Instantly recognizable to science fiction fans everywhere, nebulae are clouds of dust or gas in space. The term “nebula” was originally used to denote any extended astronomical object and this included galaxies until they were discovered. It wasn’t until the invention and modification of telescopes, that research into the skies and nebula in particular really took off and it was at this point in time that the classification of nebulae began. There are many types of nebulae, each of which has a particular defining characteristic.
Dark nebulae are extremely difficult to see. This is due to their ability to absorb the light from behind them and then re-emit this light on the infrared spectrum. They can be extremely beautiful objects though, with the cone nebula being a spectacular example of this. This nebula Lies 2,600 light years away in the constellation of Monoceros.
Emission nebulae are the victims of stellar radiation which allow them to emit their own light. Often found near reflection nebulae, emission nebulae are sometimes called “H2 regions” due to the hydrogen molecules being ionized. An example of an emission nebula would be the Keyhole Nebula which is a mere 8000 light years away from us.
Reflection nebulae are fairly common in our night sky. The dust and gases of this nebula are so dark they do not emit their own light. We manage to see them because as their name suggests they reflect light from nearby stellar objects. An example of a reflection nebula would be NGC 1999 which lies 1500 light years away from us in the middle of the Orion constellation.
Planetary nebulae are created when stars die and have consumed all their fuel. The hydrogen of the star has been converted to helium and the star expands to become a red giant. Then when the red giant contracts and becomes a white dwarf star the helium is left behind in a circular pattern and is ionized by the white dwarfs radiation which makes it glow. Examples of this type of nebula are NGC 7293 or the helix nebula and IC 3568 or the Lemon Slice Nebula.
Finally supernova remnants are just that. If a massive star runs out of fuel and collapses under its own weight to become a black hole or a white dwarf star pulls in material from other stars and reaches critical mass the resulting massive explosions form supernova remnant nebulae. These are usually very colourful when photographed and an example of this would be the supernova remnant N49 in the large Magellanic cloud.
Of course each of these types of nebulae also has internal classifications and distinctions but these are considered the basic types. Thanks to instruments like the Hubble telescope we can now begin to see just how beautiful the galaxy really is.