Convection inside a star

Now, I was told a long time ago that stars like the Sun, and bigger,
can't burn all of their hydrogen fuel, and they therefore die after only
a few billion years, if not after a few million years. That is because
they only burn the hydrogen in their cores, and hydrogen outside of this
region is not dense enough to begin fusion.

On the other hand, small red dwarf stars will last hundreds of billions
of years if not trillions, because they are able to burn most of the
hydrogen within them because more hydrogen keeps coming into their cores
from the atmosphere through convection. Can someone explain how it is
that convection can penetrate the core of a red dwarf, but not the
bigger stars?

Is it because the core of a red dwarf is less dense than that of a main
sequence star? And if that's the case, then it means that fusion doesn't
require such a dense core. So if a less dense core will do, then why
aren't there lower-density outer layers of the main sequence star cores
where convection can penetrate like those inside red dwarfs, thus
extending main sequence lifetimes?

Yousuf Khan

On 4/3/2008 11:28 PM Yousuf Khan brightened our day with:
Now, I was told a long time ago that stars like the Sun, and bigger,
can't burn all of their hydrogen fuel, and they therefore die after
only a few billion years, if not after a few million years. That is
because they only burn the hydrogen in their cores, and hydrogen
outside of this region is not dense enough to begin fusion.

On the other hand, small red dwarf stars will last hundreds of
billions of years if not trillions, because they are able to burn most
of the hydrogen within them because more hydrogen keeps coming into
their cores from the atmosphere through convection. Can someone
explain how it is that convection can penetrate the core of a red
dwarf, but not the bigger stars?

Is it because the core of a red dwarf is less dense than that of a
main sequence star? And if that's the case, then it means that fusion
doesn't require such a dense core. So if a less dense core will do,
then why aren't there lower-density outer layers of the main sequence
star cores where convection can penetrate like those inside red
dwarfs, thus extending main sequence lifetimes?

Yousuf Khan
convection doesn't play that big of a role in red dwarfs. They live so
long because they burn hydrogen more slowly. If there was zero
convection in a red dwarf they'd still last a really long time.

--
"Out here on the perimeter there are no stars"

Steve --Inglo--

In article ,
Yousuf Khan writes:
On the other hand, small red dwarf stars will last hundreds of billions
of years if not trillions, because they are able to burn most of the
hydrogen within them because more hydrogen keeps coming into their cores
from the atmosphere through convection.

As someone else pointed out, that last is a minor reason. The major
reason is that low mass stars are much less luminous than high mass
stars. Thus their fuel lasts a long longer.

Can someone explain how it is that convection can penetrate the
core of a red dwarf, but not the bigger stars?

It's because the temperature is lower. Lower temperature means the
radiative opacity ("Kramers opacity") is higher, and the energy has
to flow outward by convection instead. The Sun has a convective
layer but only near the surface. Lower mass stars are cooler
throughout, and the "surface" convective layer extends deeper into
the star. For stars that have low enough mass, the convective zone
extends all the way to the center.

--
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.)

On Apr 4, 10:24*pm, (Steve Willner) wrote:
In article ,
*Yousuf Khan writes:

On the other hand, small red dwarf stars will last hundreds of billions
of years if not trillions, because they are able to burn most of the
hydrogen within them because more hydrogen keeps coming into their cores
from the atmosphere through convection.

As someone else pointed out, that last is a minor reason. *The major
reason is that low mass stars are much less luminous than high mass
stars. *Thus their fuel lasts a long longer.

Can someone explain how it is that convection can penetrate the
core of a red dwarf, but not the bigger stars?

It's because the temperature is lower. *Lower temperature means the
radiative opacity ("Kramers opacity") is higher, and the energy has
to flow outward by convection instead. *The Sun has a convective
layer but only near the surface. *Lower mass stars are cooler
throughout, and the "surface" convective layer extends deeper into
the star. *For stars that have low enough mass, the convective zone
extends all the way to the center.

--
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.)

Geodynamics is inclined to suffer the same fate as stellar dynamics
insofar as the rotation of a composition in a viscous state generates
diffferential rotation and there is no reason to exempt the 40 km
deviation of the Earth from generalised rotational dynamics based on
the correlation between maximum equatorial speed,differential rotation
and sphericity.The first concept that has to be jettisoned in
'convection cells'.

I am not too surprised that when stellar dynamicists correlate the
greater Equatorial speeds with greater deviation in sphericity,they
omit the variations in differential rotation rates which occur
however it is a greater surprise that they do not make the leap to
geodynamics by generalising the principles of differential rotation
and apply it,along with the viscous internal composition of the
Earth,as the reason for the 40 KM deviation.

The problem is that 'convection cells' are geostationary
notions,having no correlatioin to rotaional dynamics and its effects
such as sphericity,the same goes for the viscous composition observed
in stars insofar as there is room for either observed differential
rotation or speculative convection cells but not both.

On Apr 5, 7:02*am, oriel36 wrote:
On Apr 4, 10:24*pm, (Steve Willner) wrote:





In article ,
*Yousuf Khan writes:

On the other hand, small red dwarf stars will last hundreds of billions
of years if not trillions, because they are able to burn most of the
hydrogen within them because more hydrogen keeps coming into their cores
from the atmosphere through convection.

As someone else pointed out, that last is a minor reason. *The major
reason is that low mass stars are much less luminous than high mass
stars. *Thus their fuel lasts a long longer.

Can someone explain how it is that convection can penetrate the
core of a red dwarf, but not the bigger stars?

It's because the temperature is lower. *Lower temperature means the
radiative opacity ("Kramers opacity") is higher, and the energy has
to flow outward by convection instead. *The Sun has a convective
layer but only near the surface. *Lower mass stars are cooler
throughout, and the "surface" convective layer extends deeper into
the star. *For stars that have low enough mass, the convective zone
extends all the way to the center.

--
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.)

Geodynamics *is inclined to suffer the same fate as stellar dynamics
insofar as the rotation of a composition in a viscous state *generates
diffferential rotation and there is no reason to exempt the 40 km
deviation of the Earth *from generalised rotational dynamics based on
the correlation between maximum equatorial speed,differential rotation
and sphericity.The first concept that has to be jettisoned in
'convection cells'.

I am not too surprised that when stellar dynamicists correlate the
greater Equatorial speeds with greater deviation in sphericity,they
omit the variations in differential rotation rates which occur
however it is a greater surprise that they do not make the leap to
geodynamics by generalising the principles of differential rotation
and apply it,along with the viscous internal composition of the
Earth,as the reason for the 40 KM deviation.

The problem is that 'convection cells' are geostationary
notions,having no correlatioin to rotaional dynamics and its effects
such as sphericity,the same goes for the viscous composition observed
in stars insofar as there is room for either observed differential
rotation or speculative convection cells but not both.- Hide quoted text -

- Show quoted text -

The generalised principles which correlate maximum equatorial speed
and differential rotation with deviation in sphericity can be observed
in rotating stars -

http://www.youtube.com/watch?v=iwCpcoS0jKc

The term 'generalised principles' is applied to any rotating celestial
body with a composition in a viscous state and the Earth's internal
structure contains such a state.While the 40 Km deviation and
rotational dynamics has been known for centuries,the specifics are
quite vague,if a person wishes to provide an alternative mechanism to
differential rotation then so well and good however the leap from
stellar rotational dynamics to geodynamics looks credible.

The secondary observation is that should differential rotation supply
the cause for deviation in sphericity,it would also provide a more
productive mechanism for plate tectonics and crustal motion/evolution.

On Apr 4, 11:25 am, LymanAlpha ioo@??.¿¿¿ wrote:
convection doesn't play that big of a role in red dwarfs. They live so
long because they burn hydrogen more slowly. If there was zero
convection in a red dwarf they'd still last a really long time.

Okay, what is expected to happen to red dwarves at the end of their
lives? Do they start burning heavier elements like helium? Or do they
go straight to nova and white dwarf state?

Yousuf Khan

Yousuf Khan wrote in :

Now, I was told a long time ago that stars like the Sun, and bigger,
can't burn all of their hydrogen fuel, and they therefore die after
only a few billion years, if not after a few million years. That is
because they only burn the hydrogen in their cores, and hydrogen
outside of this region is not dense enough to begin fusion.

On the other hand, small red dwarf stars will last hundreds of
billions of years if not trillions, because they are able to burn most
of the hydrogen within them because more hydrogen keeps coming into
their cores from the atmosphere through convection. Can someone
explain how it is that convection can penetrate the core of a red
dwarf, but not the bigger stars?

Is it because the core of a red dwarf is less dense than that of a
main sequence star? And if that's the case, then it means that fusion
doesn't require such a dense core. So if a less dense core will do,
then why aren't there lower-density outer layers of the main sequence
star cores where convection can penetrate like those inside red
dwarfs, thus extending main sequence lifetimes?

You've raised an excellent question, because fluid effects are present
throughout the entire star. There is no magic law of science asserting
that fluids stop behaving as fluids in that region where nuclear fusion
dominates.

There is still plenty of work to be done in theoretical modelling of the
plasma physics of the sun, as evinced by the fact that no simple
equation exists for the flipping of the solar magentic axis, into which
we would like to put the parameters of the solar meterial, and out of
which we would get the answer of 11.3 years. So far the phenomenon can
only be described by computer modelling.

That physics is quite probably much simpler than what occurs in the
solar core. There's no reason to suppose that the flows in the core
aren't highly turbulent, and again, turbulence is an unsolved problem
even in the simpler much domain of incompressible, non-ionized flows,
not to mention plasma physics at such high densities.

I suspect that you've probably hit upon an area where the research is
still incomplete, and there is still some interesting modelling to be
done, before the entire system is completely understood. If you have
the urge to know more, you might want to try to write down the starting
equations that describe what happens in the solar core, to see what
would be required to examine the interplay between nuclear processes,
radiation and convection, the solar core.

Of course, there is already a large body of published work in this area,
and before you try to pursue your own ideas, you may want to invest a
little library time, looking at the approaches that other people have
already examined.

In article ,
YKhan writes:
Okay, what is expected to happen to red dwarves at the end of their
lives? Do they start burning heavier elements like helium?

It depends on mass. The Sun will burn helium and thus go through a
giant stage, but the lowest-mass stars won't. I'm not sure exactly
where the dividing mass is, but somebody probably knows it. After
nuclear burning is over, the star becomes a white dwarf, supported by
electron degeneracy pressure, then gradually cools.

Or do they go straight to nova and white dwarf state?

Novae involve binary stars.

By the way, while I'm a big Tolkien fan too, most astronomers prefer
"dwarfs" for the plural of "dwarf."

--
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.)

On 4/10/08 2:44 PM Steve Willner brightened our day with:
In article ,
YKhan writes:

Okay, what is expected to happen to red dwarves at the end of their
lives? Do they start burning heavier elements like helium?


It depends on mass. The Sun will burn helium and thus go through a
giant stage, but the lowest-mass stars won't. I'm not sure exactly
where the dividing mass is, but somebody probably knows it. After
nuclear burning is over, the star becomes a white dwarf, supported by
electron degeneracy pressure, then gradually cools.


Or do they go straight to nova and white dwarf state?


Novae involve binary stars.

By the way, while I'm a big Tolkien fan too, most astronomers prefer
"dwarfs" for the plural of "dwarf."


Here's a good paper on "the end of the main sequence"
http://www.journals.uchicago.edu/doi/pdf/10.1086/304125

--
"Out here on the perimeter there are no stars"

Steve --Inglo--

On 4 apr, 23:24, (Steve Willner) wrote:
In article ,
Yousuf Khan writes:

On the other hand, small red dwarf stars will last hundreds of billions
of years if not trillions, because they are able to burn most of the
hydrogen within them because more hydrogen keeps coming into their cores
from the atmosphere through convection.

As someone else pointed out, that last is a minor reason. The major
reason is that low mass stars are much less luminous than high mass
stars. Thus their fuel lasts a long longer.

Can someone explain how it is that convection can penetrate the
core of a red dwarf, but not the bigger stars?

It's because the temperature is lower. Lower temperature means the
radiative opacity ("Kramers opacity") is higher, and the energy has
to flow outward by convection instead. The Sun has a convective
layer but only near the surface.

The temperature near the bottom is said to be about 2 millions of
kelvins.

Lower mass stars are cooler
throughout, and the "surface" convective layer extends deeper into
the star. For stars that have low enough mass, the convective zone
extends all the way to the center.

Which is sometimes hotter than 6 millions of kelvins.

Why is the bottom of Sun´s convective layer too cool for fusion, and
why are the red dwarfs convective to higher temperatures?