Complex sentence: Spectroscopic observations show that all planetary nebulae are expanding, and so the idea arose that planetary nebulae were caused by a star's outer layers being thrown into space at the end of its life.
Split into simpler sentences: Spectroscopic observations show that all planetary nebulae are expanding. This led to the idea arose that planetary nebulae were caused by a star's outer layers being thrown into space at the end of its life.