Nebulous nebulas

Nebulas are cool.  There are really three types, in my mind.  My three types are planetary nebulas, stellar nebulas and stellar dust.  Historically they were different.  People used to call any blob in the sky a nebula and this worked well until telescopes got better and people could see what they actually were.  Fifty or sixty years ago, people only knew about one galaxy; the one we live in and there was nothing else.  Then telescopes got better and the fuzzy Andromeda Nebula resolved into a huge galaxy.  Other nebulas became other galaxies, but some nebulas remained, but were reclassified as planetary or stellar depending on their cause.

Planetary Nebulas are the deaths of stars.  When a star dies, it throws off its outer gas layer and the stellar remains at the core of the gas illuminates or ionizes the departing gasses.  Violent deaths illuminate more of the gas and the nebula appears to be brighter.

Stellar nebula are the birth places of stars.  The space dust, mostly gas, clumps together and gravitationally attracts more gas until the region becomes dense enough to start fusion reactions and stars are born.  While it is conceiveable for only one star to be born, usually a whole cluster of stars are created.  When the stars are born they create a breeze in space, the particles they energize or the light coming from the star, this pushes the gas and dust away from the new star and when the cloud starts to break up from this wind, we can start to see the nebula.  The light from the new stars ionizes the particles and they start to reflect and emit light.  They glow.  

Hydrogen gas when ionized, emits red light, oxygen emits green.  What happens is the atoms in the outer shell become energized and they temporarily move to a higher energy state.  Once in the higher state the atom becomes unstable and the electron moves back to the lower energy state and emits the light back into the environment.

Stellar dust, galactic dust, is just the gas that has not been used in star formation.  It was once in a nebula or it is part of the leftover matter that was created in the big bang but has not become anything yet.  One day it will, but not just yet.