Why Do Old Stars Expand ?

Phineas T Puddleduck wrote in
news
In article ,
(Luminoso) wrote:

As the outer layers cool, they expand.

Odd ... everywhere ELSE in the universe, cooling
gasses SHRINK.

The gravity of the star will remain about the same, the
mass of the gas molecules will remain the same. If the
temperature diminishes then the gas layer will be pulled
CLOSER to the stars core, not pushed away. If you cool
the earth, the atmosphere becomes denser and its outer
boundary doesn't stick out so far into space.

You may have gotten that one wrong.


No.

Add in the extra radiation pressure from the new helium burning core as
well.



The key point is that the total energy production in the core sharply
increases when He burning starts. The star must expand to maintain
equilibrium. You have the combined effect of approx black body radiation
law vs surface area versus modified gas laws.

Klamzon.

Llanzlan Klazmon the 15th wrote:
Scott Miller wrote in news:f1qpvk$269e$1
@news.louisville.edu:


Luminoso wrote:

Someone once implied that a large star fuses every last atom
of hydrogen, then rapidly contracts until there's enough
pressure/heat to start on the helium - but the helium yeilds
less energy than the hydrogen. Even larger stars employ
a 'CNO' cycle that further blurs things.

First of all, why would the transition from hydrogen to
helium be so abrupt ? It seems like the hydrogen would
go through a stage where it was getting rarer and rarer,
cooling the star and causing it to slowly contract until
pressures were in the range where helium could begin to
be used. No reason for it to all happen in a flash, unless
there's a huge energy gap in the temperature/pressure
required for the first heliums to fuse -vs- the hydrogens.
Only then would a (relatively) abrupt contraction be necessary.

Meanwhile though, old stars are supposed to EXPAND. What's
up with this ? It's getting less energy from the last reserves
of hydrogen, then it's getting less energy from the helium.
Seems the star should get SMALLER, not larger. Maybe there
would be an oscillation right when the hydrogen quit and
the helium lit. Stars the size of our sun may not even
be able to produce enough umph to sustain helium fusion,
so when the hydrogen is gone that's the end and the star
shrinks down to a dwarf (perhaps with a bit of a bang).

Yet I keep seeing projections that our sun will expand
until it encompasses the orbit of the earth, maybe even
mars. What's supposed to make a cooling, dying star of
that size increase its diameter ???


I'll try to keep this simple.

Hydrogen fusion can occur in the core of a star where the temperatures
and densities are such that it is an efficient form of energy. In the
case of the Sun, that is about the inner thirty percent of the interior
of the Sun which may involve about 10% of the Sun's mass. Hydrogen
fusion creates helium as an inert, degenerate matter accumulating in the
core.

Eventually, there is insufficient hydrogen to fuse into helium, so that
there is no energy supporting the core of helium. It reacts to gravity
and contracts.

Since the helium is inert (degenerate in nature), its contraction simply
raises the temperature in the core, allowing hydrogen fusion to begin in
a shell just outside the collapsing core of helium. Since this new
hydrogen fusion source is closer to the outer two-thirds of the star, it
is a heat source for the cooler outer parts of the interior. The
reaction to such a heat source is to expand. As the outer layers of the
interior expand, they cool - you have formed a red giant.


That explanation isn't correct. It has nothing to do with the heat source
being closer. The reason for the expansion is that the total energy
produced by the fusing shell around the helium core is greater than what
was earlier produced in the just the core and this heat production
gradually increases as the star evolves. The star must radiate the extra
energy to stay in equilibrium. This effectively causes the star's outer
layers to expand.

Klazmon

From "Astronomy: Journey from the Cosmic Frontier", 4th edition by John
Fix, p. 450.

"After a hydrogen "burning" shell developes in a star, its structure and
appearance begin to change rather quickly...The shell within which
hydrogen is being converted to helium burns itself outward through the
mass of the star like a prairie fire. The helium 'ash' left behind
settles into the core of the star, making the core denser and more
massive. At the same time, the surface of the star expands and cools..."

From "Universe", 5th edition, by William Kaufmann and Roger Freedman,
pp. 520-521.

"At the end of the star's main-sequence lifetime, all the hydrogen in
its core has been used up and hydrogen burning ceases there. In this
new stage, hydrogen burning still continues but only in the
hydrogen-rich material surrounding the core...this process occurs only
in the hottest region just outside the core, where the hydrogen fuel has
not yet been exhausted...As the core contracts, its temperature again
increases, and heat begins to flow outward from the core even though no
nuclear reactions are taking place there...This new flow of heat warms
the gases around the core, increasing the rate of shell hydrogen burning
and making the shell eat further outward into the surrounding
matter...During this post-main-sequence phase, the star's outer layers
expand just as dramatically...

Both of these quotes say essentially what I said. As I said at the
beginning of my post, I was not going to go into lots of detail - keep
it simple. And what I said was essentially correct.

Luminoso wrote:

Odd ... everywhere ELSE in the universe, cooling
gasses SHRINK.

The gravity of the star will remain about the same, the
mass of the gas molecules will remain the same. If the
temperature diminishes then the gas layer will be pulled
CLOSER to the stars core, not pushed away. If you cool
the earth, the atmosphere becomes denser and its outer
boundary doesn't stick out so far into space.

You may have gotten that one wrong.

He was essentially correct. The core is not "ordinary" matter as we
think of it. It is in the form of what is called degenerate matter.
Degenerate matter acts differently than ordinary matter when heated. In
particular, there is no "safety valve" like ordinary matter when you
heat the stuff. It will continue to contract gravitationally, and that
will increase its density and temperature. In the case of the helium
core, this can eventually lead to the temperatures and densities
necessary to break the degeneracy and initiate helium fusion. That is
what is known as the helium flash.

Scott Miller wrote in
:

Llanzlan Klazmon the 15th wrote:
Scott Miller wrote in news:f1qpvk$269e$1
@news.louisville.edu:


Luminoso wrote:

Someone once implied that a large star fuses every last atom
of hydrogen, then rapidly contracts until there's enough
pressure/heat to start on the helium - but the helium yeilds
less energy than the hydrogen. Even larger stars employ
a 'CNO' cycle that further blurs things.

First of all, why would the transition from hydrogen to
helium be so abrupt ? It seems like the hydrogen would
go through a stage where it was getting rarer and rarer,
cooling the star and causing it to slowly contract until
pressures were in the range where helium could begin to
be used. No reason for it to all happen in a flash, unless
there's a huge energy gap in the temperature/pressure
required for the first heliums to fuse -vs- the hydrogens.
Only then would a (relatively) abrupt contraction be necessary.

Meanwhile though, old stars are supposed to EXPAND. What's
up with this ? It's getting less energy from the last reserves
of hydrogen, then it's getting less energy from the helium.
Seems the star should get SMALLER, not larger. Maybe there
would be an oscillation right when the hydrogen quit and
the helium lit. Stars the size of our sun may not even
be able to produce enough umph to sustain helium fusion,
so when the hydrogen is gone that's the end and the star
shrinks down to a dwarf (perhaps with a bit of a bang).

Yet I keep seeing projections that our sun will expand
until it encompasses the orbit of the earth, maybe even
mars. What's supposed to make a cooling, dying star of
that size increase its diameter ???


I'll try to keep this simple.

Hydrogen fusion can occur in the core of a star where the temperatures
and densities are such that it is an efficient form of energy. In the
case of the Sun, that is about the inner thirty percent of the interior
of the Sun which may involve about 10% of the Sun's mass. Hydrogen
fusion creates helium as an inert, degenerate matter accumulating in
the core.

Eventually, there is insufficient hydrogen to fuse into helium, so that
there is no energy supporting the core of helium. It reacts to gravity
and contracts.

Since the helium is inert (degenerate in nature), its contraction
simply raises the temperature in the core, allowing hydrogen fusion to
begin in a shell just outside the collapsing core of helium. Since
this new hydrogen fusion source is closer to the outer two-thirds of
the star, it is a heat source for the cooler outer parts of the
interior. The reaction to such a heat source is to expand. As the
outer layers of the interior expand, they cool - you have formed a red
giant.


That explanation isn't correct. It has nothing to do with the heat
source being closer. The reason for the expansion is that the total
energy produced by the fusing shell around the helium core is greater
than what was earlier produced in the just the core and this heat
production gradually increases as the star evolves. The star must
radiate the extra energy to stay in equilibrium. This effectively
causes the star's outer layers to expand.

Klazmon

From "Astronomy: Journey from the Cosmic Frontier", 4th edition by John
Fix, p. 450.

"After a hydrogen "burning" shell developes in a star, its structure and
appearance begin to change rather quickly...The shell within which
hydrogen is being converted to helium burns itself outward through the
mass of the star like a prairie fire. The helium 'ash' left behind
settles into the core of the star, making the core denser and more
massive. At the same time, the surface of the star expands and
cools..."

As I said, the cause of the expansion is the far greater rate of heat
production in the core (helium burning). The 'expand and cool' is a
reaction to the greater heat production in the core as it is the only way
the star can get rid of the extra heat.



From "Universe", 5th edition, by William Kaufmann and Roger Freedman,
pp. 520-521.

"At the end of the star's main-sequence lifetime, all the hydrogen in
its core has been used up and hydrogen burning ceases there. In this
new stage, hydrogen burning still continues but only in the
hydrogen-rich material surrounding the core...this process occurs only
in the hottest region just outside the core, where the hydrogen fuel has
not yet been exhausted...As the core contracts, its temperature again
increases, and heat begins to flow outward from the core even though no
nuclear reactions are taking place there...This new flow of heat warms
the gases around the core, increasing the rate of shell hydrogen burning
and making the shell eat further outward into the surrounding
matter...During this post-main-sequence phase, the star's outer layers
expand just as dramatically...

Both of these quotes say essentially what I said. As I said at the
beginning of my post, I was not going to go into lots of detail - keep
it simple. And what I said was essentially correct.

The part I was objecting to was your saying about the heat source being
closer. That can only be a transitory effect when we are talking of a star
near equilibrium.

Klazmon.

Llanzlan Klazmon the 15th wrote:

The part I was objecting to was your saying about the heat source being
closer. That can only be a transitory effect when we are talking of a star
near equilibrium.

Klazmon.

If the hydrogen shell is moving outward, that would make it closer as a
heat source than before that shell fusion was initiated. The outer
layers are reacting to that source and moving away. You are picking
nits where there are few to be picked.

On May 10, 10:34 pm, Scott Miller wrote:
Llanzlan Klazmon the 15th wrote:



The part I was objecting to was your saying about the heat source being
closer. That can only be a transitory effect when we are talking of a star
near equilibrium.

Klazmon.

If the hydrogen shell is moving outward, that would make it closer as a
heat source than before that shell fusion was initiated. The outer
layers are reacting to that source and moving away. You are picking
nits where there are few to be picked.

The star has to get rid of the heat whether it is produced in the core
or in a shell around the core. Where the heat is produced is not
relevant to the expansion, which is a simple gas law/radiation
pressure effect versus Stefan-Botzmann radiation from the star's
photosphere. Again it is the increased quantity of heat production
versus what is produced during the main sequence phase that causes the
star to expand. Of course things aren't that simple in the core itself
where the gas laws break down due to degeneracy.

Bill

On 10 May 2007 11:53:28 +1200, Llanzlan Klazmon the 15th
wrote:

Phineas T Puddleduck wrote in
news
In article ,
(Luminoso) wrote:

As the outer layers cool, they expand.

Odd ... everywhere ELSE in the universe, cooling
gasses SHRINK.

The gravity of the star will remain about the same, the
mass of the gas molecules will remain the same. If the
temperature diminishes then the gas layer will be pulled
CLOSER to the stars core, not pushed away. If you cool
the earth, the atmosphere becomes denser and its outer
boundary doesn't stick out so far into space.

You may have gotten that one wrong.


No.

Add in the extra radiation pressure from the new helium burning core as
well.



The key point is that the total energy production in the core sharply
increases when He burning starts. The star must expand to maintain
equilibrium. You have the combined effect of approx black body radiation
law vs surface area versus modified gas laws.


But then the expansion due to the heat will decrease
the density of the core and the helium reaction will
immediately die out. For the helium to keep burning
it MUST be kept under a certain minimum of pressure.

It's not even clear how hydrogen stars survive their
initial start-up. You'd think that when the fusion
threshold is reached the sudden heat would pretty
much blast apart the star, throwing much of the
material off into space. Indeed you can SEE such
a displacement of material in some astrophotos of
'stellar nurseries' - a big clear ring forms around
the new star, cast out by the burst of heat and the
radiation pressure.

Can gravity alone pull back the material again and
again, even as it's heated more and more by each
successive burst of fusion ? That would make a
stellar start-up kind of like a choo-choo train
picking up speed :-)

(Luminoso) wrote in news:46438d7f.38393546
@news.east.earthlink.net:

On 10 May 2007 11:53:28 +1200, Llanzlan Klazmon the 15th
wrote:

Phineas T Puddleduck wrote in
news
In article ,
(Luminoso) wrote:

As the outer layers cool, they expand.

Odd ... everywhere ELSE in the universe, cooling
gasses SHRINK.

The gravity of the star will remain about the same, the
mass of the gas molecules will remain the same. If the
temperature diminishes then the gas layer will be pulled
CLOSER to the stars core, not pushed away. If you cool
the earth, the atmosphere becomes denser and its outer
boundary doesn't stick out so far into space.

You may have gotten that one wrong.


No.

Add in the extra radiation pressure from the new helium burning core as
well.



The key point is that the total energy production in the core sharply
increases when He burning starts. The star must expand to maintain
equilibrium. You have the combined effect of approx black body radiation
law vs surface area versus modified gas laws.


But then the expansion due to the heat will decrease
the density of the core and the helium reaction will
immediately die out. For the helium to keep burning
it MUST be kept under a certain minimum of pressure.

The core is degenerate. It is no longer a gas.

Klamzon



It's not even clear how hydrogen stars survive their
initial start-up. You'd think that when the fusion
threshold is reached the sudden heat would pretty
much blast apart the star, throwing much of the
material off into space. Indeed you can SEE such
a displacement of material in some astrophotos of
'stellar nurseries' - a big clear ring forms around
the new star, cast out by the burst of heat and the
radiation pressure.

Can gravity alone pull back the material again and
again, even as it's heated more and more by each
successive burst of fusion ? That would make a
stellar start-up kind of like a choo-choo train
picking up speed :-)