After a star like our Sun (and other "low mass" stars) runs
out of fuel to "burn" in its core, the star must die! Low
mass stars (like our Sun) "die" by simply blowing their
outer parts off and leaving behind their hot core which becomes a
White Dwarf. The blown-off material is enriched with heavy
elements (like carbon, oxygen, and nitrogen), and is heated up by the
central White Dwarf which causes it to glow (just like a neon
light). This glowing remnant of a dead star is called a Planetary
Nebula (because they looked like the disk of a planet through
early telescopes).
The following are a few examples of the variety of Planetary
Nebula. (Incidentally, the upper right picture at the top of this
page is also of a Planetary Nebula.)
