Stellar Evolution In post-Main-Sequence evolution, what you see on the surface is not a good indicator of what is happening deep in the interior

Core contraction occurs, core smaller and hotter Core contraction occurs, core smaller and hotter. Star “ascends the Giant Branch” Question: Why don’t The Helium nuclei Begin to fuse?

In low mass PMS stars, the “Helium Flash” occurs In low mass PMS stars, the “Helium Flash” occurs. Nuclear reactions involving Helium – Carbon. Also called the “Triple Alpha Process” 3He C

At time of the Helium Flash, a star evolves to the right on the HR diagram, at near constant luminosity

Helium Flash converts Helium core to Carbon, Oxygen Stellar evolution making chemical structure of the star more and more complicated

As star evolves further, it develops a Helium-burning shell All the action is deep in the tiny core of the star

As Helium shell burning stars, the star moves onto the “Asymptotic Giant Branch”

The Helium shell-burning is unstable and sends thermal pulses through the star, throwing off the outer layers of the star into space. As the outer layers are peeled back, it reveals the extremely hot, ultraviolet-emitting carbon and oxygen core which ionizes the stellar wind

Hot core photoionizes the surrounding stellar wind or ejecta. Resulting object is a Planetary Nebula, like M57

An incredible yarn, but all the pieces are there in the night sky Red Giants Horizontal Branch Giants Asymptotic Giant Branch Stars Mira Variables Planetary Nebulae White Dwarf Stars