Red Giants and Supergiants (1 Viewer)

[Chinmoku03]

Ok, while doing the investigation, I've been reading books talking about how a main sequence star transforms into a red giant.

From what I understand, when the hydrogen in the star core is completely used up, no outward radiation pressure can balance the inward gravitational pressure, loss in gravitational potential energy equates to gain in kinetic energy in the form of heat. The temperature increases high enough for helium nuclei to start fusing to produce carbon.

Now here's the bit I don't get. It says a shell where hydrogen fusion occurs develops around the inner core. If the hydrogen was used up in the core when becoming a red giant, where did the extra hydrogen that initiates the hydrogen fusion in the shell come from?

Thanks in advance.

 

[alcalder]

While the core is burning/fusing hydrogen to form helium, there is an outer layer of the star that is non-reacting hydrogen.

When the star begins to collapse, because the fusing core has used up all the hydrogen in the core only, the outer layer of non-reacting hydrogen begins to heat up and it also starts fusing. The heat in the outer layer causes the star to expand and it then becomes a red giant.

 

[Chinmoku03]

I see, thanks very much ^.^