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?

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

In article <47f5ca9...@news.bnb-lp.com>,
 Yousuf Khan <bbb...@yahoo.com> 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     swill...@cfa.harvard.edu
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, will...@cfa.harvard.edu (Steve Willner) wrote:





In article <47f5ca9...@news.bnb-lp.com>,
 Yousuf Khan <bbb...@yahoo.com> 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     swill...@cfa.harvard.edu
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 -

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.

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?

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?

In article <d4acb518-be9c-42fb-86b8-2337f589f39c@d45g2000hsc.googlegroups.com>,
YKhan <yjkhan@gmail.com> 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"

In article <47f5ca9...@news.bnb-lp.com>,
Yousuf Khan <bbb...@yahoo.com> 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.

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.

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

Novae involve binary stars.

On Apr 10, 5:44 pm, will...@cfa.harvard.edu (Steve Willner) wrote:

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

Novae involve binary stars.


Does it? I always assumed that all stars go nova (or supernova) once
they've reached their last stage of fuel burning, after which they
become white dwarfs. Just to blow off their last shells of non-
burnable atmosphere.

Yousuf Khan

Stars like the sun blow off their outer layers but it's gradual, not

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

Why is the bottom of Sun=B4s convective layer too cool for fusion, and
why are the red dwarfs convective to higher temperatures?

I always assumed that all stars go nova (or supernova) once
they've reached their last stage of fuel burning,