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Thread: Ionized Hydrogen Regions and Light

  1. #1
    Join Date
    Jun 2008
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    Question Ionized Hydrogen Regions and Light

    How do H II regions emit light? My understanding is that once a hydrogen ion recombines with a free-floating electron, it emits light in the visible spectrum (unlike normal hydrogen which is not in the visible spectrum). Is this what causes nebulae like Orion to glow reddish?
    “Of all the sciences cultivated by mankind, Astronomy is acknowledged to be, and undoubtedly is, the most sublime, the most interesting, and the most useful. For, by knowledge derived from this science, not only the bulk of the Earth is discovered, but our very faculties are enlarged with the grandeur of the ideas it conveys, our minds exalted above their low contracted prejudices.” - James Ferguson

  2. #2
    Join Date
    Oct 2005
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    Neutral hydrogen emits visible light only when higher excitation states decay into states that are not the ground state. The rate of those decays is set by the atomic physics, it doesn't matter how the hydrogen came to be excited. The rates to the ground state are higher, so you get lots of UV, but you will still get visible light too. What you mean by "normal" hydrogen is hydrogen that is very likely in its ground state and is probably not all that likely to get excited at all. Ionized hydrogen, on the other hand, is getting ionized somehow, so there's lot's of excitation. That gives lots of opportunities to get the atoms to high excitation levels, either by exciting them from the ground state or recombining into the excited levels, so that's why you see a lot of H alpha from HII regions.

    As for contrasting the visible to the ultraviolet, how bright it looks in each will depend on whether or not the source is "optically thick" (most of the emitted light is reabsorbed) or "optically thin" (most of the emitted light escapes to be observed). In HII regions, the H lines in the UV are often optically thick, so heavily reabsorbed, while in H alpha they are optically thin, so escape more easily and that's why we see so much optical light from them (creating that red glow).
    Last edited by Ken G; 2019-Dec-14 at 05:46 PM.

  3. #3
    Join Date
    Sep 2006
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    1,840
    In regions near hot stars, some hydrogen atoms will be excited to energy states of N=3 or higher (where N=1 is the state with lowest energy, also called the ground state). As each atom drops down in energy, from one state to a lower state, it emits a photon with the energy difference between those two states. If an atom drops down from N=3 to N=2, then it will emit a photon with an energy of 1.89 electron volts, which corresponds to a wavelength of 656.3 nm. Humans perceive such photons as red, which is why HII regions often appear red in pictures.

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