Planetary Nebulae 

As the outer layers of an AGB star expand, their density drops and the eventually become optically thin, exposing the C/O core left behind. At this point the star has turned into a planetary nebula.

Note: Planetary nebulae have nothing to do with planets!

The C/O core is initially hot (100,000 K) and luminous. It will gradually cool over time (no nuclear reactions!) and become a white dwarf like Sirius B. More on white dwarfs later...
 
 

Meanwhile, the ejected outer layers flouresce (shine) and become visible as the planetary nebula itself


Typical properties of planetaries:

What causes a planetary nebula to shine?

Why do planetaries look like rings?
 
 


Not all planetaries look like rings:

Animation of possible evolution
 
 

Planetary Nebula Links from the Space Telescope Science Institute:
(note extensive use of false color)