Planetary nebulas are envelopes expelled from stars in the late stages of their lives.  During most of its life, a star shines by changing the hydrogen in its core into helium.  When the core helium has been used up, the changes continue.  Thousands of years later, the outer part swells up and the star becomes a red giant.  At least some of the outer part must escape into space.  It is believed that a planetary nebula is formed at that time.  The remaining part of the star then settles down and becomes a white dwarf, which is a whitish star in a latter stage of development.  A white dwarf is small in size, has low light, yet great density.  The middle star of the planetary nebula has very high temperatures - up to 20,000°Fahrenheit.  As it progresses towards the white dwarf stage, it radiates energy of heavy particles, since little, if any, nuclear fuel (the core helium) is left.  Then, a planetary nebula is born from its central star, while the “leftover” becomes a diffuse nebula.