Nebulae!

It's only my opinion, but stellar nebulae are some of the most beautiful occurrences in astronomy. There are several different kinds of nebulae including Emission Nebulae, Dark Nebulae, and Planetary Nebulae.

Emission nebulae are found near very hot, very bright stars (usually O and B stars). They occur when O and B stars give off large amounts of ultraviolet radiation. When cold atoms of gas in the interstellar medium absorb the ultraviolet radiation, an emission nebula is formed!


A Reflection nebula is a bluish haze that surrounds a star. They are caused by very fine grains of dust around new stars. The light we see coming from the nebula is starlight that has been scattered and reflected by the dust grains.


A Dark nebula is an area that is so densely packed with dust that it literally blocks out all light from behind it. The dust scatters and absorbs light very efficiently. Dark nebulae are cool enough (10K to 100K) for hydrogen atoms to form molecules.


Planetary nebulae, unlike the other types of nebula, occur when a star is at the end of its life span. Low-mass stars (like the Sun) gently expel its outer layers into space when it dies. The ejected gas forms a glowing cloud (the planetary nebula).