Thread: fusion stops in Sun. It expands cools and Earth gets... hotter?

In pop astronomy when the sun expands in its last throes the Earth gets hotter.

But this can't be right.

The sun's surface radiates as much as it does due to its heat energy escaping through it's surface. Its heat is stable over time because the escaped energy is being replaced by fusion.

But when it expands and the fusion stops the heat energy has to be distributed over a larger volume so the surface tempreture goes down while the surface area goes up. The energy radiated is no longer being replaced but the sun still has a lot of internal heat so the surface access to the remaining heat. If that were the only consideration the volume of the expanded sun and its new surface would radiate only a little less to the Earth as it did before.



But The heat energy of the volume now has a lower surface area to volume ratio so the energy has a smaller route from which to escape the sun. It takes longer to escape from an expanded sun than it did from the smaller Sun.

So unless the Earth comes into direct contact with the surface of the red expanded Sun The radiant energy from the Sun to the Earth should decrease not increase. It would be as if, by expanding, the Sun is wraping itself in insulation preventing as much energy from escaping as did when it was smaller.

Pop astronomy? Does that include Phil Plait's second book?

I only have his first book here, but I do have the other at home.

The Sun would not expand unless its heat output had increased. Think about it. Yes, the surface is dimmer per unit area, but the total irradiance is greater for a red giant.

Originally Posted by RayTomes

The Sun would not expand unless its heat output had increased. Think about it. Yes, the surface is dimmer per unit area, but the total irradiance is greater for a red giant.

No. The Sun expands because it has lost so much mass due to fusion that it can no longer maintain the gravitational confinement and compression of its mass. It doesn't get hotter. It gets cooler because it no longer has the gravitational compression or hydrogen fuel to run its fusion engine. It's losing mass right now, its gravity is slowly weakening and its expanding right now.

Originally Posted by astrotech

So unless the Earth comes into direct contact with the surface of the red expanded Sun The radiant energy from the Sun to the Earth should decrease not increase. It would be as if, by expanding, the Sun is wraping itself in insulation preventing as much energy from escaping as did when it was smaller.

As I recall the scenario, the expanding sun will engulf the earth...

Originally Posted by David E. Eaton Sr.

As I recall the scenario, the expanding sun will engulf the earth...

That's one scenario. The most common one is that the Sun doesn't get as big as the orbit of Earth.

Fusion does not stop in the sun during the red giant phase. Hydrogen in the core becomes exhausted, which collapses, heats from 15 million kelvin to 100 million kelvin, and shifts to helium fusion, forming light elements. The increased heat stimulates hydrogen fusion outside the core, and the whole star expands about 200 radii. While the outer portions become cooler, 40,000 times as much surface area means it radiates a lot more energy. Helium fusion is unstable, sensitive to small variations in temperature, so the star becomes highly variable. Eventually the pulsations become violent enough to blow off the star's atmosphere, which exposes the immediate area of the sun to direct ultraviolet radiation from the core. The core then becomes a white dwarf, and fusion really has stopped at that point.

Originally Posted by Larry Caldwell

Fusion does not stop in the sun during the red giant phase. Hydrogen in the core becomes exhausted, which collapses, heats from 15 million kelvin to 100 million kelvin, and shifts to helium fusion, forming light elements. The increased heat stimulates hydrogen fusion outside the core, and the whole star expands about 200 radii. While the outer portions become cooler, 40,000 times as much surface area means it radiates a lot more energy. Helium fusion is unstable, sensitive to small variations in temperature, so the star becomes highly variable. Eventually the pulsations become violent enough to blow off the star's atmosphere, which exposes the immediate area of the sun to direct ultraviolet radiation from the core. The core then becomes a white dwarf, and fusion really has stopped at that point.

That is as I would understand it.. well said, Larry Caldwell. ... Extrapolate the effect forward.. It gets hot round here.

Originally Posted by Larry Caldwell

Fusion does not stop in the sun during the red giant phase. Hydrogen in the core becomes exhausted, which collapses, heats from 15 million kelvin to 100 million kelvin, and shifts to helium fusion, forming light elements. The increased heat stimulates hydrogen fusion outside the core, and the whole star expands about 200 radii. While the outer portions become cooler, 40,000 times as much surface area means it radiates a lot more energy. Helium fusion is unstable, sensitive to small variations in temperature, so the star becomes highly variable. Eventually the pulsations become violent enough to blow off the star's atmosphere, which exposes the immediate area of the sun to direct ultraviolet radiation from the core. The core then becomes a white dwarf, and fusion really has stopped at that point.

That scenario introduces even more conficting situations (though it is a reasonable paraphrasing of the wikipedia article). What is the mechanical nature of the core "collapse". Cooling and shrinking? How does it cool and shrink yet the tempreture increase? Fusion itself doesn't cause any resistance to gravitational compression. What kind of weasle word is "collapse" anyway? What? The hydrogen fuel is "exhausted" and the core collapses like an empty soda can?

Helium fusion and fusion of "light" elements all the way to iron is happening right now anyway. Those fusions are not self sustaining. They are powered by the heat of the hydrogen fusion and the gravitational compression that exists now. Your description is more like a description of the early stages of a supernova.

Originally Posted by astrotech

Fusion itself doesn't cause any resistance to gravitational compression.

JOOC, why not?

What kind of weasle word is "collapse" anyway? What? The hydroVgen fuel is "exhausted" and the core collapses like an empty soda can?

what kind of weasel word is "weasel"?

What's wrong with the word "collapse"? Pressure holds it out, and when the pressure disappears, it falls in on itself.

Helium fusion and fusion of "light" elements all the way to iron is happening right now anyway.

You mean in the sun?