Interstellar Medium, Part IV, Ionized Gas
actually see the interface of the ionized gas with the neutral and molecular gas. The pillars of dark molecular gas are being slowly ablated by ionizing radition from bright blue stars (off the top of this photograph). The bright yellowish (in false color) streams coming off the pillars are actually the newly ionized gas in a region called the ionization front.
The ionization occurs when an ultraviolet photon from a star strikes a neutral atom, say hydrogen. The photon has enough energy to kick the electron completely away from the proton. However, the electron and proton (or another proton) quickly recombine into a neutral atom. When they do so, they must release energy in the form of a photon. Usually the recombination occurs at one of the upper energy levels of hydrogen and a low energy photon low energy photon is released. Very quickly, however, the electron cascades down the energy levels toward the ground state, releasing photons of higher and higher energy. When the electron falls from the 3rd energy level to the 2nd energy level it emits an H-alpha photon, which is very red. This very common transition gives diffuse nebulae their red glow. Many other transitions, including those of other elements can also be seen at different wavelengths, but hydrogen is the most common element, and H-alpha is the most common transition visible in the optical, and this is why in most photographs, HII regions appear red.
Ionized gas is also common in supernova remnants. Supernovae are the dramatic and explosive ends to the lives of massive stars. The explosions are so violent that they literally blow bubbles in the interstellar medium. Within those bubbles the gas can be hot enough to emit strongly in the X-ray (millions of degrees). These last remnants of the stellar envelope plow outward at hundreds of kilometers per second into the surrounding interstellar medium, forming a supersonic shock. The shocking heats and ionizes the gas, which, in turn, radiates in the optical, as seen in this photograph of the Vela Supernova Remnant. The red color derives once again from the H-alpha transition. So, inside the bubble, you have very hot, very thin gas plowing into the otherwise dolcent ISM. At the interface between the two, the ISM gas becomes shock heated and ionized, then radiates energy away as light.
Ionized gas certainly produces some of the prettiest pictures astronomy has to offer, but also occurs in dramatic and dynamic situations, including star forming regions and supernova remnants. Very thin ionized gas also fills most of the volume of the Galaxy.
...This concludes my four part
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