Planetary nebulae are the remnants of the external layers of sun-like star. In other words, what we are seeing is the future of our sun. When a sun-like star have burned all the hydrogen to helium in its core, becomes a red giant burning also the carbon to oxygen in the nuclear reactions. After that, the outer layers expand and the instabilities cause the ejection of a significant part of the mass star. In the core, remains only a small central star.

An emission nebula is a cloud of ionized gas emitting light of various colours. Among the different kind of emission nebulae are the HII region in which young star formation is taking place.

Supernova remnants are the result of the ejected material expanding from the explosion.

Dark nebulae are unilluminated molecular clouds that can be detected when they obscure stars or emission nebulae.