The central stars of planetary nebulae are very hot. Their luminosity, though, is very low, implying that they must be very small. A star can collapse to such a small size only once it has exhausted all its nuclear fuel, so planetary nebulae came to be understood as a final stage of stellar evolution. 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.