Why Is It So?

[Baddad]

The current and accepted theory of the life of our Sun is that it is about 4 or 5 billion years old. In about another 4 to 5 billion years time it will dramatically change.

As hydrogen fuel is consumed the Sun will expand and maybe engulf the Earth. It will eventually reach a point of maximum and then compact into a dwarf.

What is the physics behind the initial expansion?

Currently there is a balance that maintains the Sun's size. Without the huge energy output the Sun would collapse to a fraction of its current size under gravitational pressure. This is what confuses me. Once H2 becomes depleted I imagine the Sun would shrink but the converse is what scientists believe is to happen.

Why is it so?

Cheers:)

[multiweb]

My (limited) understanding is that the Sun is in equilibrium. Gravity pulls in, Radiation pressure pushes out. Gravity forces are a function of its mass which decreases as it burns its fuel. So as gravity decreases its atmosphere layers will expand outwards during the red giant phases. Then collapse again when nothing's left to burn. :question:

[el_draco]

Simplistically..... My understanding is that as the Hydrogen is converted into Helium, the Helium concentration increases in the core and Hydrogen "burning" moves outward. Contraction around the core causes a massive increase in pressure which raises temperature and ignites the Helium fusion process. Lots more energy produced and the star expands. Repeat for successive elements.

[Baddad]

One source I found explains the same as what you guys have described. Also the H2 is forced to the surface where it continues fusing and the temperature increases, higher energy output which leads to the expansion.

Once the H2 is "burnt" the whole thing collapses on itself. I think I'm beginning to understand what is happening here. Thanks for your help gentlemen.

Fascinating that with more mass a black hole was possible.

Cheers:)