Planetary nebulae
What happens to hydrogen in HTI regions?
Nebula
Planetary nebulae form from the gaseous shells that are ejected from low-mass asymptotic giant branch stars when they transform into white dwarfs. They are emission nebulae with spectra similar to those of emission nebulae found in star formation regions. Technically they are HII regions, because most hydrogen will be ionized, but they are denser and more compact than the nebulae in star formation regions. Planetary nebulae were given their name by the first astronomical observers who became able to distinguish them from planets, who tended to confuse them with planets, of more interest to them. Our Sun is expected to spawn a planetary nebula about 12 billion years after its formation.