Planetary Nebulae: Astonishing Facts
At right: Diagram of the formation of a planetary nebula. Image by Kurgus, Wikimedia Commons.
Planetary nebulae come in many different shapes. Common shapes include spheres, disks, and double-ended (bipolar) forms. Many planetary nebulae, especially young ones, have complex shapes that are hard to describe, and some are asymmetrical.
The wide variety of shapes has been a puzzle for astronomers. Bipolar nebulae may be the result of companion stars. The gas cloud is ionized, so magnetic fields from the core star may affect the shape. Other possibilities are "interactions between material moving away from the star at different speeds [and] multiple ejection events." (Internet Encyclopedia of Science)
In the early universe, the only abundant elements were hydrogen and helium. The gas cloud of a planetary nebula is made of heavy elements that are the result of helium fusion during the red giant phase. As a planetary nebula "evaporates" into space, it adds these heavy elements to the interstellar medium. Later stars that are born out of this material will have more heavy elements than the first stars had. Evaporation of planetary nebulae is thought to be the source of the heavy elements in the universe, including those on Earth.
"Planetary nebula." Internet Encyclopedia of Science article.
"Planetary Nebulae." Students for the Exploration and Development of Space website.