Planetary nebula are the remains of a star that didn't quite supernova. The star blew off its outer layers of gas into space as its core collapsed into a white dwarf. It contains hydrogen, helium, oxygen, sulfur, and other heavy elements but nothing much past carbon. These stars were not massive enough to produce anything heavier. The different shapes of the nebula could come from a number of factors. It is believed that some may be caused by large planets, brown dwarfs, or stars shaping the star's explosion through gravity. Magnetic fields and rotation rates of the stars may also provide some disturbance of the star to create the interesting shapes.

http://web.utah.edu/astro/nebula.html

Witch Head

Carina

Ring

Hand

Rosette

Ghost

Trifid