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Interstellar Medium, Part IV, Ionized Gas

© Wesley Colley

The final phase of the interstellar medium is the ionized gas, gas so hot that it mainly exists as separate electrons and nuclei, rather than as neutral atoms. Most of the ionized gas is that which fills most of the volume of the Galaxy, but is very rare, with densities of the order 0.01 electrons per cubic centimeter. However, the ionized gas also occurs in more dense and far more exciting places, such as star formation regions and supernova remnants.

The ionized gas that does little other than fill the volume of the Galaxy is, in some senses, quite boring. Very little happens here. Mainly it just fills the void. It is this gas that extends the farthest from the center of the galaxy and eventually blends into the still rarer intergalactic medium, also ionized, often even hotter, and as rare as a particle per thousands of cubic centimeters.

The most important role of the ionized gas is to serve as a sort of "Galactic refrigerator," if you consider 10000 Kelvins to be cold. The ionized gas is extremely transparent, which allows light to pass through it and escape the Galaxy, where as atomic and molecular gas act to blanket radiation and cause further heating. So, if there were a hot star surrounded by molecular and atomic gas, the gas trap and reprocess the radiation from the star. However, once the gas reached about 10000 degrees, it would become transparent and the radiation would simply pass through. In this way, the gas never really exceeds about 10000 degrees. In converse, of course, if the gas for some reason ceases to be heated (the star moves away, for instance), it will condense back into neutral hydrogen. This mechanism keeps the gas in the Galaxy at an approximate equilibrium in a global sense. We're neither blowing all of our gas away with our stars, nor is all of our gas collapsing into dense molecular clouds.

These same effects occur in very locallized regions, called HII regions (HII, pronounced aytch-too, is the symbol for single ionized hydrogen--HI is neutral hydrogen). Typical HII regions are the centers of star formation. The Orion Nebula, Lagoon Nebula, and Eagle Nebula are famous examples. In each of these objects, the brightly luminescent gas is ionized by intense ultraviolet radiation from young blue stars within the nebula. The stars are easily visible in the Lagoon and Eagle Nebulae, and if one looks closely, one can see the bluish white stars which power the Orion Nebula.

If we zoom into the Eagle Nebula with the Hubble Space Telescope, we can

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