The word "nebula" is derived from the Latin word for cloud, and that's no coincidence. Nebulae are like vast clouds in space, ranging from dozens to even hundreds of light years across. They are made up of gases and tiny dust particles, deriving their magnificent light from their own radiation or by reflecting the light from stars. Most nebulae are about 90% hydrogen, 10% helium and 0.1% other heavier elements including anything from carbon to iron.
The Triangulum Emission Garren Nebula (NGC 604)
Nebulae are classified into five different types: emission nebulae, reflection nebulae, dark nebulae, planetary nebulae, and supernova remnants. Emission, reflection and dark nebulae are all classified under diffuse nebulae because they all have similar compositions and tend to be fuzzy, lacking any specific shape.
Emission nebulae are made up of high temperature gases, mostly ionized hydrogen, that emit red light. Stars that are sometimes hidden within the nebula energize the gases and cause them to emit this light. They work a lot like neon lights, energy is applied to a gas which releases that energy as light.
Orion nebula, an emission nebula, actually visible with the naked eye
Reflection nebulae have the same composition as emission nebula, but instead of being hot and radiating light, they are cold and usually reflect light from a star or cluster of stars. Much of the light from reflection nebulae is blue because it reflects better. Reflection nebulae are some of the better places for star formation because of their relatively low energies.
The Trifid nebula, a reflection nebula
Dark nebulae, classified under diffuse nebulae with emission and reflection, have the same sort of composition as their brethren, but instead of reflecting light, their position means that they block light.
Horsehead nebula, a famous dark nebula
Planetary nebulae, despite what their name might suggest, have nothing to do with planets. As a star begins to die or turn into a white giant, they will leave a shell of gas. These nebulae used to be mistaken for planets, hence the name. Planetary nebulae have the same composition as diffuse nebulae, but much more dense.
The Ring Nebula, a good example of planetary nebulae
And finally, supernova remnants. These nebulae are exactly what their name would suggest, remnants of supernovae. When a star dies, it spews out large amounts of gas and debris which can glow from their own supernova energy or be illuminated like diffuse nebulae.
The Crab nebula, a supernova remnantThere is a saying that nebulae are stellar nurseries, and this is exactly true. All the stars and planets in our universe were likely created in nebulae. A nebula can sit undisturbed for thousands of years, but if a shock wave from a supernova or any other event disturbs it, there will likely be a cascading event. Particles begin moving closer to each other and adhering because of gravity. Eventually they will form clumps that reach critical mass, where gas and particles begin heating up due to pressure and friction enough that they ignite, creating protostars. The remaining matter can often coalesce into planets or other space rocks. And thus, a new solar system is born.
And now for some cool pictures.
The Pillars of Creation
The Eagle Nebula
The Flame Nebula
And my favorite:
The Carina Nebula