<p>HII regions or emission nebulae are associated with the presence of massive stars that ionize the gas. The strongest emission line from an HII region comes from H-alpha. What happens in this case is that the hydrogen atom becomes ionized. Next, the electron and proton recombine to form a hydrogen atom again; however, the electron may be at any energy level. About half of the time, the subsequent downward cascade will be from transition n=3 to n=2. This emits H-alpha light which is in the deep red visible part of the spectrum. This line is one of the most common star formation tracers. I'm not sure if that completely answers your question, but most astronomers use H-alpha to detect HII regions, which is why I described its emission.</p>
<p>To add (in an attempt to clarify the various ways emission is detected), matter that is completely ionized is typically detected through free-free emission. In this case, there is no downward cascade of the electron to emit light the &quot;typical&quot; or most familiar way. Instead, emission is produced through free electrons scattering off of ions. Perhaps this is what you are referring to?</p>
<p>There is also synchrotron radiation, when charges particles accelerate in a magnetic field.</p>
<p>In any galaxy, a variety of emission types may contribute to a specific part of a galaxy, but there is typically one that dominates or is favored for detection purposes. For HII regions, H-alpha is studied because it is the strongest hydrogen emission line.</p>
<p>Oh, and H-alpha has a wavelength of 656.28 nm in air.</p>

CC BY-SA 4.0
2021-08-04T02:19:59.250Z
Interesting that you mention the [wavelength in air](https://astronomy.stackexchange.com/a/45001/7982)!
uhoh
CC BY-SA 4.0
2021-08-04T03:14:35.890Z
To add, if you want to further review the various types of emission in HII regions, here is a short pdf with some good information: http://www.physics.drexel.edu/~bob/Term_Reports/Danny.pdf
Astroturf
CC BY-SA 4.0
2021-08-04T00:58:22.630Z
2021-08-04T00:58:22.630Z
Astroturf
<p>The Wikipedia page on H II regions says that they are 'indirectly' detectable by the detection of doubly-ionized oxygen atoms mixed in.... (I am presuming atoms, not diatomic molecules...)</p>
<p>But are free protons and electrons easily detectable?  At what wavelengths or frequencies?  Would they be all over the map due to not having defined energy levels, like atoms and molecules?</p>
<p>Also, if astronomers can detect free electrons more easily than protons (I am making an assumption here) do they assume that wherever there are electrons, there are protons?  And vice versa?</p>

<observational-astronomy><spectroscopy><plasma-physics><electromagnetic-spectrum><proton>
How is the H II 'region' directly detectable? By Compton or Thomson free-particle scattering? At what wavelengths?
Kurt Hikes
# How is the H II 'region' directly detectable? By Compton or Thomson free-particle scattering? At what wavelengths?

(Asked by: username_0 on 2021-08-03)

The Wikipedia page on H II regions says that they are 'indirectly' detectable by the detection of doubly-ionized oxygen atoms mixed in.... (I am presuming atoms, not diatomic molecules...)

But are free protons and electrons easily detectable? At what wavelengths or frequencies? Would they be all over the map due to not having defined energy levels, like atoms and molecules?

Also, if astronomers can detect free electrons more easily than protons (I am making an assumption here) do they assume that wherever there are electrons, there are protons? And vice versa?

**Tags:** observational-astronomy, spectroscopy, plasma-physics, electromagnetic-spectrum, proton


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## 1 answers

### Answer by username_1 on 2021-08-04. Score: 1

HII regions or emission nebulae are associated with the presence of massive stars that ionize the gas. The strongest emission line from an HII region comes from H-alpha. What happens in this case is that the hydrogen atom becomes ionized. Next, the electron and proton recombine to form a hydrogen atom again; however, the electron may be at any energy level. About half of the time, the subsequent downward cascade will be from transition n=3 to n=2. This emits H-alpha light which is in the deep red visible part of the spectrum. This line is one of the most common star formation tracers. I'm not sure if that completely answers your question, but most astronomers use H-alpha to detect HII regions, which is why I described its emission.

To add (in an attempt to clarify the various ways emission is detected), matter that is completely ionized is typically detected through free-free emission. In this case, there is no downward cascade of the electron to emit light the "typical" or most familiar way. Instead, emission is produced through free electrons scattering off of ions. Perhaps this is what you are referring to?

There is also synchrotron radiation, when charges particles accelerate in a magnetic field.

In any galaxy, a variety of emission types may contribute to a specific part of a galaxy, but there is typically one that dominates or is favored for detection purposes. For HII regions, H-alpha is studied because it is the strongest hydrogen emission line.

Oh, and H-alpha has a wavelength of 656.28 nm in air.

> **Comments:**
> **username_2**: Interesting that you mention the [wavelength in air](https://astronomy.stackexchange.com/a/45001/7982)!  
> **username_1**: To add, if you want to further review the various types of emission in HII regions, here is a short pdf with some good information: http://www.physics.drexel.edu/~bob/Term_Reports/Danny.pdf  

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