
There will be a very small energy loss (and thus contraction)
resulting from inelastic collisions in the (less dense) atmosphere of
the thus formed star, but this will be too small to be noticeable over
short periods of time.

Thomas

>
>
>
> > See my pagehttp://www.plasmaphysics.org.uk/research/starformation.htm
> > for more in this respect.
>
> > Thomas

[Craig Markwardt]

Thomas Smid <thomas.smid@gmail.com> writes:

> On 20 Jun, 22:54, Jonathan Thornburg <jonat...@helium.soton.ac.uk>
> wrote:
> >
> > True, however there *is* a fairly sharp transition when Deuterium
> > ignites, and the radiated energy from D fusion exceeds that from
> > gravitational collapse just before then. At that point the collapse
> > stops and you have a roughly quasistationary protostar.
>
> There is in fact no fusion required to stop the collapse. It simply
> will stop if the gas density becomes so high that no individual atoms
> (and thus no inelastic collisions leading to radiative energy loss)
> can exist anymore (which is of the order of 10^23 cm^-3).

Huh? Whether or not there are "individual atoms" -- which presumably
means neutral atoms -- is irrelevant. There are plenty of emission
mechanisms which can carry away energy radiatively (see free-free
emission). Even so, I suspect most (proto-)stars have mostly-neutral
atmospheres -- and probably in most cases molecular hydrogen -- since
typical stellar atmosphers are in the 3000-30000 K range, and the
dissociation temperature of H2 gas is ~50000 K.

CM

[Thomas Smid]

On 2 Jul, 21:18, Craig Markwardt
<craigm...@REMOVEcow.physics.wisc.edu> wrote:
> Thomas Smid <thomas.s...@gmail.com> writes:
> > On 20 Jun, 22:54, Jonathan Thornburg <jonat...@helium.soton.ac.uk>
> > wrote:
>
> > > True, however there *is* a fairly sharp transition when Deuterium
> > > ignites, and the radiated energy from D fusion exceeds that from
> > > gravitational collapse just before then. At that point the collapse
> > > stops and you have a roughly quasistationary protostar.
>
> > There is in fact no fusion required to stop the collapse. It simply
> > will stop if the gas density becomes so high that no individual atoms
> > (and thus no inelastic collisions leading to radiative energy loss)
> > can exist anymore (which is of the order of 10^23 cm^-3).
>
> Huh? Whether or not there are "individual atoms" -- which presumably
> means neutral atoms -- is irrelevant. There are plenty of emission
> mechanisms which can carry away energy radiatively (see free-free
> emission). Even so, I suspect most (proto-)stars have mostly-neutral
> atmospheres -- and probably in most cases molecular hydrogen -- since
> typical stellar atmosphers are in the 3000-30000 K range, and the
> dissociation temperature of H2 gas is ~50000 K.
>
> CM

I am not aware that free-free emissions would be considered to be of
any relevance for the energy loss in the context of star formation
(only collisional excitation is supposed to be responsible for this),
nor am I aware that it would be relevant for the subsequent evolution
and structure of stars. It can in fact not be relevant, because the
energy loss due to free-free emissions would strictly increase with
density, and there would thus not be a threshold density beyond which
no energy loss can take place any more (which is indicated by the
existence of the solar photosphere; see
http://groups.google.co.uk/group/sci.astro/browse_thread/thread/5e1fff3bd2e497e5/d6b6990cd022a73d
).

Thomas

[Craig Markwardt]

Thomas Smid <thomas.smid@gmail.com> writes:
> On 2 Jul, 21:18, Craig Markwardt
> <craigm...@REMOVEcow.physics.wisc.edu> wrote:
> > Thomas Smid <thomas.s...@gmail.com> writes:
> > > On 20 Jun, 22:54, Jonathan Thornburg <jonat...@helium.soton.ac.uk>
> > > wrote:
> >
> > > > True, however there *is* a fairly sharp transition when Deuterium
> > > > ignites, and the radiated energy from D fusion exceeds that from
> > > > gravitational collapse just before then. At that point the collapse
> > > > stops and you have a roughly quasistationary protostar.
> >
> > > There is in fact no fusion required to stop the collapse. It simply
> > > will stop if the gas density becomes so high that no individual atoms
> > > (and thus no inelastic collisions leading to radiative energy loss)
> > > can exist anymore (which is of the order of 10^23 cm^-3).
> >
> > Huh? Whether or not there are "individual atoms" -- which presumably
> > means neutral atoms -- is irrelevant. There are plenty of emission
> > mechanisms which can carry away energy radiatively (see free-free
> > emission). Even so, I suspect most (proto-)stars have mostly-neutral
> > atmospheres -- and probably in most cases molecular hydrogen -- since
> > typical stellar atmosphers are in the 3000-30000 K range, and the
> > dissociation temperature of H2 gas is ~50000 K.
> >
> > CM
>
> I am not aware that free-free emissions would be considered to be of
> any relevance for the energy loss in the context of star formation
> (only collisional excitation is supposed to be responsible for this),
> nor am I aware that it would be relevant for the subsequent evolution
> and structure of stars. It can in fact not be relevant, because the
> energy loss due to free-free emissions would strictly increase with
> density, and there would thus not be a threshold density beyond which
> no energy loss can take place any more (which is indicated by the
> existence of the solar photosphere; see

I believe that is a non-sequitur. You made the claim that somehow, if
"no individual atoms ... can exist", then the collapse of a star will
stop. My point was that there are plenty of mechanisms which allow a
(proto)star to emit energy, regardless of the ionization state of atoms.

Furthermore, your subsequent claim about a "threshold density beyond
which no energy loss can take place" seems to be misplaced. Since all
stars are radiating energy -- and hence "losing" energy -- your claim
seems to not be relevant.

CM

[Thomas Smid]

On 9 Jul, 07:15, Craig Markwardt
<craigm...@REMOVEcow.physics.wisc.edu> wrote:
> Thomas Smid <thomas.s...@gmail.com> writes:
> > On 2 Jul, 21:18, Craig Markwardt
> > <craigm...@REMOVEcow.physics.wisc.edu> wrote:
> > > Thomas Smid <thomas.s...@gmail.com> writes:
> > > > On 20 Jun, 22:54, Jonathan Thornburg <jonat...@helium.soton.ac.uk>
> > > > wrote:
>
> > > > > True, however there *is* a fairly sharp transition when Deuterium
> > > > > ignites, and the radiated energy from D fusion exceeds that from
> > > > > gravitational collapse just before then. At that point the collapse
> > > > > stops and you have a roughly quasistationary protostar.
>
> > > > There is in fact no fusion required to stop the collapse. It simply
> > > > will stop if the gas density becomes so high that no individual atoms
> > > > (and thus no inelastic collisions leading to radiative energy loss)
> > > > can exist anymore (which is of the order of 10^23 cm^-3).
>
> > > Huh? Whether or not there are "individual atoms" -- which presumably
> > > means neutral atoms -- is irrelevant. There are plenty of emission
> > > mechanisms which can carry away energy radiatively (see free-free
> > > emission). Even so, I suspect most (proto-)stars have mostly-neutral
> > > atmospheres -- and probably in most cases molecular hydrogen -- since
> > > typical stellar atmosphers are in the 3000-30000 K range, and the
> > > dissociation temperature of H2 gas is ~50000 K.
>
> > > CM
>
> > I am not aware that free-free emissions would be considered to be of
> > any relevance for the energy loss in the context of star formation
> > (only collisional excitation is supposed to be responsible for this),
> > nor am I aware that it would be relevant for the subsequent evolution
> > and structure of stars. It can in fact not be relevant, because the
> > energy loss due to free-free emissions would strictly increase with
> > density, and there would thus not be a threshold density beyond which
> > no energy loss can take place any more (which is indicated by the
> > existence of the solar photosphere; see
>
> I believe that is a non-sequitur. You made the claim that somehow, if
> "no individual atoms ... can exist", then the collapse of a star will
> stop. My point was that there are plenty of mechanisms which allow a
> (proto)star to emit energy, regardless of the ionization state of atoms.
>
> Furthermore, your subsequent claim about a "threshold density beyond
> which no energy loss can take place" seems to be misplaced. Since all
> stars are radiating energy -- and hence "losing" energy -- your claim
> seems to not be relevant.
>
> CM

Yes, stars are radiating (and thus lose energy). But this is only from
the relatively few atoms in the stellar atmosphere (where the density
is below the threshold). This energy loss is so small that the
resulting further contraction of the star is not noticeable over short
time scales.

Thomas