<p>This is an excellent question. Think about the way in which emission happens. <a href="https://en.wikipedia.org/wiki/H-alpha" rel="nofollow noreferrer">$\text{H}\alpha$ emission</a> happens when an electron makes a transition from the third energy level to the second, emitting a photon in the process with the energy equivalent to the difference of the energies of the two states, roughly $1.9\text{ eV}$. There's a lot of hydrogen in emission nebulae - and in nebulae in general - and so it plays an important role in the emission spectrum.</p>

<p>Emission from $[\text{O III}]$ comes from a different process: collisions. Free electrons with energies of $\sim1\text{ eV}$ - about the amount of the oxygen's low excitation potential - collide with oxygen atoms. This produces <a href="https://en.wikipedia.org/wiki/Forbidden_mechanism" rel="nofollow noreferrer">forbidden lines</a> that would otherwise have been unlikely to be created, as the ionizing photons typically would not have the right energies. Therefore, doubly-ionized oxygen (in particular) becomes extremely important in spectra, generally second only in intensity to $\text{H}\alpha$.</p>

<p>Given that $[\text{O III}]$ emission is collision-based, it makes sense that there is a temperature dependence - which there is, as is the case with $[\text{N II}]$ lines and is responsible for their high intensities. Conditions in many hot nebulae (in this case, $&gt;10,000\text{ K}$) are right for such collisions and thus forbidden line emission. Hot host stars can sometimes help achieve high electron temperatures.</p>

CC BY-SA 3.0
2017-01-08T03:07:47.063Z
Thanks. This is useful, but I'm still struggling to use the facts here to understand OIII intensity being ~75% of Hα when abundance of oxygen is a million times smaller. Should I infer from your answer that the high temperatures make the electron-Oxygen collisions ~million times more likely than the Hα electron transitions?
Arvind H
CC BY-SA 3.0
2017-01-08T07:23:27.813Z
Also ... there is a crap-ton of O III in our atmosphere (an entire layer of it). Which is what almost all spectral data collected has to look through.
LaserYeti
CC BY-SA 3.0
2017-01-08T19:36:31.620Z
@ArvindH See the edit: It's that there's a strong temperature dependence, and therefore at higher temperatures $[\text{O III}]$ becomes more important, while $\text{H}\alpha$ stays roughly the same.
HDE 226868
CC BY-SA 3.0
2017-01-07T17:25:39.520Z
2017-01-08T19:35:39.840Z
2017-01-08T19:35:39.840Z
HDE 226868
<p>Looking at spectra of emission nebulae like the <a href="https://en.wikipedia.org/wiki/Lagoon_Nebula" rel="nofollow noreferrer">Lagoon Nebula</a>, the $[\text{O III}]$ lines are prominent in intensity. However, the abundance of oxygen is minuscule compared to hydrogen. How then are the $[\text{O III}]$ spectral lines of such high intensity?</p>

<spectra><nebula><elemental-abundances>
Why are O III lines so prominent in the spectra of emission nebulae when the amount of oxygen relative to hydrogen is a million times smaller?
Arvind H
# Why are O III lines so prominent in the spectra of emission nebulae when the amount of oxygen relative to hydrogen is a million times smaller?

(Asked by: username_0 on 2017-01-07)

Looking at spectra of emission nebulae like the [Lagoon Nebula](https://en.wikipedia.org/wiki/Lagoon_Nebula), the $[\text{O III}]$ lines are prominent in intensity. However, the abundance of oxygen is minuscule compared to hydrogen. How then are the $[\text{O III}]$ spectral lines of such high intensity?

**Tags:** spectra, nebula, elemental-abundances

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## 1 answers

### Answer by username_1 on 2017-01-07. Score: 5 | Accepted answer

This is an excellent question. Think about the way in which emission happens. [$\text{H}\alpha$ emission](https://en.wikipedia.org/wiki/H-alpha) happens when an electron makes a transition from the third energy level to the second, emitting a photon in the process with the energy equivalent to the difference of the energies of the two states, roughly $1.9\text{ eV}$. There's a lot of hydrogen in emission nebulae - and in nebulae in general - and so it plays an important role in the emission spectrum.

Emission from $[\text{O III}]$ comes from a different process: collisions. Free electrons with energies of $\sim1\text{ eV}$ - about the amount of the oxygen's low excitation potential - collide with oxygen atoms. This produces [forbidden lines](https://en.wikipedia.org/wiki/Forbidden_mechanism) that would otherwise have been unlikely to be created, as the ionizing photons typically would not have the right energies. Therefore, doubly-ionized oxygen (in particular) becomes extremely important in spectra, generally second only in intensity to $\text{H}\alpha$.

Given that $[\text{O III}]$ emission is collision-based, it makes sense that there is a temperature dependence - which there is, as is the case with $[\text{N II}]$ lines and is responsible for their high intensities. Conditions in many hot nebulae (in this case, $>10,000\text{ K}$) are right for such collisions and thus forbidden line emission. Hot host stars can sometimes help achieve high electron temperatures.

> **Comments:**
> **username_0**: Thanks. This is useful, but I'm still struggling to use the facts here to understand OIII intensity being ~75% of Hα when abundance of oxygen is a million times smaller. Should I infer from your answer that the high temperatures make the electron-Oxygen collisions ~million times more likely than the Hα electron transitions?  
> **username_2**: Also ... there is a crap-ton of O III in our atmosphere (an entire layer of it). Which is what almost all spectral data collected has to look through.  
> **username_1**: @ArvindH See the edit: It's that there's a strong temperature dependence, and therefore at higher temperatures $[\text{O III}]$ becomes more important, while $\text{H}\alpha$ stays roughly the same.  

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