The Basics: Nebula
You may be wondering what is a Nebula? In general it is a very large structure consisting of different gasses and dust. And by large I mean a size that the human mind can’t really understand. The measurement used is a light year and it is the distance light moves in a year. How far is that? For starters light travels at about 186,000 miles per second so try to imagine the distance traveled in a minute, hour, or week. How about approximately 6 trillion miles in a year! The Orion Nebula (M42) is approximately 24 light years in diameter and that converts to 144 trillion miles across, a truly mind boggling number. It is approximately 1,334 light years away from Earth which converts to 8,004 trillion miles. As a side note, there are objects that are over 13 billion light years away from the Earth. Ok enough about measurement and back to Nebula, there are several types of Nebula; Emission, Reflection, Dark, and Planetary. These names represent how the Nebula is illuminated because the gas and dust by themselves do not have a source of illumination.
An Emission Nebula is a patch of gas and dust that is illuminated from a bright hot star or a star cluster within the nebula. The star(s) are the light source and appear behind the gas and dust. The gas and dust partially obscure the light source but by doing so the gas and dust structure can be seen. The star(s) are typically producing ultraviolet light that ionizes the gas and cause it to glow. The most abundant gas in these types of nebula is Hydrogen and when ionized the color appears red in a camera image. Check out my Lagoon Nebula for an example of an Emission Nebula. The coolest thing about Emission Nebula is that they are the birth place for all the stars we see. All of the dust and gas that spans over several light years is slowly contracting due to electro-magnetic, heat and gravitational forces. The initial force to start the process is still being researched but it is thought that a shock wave from a nearby star can overcome the expansive force of the dust cloud and cause the internal gravitational effect to begin the collapse. As more and more dust contracts it becomes hot due to the pressure created by the same volume of dust occupying a smaller space. At some point in this process the temperature rises enough to start the fusion process and a star is born. This is a very simple description but it gives you an idea of what is happening inside all of that dust. From initiation force to star formation is estimated to take 40-50 million years.
A Reflection Nebula is also a patch of gas and dust that is illuminated from a bright hot star or a cluster of stars. The main difference is that the dust and gas are not directly in the line of sight between the Earth and the source star(s). Instead this part of the dust cloud lies to the side and the light from the source star(s) is scattered amongst the dust and appear blue. This scattering affects the shorter wavelength light which happens to be blue, thus the gas appears blue. The same effect happens on Earth and explains why our sky appears blue. Check out my Pleiades Nebula for an example of a Reflection Nebula.
A Dark Nebula is a very dense area of dust within a nebula. They are called Dark Nebula because the dust is to dense to allow light to pass, however the surrounding light from other stars illuminates the edges of the dark nebula providing some contrast from the dark background sky. Check out my Iris Nebula for an example of a Dark Nebula.
Some Nebula are a combination of all three of the above. Check out my Trifid Nebula to see what I mean.
A Planetary Nebula is another type of Nebula but it is not a planet and has nothing to do with planet formation. Ok, this prompts an obvious question, why call it a Planetary Nebula? Astronomers in the 18th century named them this because they appeared to be a planet-like object when viewed through their small telescopes. Labels always seem to stick and this is another example. Ok, back to the original question, what is a Planetary Nebula? Simply put it is a dying star without the mass to go supernova. As the star loses its ability to fuse Hydrogen the star interior starts to collapse while the surface expands. In reality, it is a mini Emission Nebula with the shrinking core being the light source and the expanding surface being the gas. Check out my Dumbbell Nebula for and example of a Planetary Nebula.