Please explain why the sun's size will increase as it begins to burn off its fuel?

Question

Thanx.

Answer 1

Helium is heavier than hydrogen, so the helium produced by the fusion of hydrogen piles up in the core. Eventually, there will not be enough hydrogen left in the core (it will be displaced by the helium) to sustain fusion. So, without the heat to keep the core in equilibrium, the center of the sun will contract. This will make it hotter, and it will reach a point where it will be hot enough to ignite and start fusing helium. Since this helium fusion is done at a much higher temperature, the sun will therefore produce more heat, and that will dilate its outer layers, which will swell out.

Answer 2

The sun is currently fusing hydrogen into helium. This reaction occurs at the Sun's present core temperature of about 13 million Kelvins. When the supply of hydrogen starts to run low another reaction will begin to take place - fusing 3 helium nuclei into carbon or 4 helium nuclei into oxygen. These two reactions occur at a much higher temperature - around 100 million Kelvins. The higher temperature will cause the sun's outer layers to expand, in the same way that any gas expands when heated.

Answer 3

When the sun runs out of hydrogen, the core will shrink under the force of gravity. This shrinking causes the core to heat up until it gets hot enough to fuse helium into carbon. The much hotter core puts out more energy, and this extra energy exerts pressure and pushes the outer layers of the sun outward.

Answer 4

As it burns off mass, it loses density, and the loss of density means its gravitational attraction is lessened, which allows the bonds that hold the sun's mass together to loosen and as this happens the sun will increase in size. It will be larger, but less dense. An interesting phenomenon, isn't it?

Answer 5

I disagree with all the previous answers, along with the questioner's assertion. From what little I know about cosmic evolution, only certain stars (Red Giants, for example) with a mass exceeding a certain amount, will supernova. The sun is not massive enough, and its future is to become a brown dwarf till its fuel is spent, when it will just fizzle out and die.

Answer 6

I see that question is pretty well answered. Also, there is a beautiful and complete article about the present-day sun at http://www.solcomhouse.com/solar.html.