Page "Planetary nebula" Paragraph 14

The central stars of planetary nebulae are very hot.

Only once a star has exhausted all its nuclear fuel can it collapse to such a small size, and so planetary nebulae came to be understood as a final stage of stellar evolution.

Spectroscopic observations show that all planetary nebulae are expanding.

This led to the idea that planetary nebulae were caused by a star's outer layers being thrown into space at the end of its life.