Nebulae are collections of dust, helium gas, hydrogen gas, and a variety of other ionized gases forming an interstellar cloud. Though the word can also sometimes be used for groups or clusters of stars, it is primarily reserved for dust clouds. Generally, nebulae can be divided into five types and the classification of the individual nebula depend on its characteristics and how it was formed.
1 – Reflection Nebulae
The collections of dust and gas in reflection nebulae do not emit their own light and as the name suggests, they reflect the light of a nearby star or nearby stars. These nebulae are commonly sites of star formation and typically appear as a blue hue.
2 – Emission Nebulae
In this type of nebulae, the dust and ionized gases emit their own light. Ultraviolet radiation is emitted from the nearby formation of a single or multiple young stars. This radiation excites the atoms of the gases and dust present in the nebula to a higher state which causes them to give off radiation as they return to their original non-exited energy state. This emission of radiation from the excited atoms gives off unique light that is predominantly reddish hue as a result of the presence of hydrogen in the nebula.
3 – Dark Nebulae
Dark nebulae are clouds of primarily dust and some gas that block light sources from whatever stars and planets are behind it. They can be rather difficult to view as a result of the darkness in contrast with the dark background in the night sky and only a few shades of difference between the two. Light passing through this nebulae can also become scattered into the surroundings of the nebulae and make it even harder to view.
4 – Planetary Nebulae
These nebulae are formed as a result of a dying star. The star has converted most of its hydrogen to helium and to form a Red Giant. The process ejects gases into the surround space to form a nebula. As the star continues to change in its process, it forms a White Dwarf, which is a very hot star. This new formation causes all of the gases that are still being released to become ionized and start glowing.
5 – Supernova remnant
The last of the nebulae types is aptly named. As a star goes supernova it ejects a portion of its contents into space that forms a nebula.
Nebulae form as a result of different reasons. Some are just dust and absorb light while others either reflect or produce it. They can also form as a result of new stars or even dying stars. Therefore it makes sense that the classification for nebulae has been determined by how they are formed. Regardless of how or when a nebula formed, it is always a joy to look at them or even just pictures of them as they are some of the most unique objects in the night sky.