This silly presentation that took long to make is suppose to illustrate how shell gamma rays can deposit more energy into the envelope of a star than gamma rays originating in the core of the star.

The two skyscrapers at the left represent two stars. The floors below ground level represent the core and the floors above ground level represent the envelope.

The runners represent two gamma rays created by fusion. The runner in the basement is a core gamma ray, while the runner starting at the ground floor is a shell gamma ray. They are racing to the top.

The race begins

The basement runner has to first climb the basement stairs to reach the ground floor.

In the process he loses some energy.

In a similar manner, core gamma rays deposit some of their energy in the core and enter the envelope with less energy.

The runner that started on the ground floor has much more energy as he runs through the “envelope”

So the shell gamma rays, that do not have to bounce out of the core, bring more energy into the envelope.

The basement runner has lost much of his energy and has only a small amount left to use.

So core gamma rays lose most of their energy exiting the core and lower envelope and have only a small amount to give to the outer layers of a star.

The runners approach the top of the skyscraper.

Both are tired.

The runner from the ground floor lost his energy in the envelope.

While the runner from the basement lost much of his energy just in getting out of the basement.

So the shell gamma rays, that do not have to bounce out of the core, bring more energy into the envelope. In a star this excess energy causes the envelope to swell up and transform the star into a giant star.

Even as the core of a star collapses, the envelope can swell as energy production shifts to shell fusion. Giant stars have very tiny cores and energy generation in shells outside the core. Each time a new shell fusion event begins, the star will swell again.

In stars less massive than 5 solar masses the onset of shell Hydrogen fusion causes the star to become a giant for the first time. Hydrogen fusion in a shell begins

When shell Helium fusion begins around the now all carbon core of the star, it swells for the second time into the giant branch. Helium fusion in a shell begins

In high mass stars, the evolution is so rapid that one shell fusion follow close on the heels of the previous shell fusion and the star moves into the giant region and stays there until its explosive end.