American science Rejecto!?

Question

Im curious on this, in astronomy class I have. Im confused about it.

It requires more heat to fuse helium than to fuse hydrogen, yet when a star has depleted its supply hydrogen it cools down! How does the star warm up again enough to fuse helium in its core?

Answer 1

I suspect you are confused by the idea of relative temperature. If the exterior surface gases are less hot,
does that necessarily mean that the interior core temperature is insufficient to fuse helium...? I don't think so.

Check any site you wish to refer to, and see what the interior temperature of the Sun is estimated to be...That temperature is well within the limits required for helium fusion even if reduced by one half, or more. It is all a matter of relative heat. What is hot, and what is cool? Cooler is less hot, that is all.

Now, where is this less hot area? That is what is important.

The surface areas of a star could cool off "relatively" fast because of the slowing deposits of super heated helium gas into the atmosphere by hydrogen fusion within the core.

Within the core of a star, hydrogen is fused into helium, right?Now, when helium is being fused, it is being fused, it is fused into what? Interesting, eh? Nobody talks about that... Would it be the next element in the atomic chart with three protons - Lithium? Ooooh, that is unstable. Maybe the next one, with four protons - Beryllium? Ooooh, another wild material. See, all those elements are on the same side of the chart as Na, Sodium, and K, Potassium; wildly unstable elements. Look at the next ones on the same side of that chart...Strontium and Cesium...atomic bomb materials...Wow! Who knows? I really don't know what material helium is fused into next but it is probably a real wild material, and in a liquid hot, molten state most likely very unstable. Stable elements are way over onto the right hand side of the atomic chart. See things like Helium, Neon, Argon, and Xenon.

However, whatever it is being fused into must be heavier than helium and hydrogen, right (see atomic weight charts)? If so, that materiel must sink down under the left over hydrogen and all that helium and remain in the hot, hot zone. So... seems like all the heat stays within the core, thus slowing the core temperature drop fairly well. The surface might cool off rather quickly in a relative manner of speaking. Note here that we are talking in terms of thousands, and thousands of years, not over night.

And, thousands compared to millions is "less." Or, as I suggested, quicker than the normal millions and millions of years for a significant change to take place.

I cannot testify that any of this is correct, however. It is only my idea of what happens, and might well be full of holes as some theories go.

Regards,
Zah

Answer 2

When a star stops fusing hydrogen it contracts, as the inward pull of gravity is no longer balanced by the radiation of the fusion reaction. This contraction causes the temperature and pressure in the core to increase. In a sufficiently large star, it will get hot enough to begin fusing helium. The tremendous heat of helium fusion causes the outer layers of the star to expand. This, paradoxically, makes the surface of the star cooler than it was before. Even though the star is radiating more energy than before, it is doing so over a much larger surface area, so the radiation per unit surface, and thus the surface temperature, is less now in the red giant phase than it was when the star was on the main sequence.

Answer 3

I think you have it backwards. Fusing hydrogen gives off more heat and energy than fusing helium. So even though a star has a conservastion of matter, it is being converted into a form that gives off less and less energy when fused. As the hydrogen supply wears out, eventually there not enough energy being given off to fuse heavier elements.

What causes the hydrogen to fuse in te first place is the pressure of gravity,