Sun's Expansion: Why Inner Planets Will Be Affected

[shihab-kol]

I read in some books that as the sun approaches the end of the fusion process(the complete one) , it will expand and engulf some of the inner planets.
But why will it expand?

 

[Astronuc]

I read in some books that as the sun approaches the end of the fusion process(the complete one) , it will expand and engulf some of the inner planets.
But why will it expand?

One should be able to find discussions on the topic of expansion by searching with "stellar evolution, expansion"

One such discussion:

The next stage of solar fuel consumption starts when hydrogen burning in the core ceases and ignites hydrogen burning in the star's outer layers. When hydrogen burning ceases in the star's core, it begins to collapse again. At this point, the star converts gravitational energy into thermal energy because it must maintain thermal equilibrium.

Stars sustain thermal equilibrium within their interiors through the ignition of helium burning. The collapse of the outer hydrogen burning shell upon the core raises the temperature and pressure in the core and begins helium burning. Because the temperature and energy needed to ignite helium fusion is greater than that of hydrogen, the energy released by helium fusion in the star's core is greater than needed to support the weight of the outer layer.

This excess energy expands the star's outer layers beyond its previous radius and star's volume increases. A star going through this stage of fuel consumption (collapse and expansion) is a Red Giant. The following diagram shows the dramatic expansion of a main sequence star as it begins helium fusion.

Ref: http://umich.edu/~gs265/star.htm

Another example: http://www.astro.utu.fi/~cflynn/Stars/l10.html

And - http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/redgia.html

 

[newjerseyrunner]

Because the core has run out of hydrogen fuel and is starting to burn helium. This creates far more heat, which causes it to puff up,

 

[stefan r]

Because the core has run out of hydrogen fuel and is starting to burn helium. This creates far more heat, which causes it to puff up,

It is hydrogen burning in a shell around the core that puffs it up. The helium flash could make the sun shrink temporarily. Helium will not be burned in the sun for hundreds of millions of years after leaving the main sequence and becoming a red giant.

 

[Drakkith]

My understanding is that as the core collapses, the density increases, exerting a greater gravitational force on the layers above the core. The first layer outside of the core is a shell of hydrogen undergoing fusion. As the force of gravity increases, temperature and pressure in the shell increases, forcing the rate of fusion in the shell has to increase as well. Additionally, the increase in gravity also increases the size of the shell by bringing more of the hydrogen into a region where it can undergo fusion. This increase in the rate of fusion in the hydrogen-burning shell (and increasing the size of the shell) greatly increases the energy released by the star, and is the source of the energy that puffs up the outer layers.

I'm sure I've gotten some of the details a bit incorrect, but I think that's the gist of it.

 

[snorkack]

But Kirchhoff´s Law merely specifies that luminosity increases with radius (second power) and surface temperature (fourth power).
If fusion power in star core increases, under Kirchhoff´s Law, the star might:

1. Shrink and heat up
2. Remain unchanged in size and heat up
3. Expand and heat up
4. Expand and remain at constant temperature
5. Expand and cool

Which of these 5 happens, and why this one out of the 5? Kirchhoff´s Law would allow any of the 5.

 

[stefan r]

... Kirchhoff´s Law would allow any of the 5.

Right. Some examples:

1, pre main sequence, Henyey track . Also the jump from red giant to red clump or horizontal branch.
1.5a, Hayashi tract shrinks and stays nearly the same temp in small stars.
1.5b, helium flash same size and temp.
2. Accretion of metal. Not sure how that would happen sans science fiction. You can compare two stars with same radius/mass. The more metallic will be hotter.

3, Nova
4, Rarely does anything real stay constant while you are changing it. Main sequence stars come very close. Over the next billion years the sun will expand and become more luminous but the surface temperature should be slightly under 5800K.
5, Red giant and asymptotic giant branch.

 

[nikkkom]

I read in some books that as the sun approaches the end of the fusion process(the complete one) , it will expand and engulf some of the inner planets.
But why will it expand?

Whether the star would expand or contract in a particular stage of its evolution is heavily dependent on equation of state of the matter in its many layers. If layers are compositionally complex, you may need a supercomputer to accurately predict what would happen.

In the particular case of the main sequence -> red giant transition for a Sun, it's not that complex. The following happens.

As Sun burns hydrogen in its core, core very slowly becomes denser (because otherwise fusion rate would decline proportionally to the depletion of hydrogen, temperature and pressure would fall, while the gravity of the Sun's mass would stay the same, and Sun would be collapsing, which obviously isn't happening). The outer layers of the Sun do not become denser - they retain the same composition and remain in essentially the same balance of gravity versus gas pressure.

As Sun nears the end of its main sequence, core has contracted rather significantly compared to young Sun and now the gravity exerts larger pressure within it. Sun is about 40% more luminous now than it was ~9 billion years ago, and correspondingly its outer layers expanded a little, since now more energy is escaping from the inside.

Now when core burns its last percents of hydrogen, it shrinks A LOT, and quickly (within some thousands of years). The core now looks like a helium white dwarf (a very small, some 1% of Sun's diameter, very dense object) sitting inside the Sun. The gravity-induced pressure on the surface of the core ("surface of the core" being the boundary between 100% helium core and not-yet-100% helium plasma above it) is rising quickly, and the temperature of the gas has to rise to maintain the balance. Which it does by increasing the rate of fusion in the not-yet-100% helium layer adjacent to the core.

The outer layers of the Sun still retain the same composition as they were ~9 billion years ago. But insides get hotter and hotter, more and more energy needs to escape from the inside. Outer layers expand, this time significantly.