Do u know why a red giant expands, i dont know either. The fact is that a star is not being blown off due to its gravity, though its a 5 billion year old nuke bomb - an continuous on going explosion.

When the energy level of this system gets reduced it should shrink, as the gravity is maximum at the surface and pressure is max at the center of the star.

Any comments ?

IIRC, the red giant expansion occurs when a star swiches over from hydrogen burning to helium burning. Now while the energy is less per fusion event for helium burning, the reaction rate is much higher, so the "wattage" of a helium burning star is higher and it expands into a red giant.

Do u know why a red giant expands, i dont know either. The fact is that a star is not being blown off due to its gravity, though its a 5 billion year old nuke bomb - an continuous on going explosion.

When the energy level of this system gets reduced it should shrink, as the gravity is maximum at the surface and pressure is max at the center of the star.

Any comments ?

The energy output actually increases with a red giant. (this is what drives the expansion.)

As the star uses up its hydrogen at the core, more and more helium is produced. Finally, this heium build up "chokes out" the fusion at the core. This Helium core begins to contract. As the it contracts, it converts gravitational potential into heat and grows hotter, when it gets hot enough, fusion starts again, but not at the core. Instead, it starts in the layer of hydrogen just above the Helium core.

Since the volume of this shell is larger than the region of the orginal fusion at the core, it actually fuses hydrogen at a faster rate and produces more energy. This energy causes the outer layers to expand, and the star becomes more luminous. The increase in surface area outpaces the increase in energy output, so even though the total energy output of the star is more than is was before, the energy per m² of the surface is less. the surface of the star is cooler than it was before and it reddens. You get a red giant.

Janus58 , u sound like a real Astrophysicist.

I will have to think about your comments after i am being able to grasp it.

But theres one question for u. If u say heavier element sink at the core then by that standard, dont u think that all the lighter gases should be in the upmost layers of the earths atmosphere and all the heaviest gases should be near the earths surface ?

Janus58 , u sound like a real Astrophysicist.


But theres one question for u. If u say heavier element sink at the core then by that standard, dont u think that all the lighter gases should be in the upmost layers of the earths atmosphere and all the heaviest gases should be near the earths surface ?

Let's see, our atmosphere's composition is:

78% nitrogen
21% oxygen
.93% argon
.04% carbon dioxide

The remaining .03% are traces of radon, methane, helium etc.

As to why they do not separate out into layers,

One reason is that the two major components nitrogen and oxygen are very close to each other in atomic weight with Oxygen just slighty heavier as 16 than Nitrogen at 14 (in fact , Oxygen and Nitrogen are right next to each other on the periodic table.) So the tendancy for them to separate out into layers is small. Small enough that air currents in the atmosphere are enough to keep them stirred up.

The mixing effect affects the rest of the gases also.

Any gases such as helium that are light enough to overcome this mixing effect do float up to the top of the atmosphere, and are lost to space. But they are constantly being replaced by natural processes. (helium, for instance, is a natural by-product of radioactive decay.)

Compare this to the core of the Sun, where the helium formed by fusion has four times the atomic weight of hydrogen. Even with mixing through convection, this is enough of a difference for the helium to tend to settle at the center. (Consider that in the Earth's atmosphere, Nitrogen has 3.5 times the atomic weight of Helium, and even with with atmospheric mixing, the helium rises.)

This is new for me, but this link say other wise http://www.chemprofessor.com/mix.htm,

Am i confused ?

The energy output actually increases with a red giant. (this is what drives the expansion.)

Why is this so ?


As the star uses up its hydrogen at the core, more and more helium is produced. Finally, this heium build up "chokes out" the fusion at the core.

So that should make the star colder.

This Helium core begins to contract. As the it contracts, it converts gravitational potential into heat and grows hotter, when it gets hot enough, fusion starts again, but not at the core. Instead, it starts in the layer of hydrogen just above the Helium core.

Why doesnt the heat fuse the helium too ?


Since the volume of this shell is larger than the region of the orginal fusion at the core, it actually fuses hydrogen at a faster rate and produces more energy.

As soon as the volume increases the G will be reduced and that will reduce the fusion.


This energy causes the outer layers to expand, and the star becomes more luminous. The increase in surface area outpaces the increase in energy output, so even though the total energy output of the star is more than is was before, the energy per m² of the surface is less. the surface of the star is cooler than it was before and it reddens. You get a red giant.

When the fusion is reduced the star will get cold and shrink back.

I think the following. Correct me if I am wrong:

Hydrogen burning still occurs in a red giant star, but it occurs in a shell rather than in the core like the original star. Because there is now enough helium in the core to initiate helium burning (Lawson criterion, triple-alpha process?). But we miss something crucial before this happens.

As fusion occurs, the mass difference as Janus pointed out is very large between the helium and hydrogen. This can only mean that once it is produced, helium sinks and accumulates in the core region where D-T fusion still occurs. Because the density is extremely great in the core region, something (I'm not quite sure specifically as to what) happens that turns the helium in and around the core upside down into a new inert core reservoir, waiting to be fused. The helium requires more energy to fuse so it hasn't "ignited" yet, but it keeps getting dumped by the shell of D-T fusion surrounding it. Eventually, because the star does not lose much mass up to this point (in proportion to itself), the gravity is more or less the same but the Kelvin-Helmholtz equilibrium is not satisfied once the helium core forms, and it starts to contract. Contraction occurs because non-fusing helium behaves as a compressible gas.

Whether the star is snuffed out completely between the stages is not within my knowledge. To this point I've always assumed that hydrogen burning still occurs while the contraction happens. Does it?

I think the following. Correct me if I am wrong:

Hydrogen burning still occurs in a red giant star, but it occurs in a shell rather than in the core like the original star. Because there is now enough helium in the core to initiate helium burning (Lawson criterion, triple-alpha process?). But we miss something crucial before this happens.

As fusion occurs, the mass difference as Janus pointed out is very large between the helium and hydrogen. This can only mean that once it is produced, helium sinks and accumulates in the core region where D-T fusion still occurs. Because the density is extremely great in the core region, something (I'm not quite sure specifically as to what) happens that turns the helium in and around the core upside down into a new inert core reservoir, waiting to be fused. The helium requires more energy to fuse so it hasn't "ignited" yet, but it keeps getting dumped by the shell of D-T fusion surrounding it. Eventually, because the star does not lose much mass up to this point (in proportion to itself), the gravity is more or less the same but the Kelvin-Helmholtz equilibrium is not satisfied once the helium core forms, and it starts to contract. Contraction occurs because non-fusing helium behaves as a compressible gas.

Whether the star is snuffed out completely between the stages is not within my knowledge. To this point I've always assumed that hydrogen burning still occurs while the contraction happens. Does it?

The fact that G doesnt change and infact it must increase due to lessened volume of the star. But that doesnt mean that outer surface should expand. The G will act as it did before or even better at shrinking the star.

So i think the that red giant are red giant stars and not expanded ones.