Planetary Nebulae: The Very Basics

The name "Planetary nebula" is misleading. When these objects were first resolved, in the primitive optics of the times, they had the appearence of a disk, much like a planet does, and since pretty much everything that wasn't a known planet was being called a "nebula" at the time, the name stuck. It wasn't until later that they were discovered to be bubbles of gas. Stars close to our own sun in mass, between one and 5 solar masses, live out their lives in relative peace, consuming their hydrogen fuel slowly, fusing it to helium by the heat and pressure present in the core. This process creates an equilibrium, in that the pressure of the mass from without balances nicely with the energy expended in the fusion reactions within. This balance goes on for billions of years, until finally the star begins to run out of fuel to sustain the fusion reactions. As the fuel starts to run out, the outer atmosphere of the star starts to expand, due to the decreased gravitational attraction. As the star expands, the outer layers cool. The star enters a red giant phase, and often becomes variable, expanding and contracting, regularly increasing and decreasing its luminosity and temperature as it changes its size. When the core can no longer maintain fusion due to lack of material, it collapses, bringing with it much of the mass of the outer atmosphere. This also causes a shock wave which in turn ejects large amounts of matter, on the order of tenths of a solar mass. This shell of material spreads outward at high speeds. The distribution can be even or irregular, depending on the condition of the stars atmosphere at the time of the collapse, its rate of spin and other factors including disks of dust surrounding the star. NGC3918 These layers continue to expand for perhaps another few million years, while the star itself gets lower and lower on fuel. After the core collapses, it maintains fusion until it once again runs out of hydrogen fuel, and the cycle starts over again. It continues to a point where most of the hydrogen has been turned to helium. Then, it may start to fuse helium to carbon. When this occurs, it occurs suddenly, in a phenomenon called "Helium Flash over". The star suddenly starts burning at a much higher temperature, but with less mass and less surface area, it's much dimmer to those observing from far away. It has become a White Dwarf star, with most of its energy being radiated in the far ultraviolet range of the spectrum. So, while not very bright to our vision, ultraviolet radiation is quite energetic, enough so that it quite easily ionizes the clouds of gas left over from all the mass ejections that took place millions of years earlier. What this means is the energy streaming from the star strips some of the electrons from the gas atoms in the clouds. When this happens, the atom changes an energy state, emitting a photon of light in the process, and the gas cloud glows. It's the same principle at work in a florescent light bulb. The different densities of the gas and the interaction of the star's solar winds is what creates the images we see in the telescope's eyepiece. Famous planetary nebulae like the Dumbbell, Ring, Helix and Saturn nebulae. Some not so famous, but still beautiful ones like the Southern sky's NGC3918 in Centaurus pictured here, a nebulae that contains so much structure, one can hardly imagine the violence of its birth, belying the delicate structures within. Or an overlooked one in Orion-NGC2022,located between Betelgeuse and Orion's head.
Here is a link to a wonderfully amazing web site by Doug Snyder . It contains the most comprehensive catalog of planetary nebulae I've ever seen. It has some photos and locations and lots of statistics on planetary nebulae, organized by constellation and position etc. Enjoy it, as I did and as always, if you have any questions, please feel free to email me.


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Revised: 8/26/10