Asymptotic Giant Branch Stars 

So we left the star on the horizontal branch, with Looks like this:

What happens when the center runs out of helium?

A very similar evolution to what happened when it runs out of hydrogen!


The End of the AGB phase

AGB stars undergo periodic instabilities In a not-well-understood fashion, these processes may conspire to drive the strong mass loss observed in many AGB stars:

The outer layers of the AGB star are very extended (several AU), and are not very strongly bound to the star. The mass loss, pulsations, and low binding energy of the outer layers can drive them off of the star, turning into....

A Planetary Nebula