What is a planetary nebula?
A red giant star has a hot, dense core and a cool, tenuous
shell. When the core grows hot enough to ignite helium
fusion, the increased radiation pressure from the increasingly
luminous core blows the cool shell away completely. The
hot core, deprived of its sheath, can be seen naked for the
first time. The outer stellar layers blow out into space, and
those departing layers are called a �planetary nebula�.