<p>You can only quantify it with a full radiative transfer calculation and a model atmosphere. You can argue it from an order of magnitude that the lower levels of visible transitions in helium are about 20 eV above the ground state, compared with 10.2 eV for the hydrogen Balmer lines.</p>
<p>In other words, for visible hydrogen absorption lines you need to have a population of atoms excited to a level 10.2 eV above the ground state. For helium, that excitation energy needs to be about 20 eV.</p>
<p>Since <span class="math-container">$k_BT$</span> in the solar photosphere is about 0.5 eV, this corresponds to a decrease of about another 20 e-foldings in the Boltzmann factor; <span class="math-container">$e^{-20} = 2 \times 10^{-9}$</span> and so the number density of He atoms in the appropriate lower levels will be <em>about</em> that factor less than the number of hydrogen atoms in the <span class="math-container">$n=2$</span> state.</p>
<p>This is crude because helium is a bit more complicated than hydrogen - an ion to consider, and also it is ten times less abundant than hydrogen (by number). But you might expect atomic helium lines to become prominent at about twice the solar temperature (B type stars), since the transition cross-sections will be similar.</p>

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2024-10-04T11:00:46.590Z
2024-10-07T13:05:37.927Z
2024-10-07T13:05:37.927Z
ProfRob
<p>It appears that N.R. Pogson was the first to observe in the solar spectrum a line of an element that later became known as helium. This was an <em>emission</em> line in the spectrum of the chromosphere during a solar eclipse (India, 1868). <a href="https://www.narit.or.th/files/JAHH/2021JAHHvol24/2021JAHH...24..629N.pdf" rel="nofollow noreferrer">B.B. Nath, W. Orchison, 2021</a></p>
<p>I wonder why helium wasn't observed as an <em>absorption</em> line in the visible part of the spectrum of the photosphere, without the need for an expedition to a location where an eclipse could be expected.</p>
<p>ProfRob writes: 'there are no helium atomic transitions in the visible part of the solar spectrum' on <a href="https://physics.stackexchange.com/questions/801617/how-do-we-know-that-the-sun-is-71-hydrogen-by-mass-from-emission-absorption-spe/801707#801707">Stackexchange Physics</a>**</p>
<p>Are the helium absorption lines very weak or absent in the 390-780 nm spectrum of the photosphere? Why? How can I quantify this, e.g. using the Saha ionisation equation?</p>
<hr />
<p>** <strong>Update</strong>: ProfRob has updated the answer. See: <a href="https://physics.stackexchange.com/posts/801707/revisions">https://physics.stackexchange.com/posts/801707/revisions</a></p>

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2024-10-08T22:29:29.123Z
You know helium is colourless? Why would there be any absorption?
Mithoron
<the-sun><spectra><helium>
Helium in solar spectra
gamma1954
# Helium in solar spectra

(Asked by: username_0 on 2024-10-03)

It appears that N.R. Pogson was the first to observe in the solar spectrum a line of an element that later became known as helium. This was an *emission* line in the spectrum of the chromosphere during a solar eclipse (India, 1868). [B.B. Nath, W. Orchison, 2021](https://www.narit.or.th/files/JAHH/2021JAHHvol24/2021JAHH...24..629N.pdf)

I wonder why helium wasn't observed as an *absorption* line in the visible part of the spectrum of the photosphere, without the need for an expedition to a location where an eclipse could be expected.

ProfRob writes: 'there are no helium atomic transitions in the visible part of the solar spectrum' on [Stackexchange Physics](https://physics.stackexchange.com/questions/801617/how-do-we-know-that-the-sun-is-71-hydrogen-by-mass-from-emission-absorption-spe/801707#801707)\*\*

Are the helium absorption lines very weak or absent in the 390-780 nm spectrum of the photosphere? Why? How can I quantify this, e.g. using the Saha ionisation equation?

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\*\* **Update**: ProfRob has updated the answer. See: <https://physics.stackexchange.com/posts/801707/revisions>

**Tags:** the-sun, spectra, helium
> **Comments:**
> **username_1**: You know helium is colourless? Why would there be any absorption?  

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## 1 answers

### Answer by username_2 on 2024-10-04. Score: 4 | Accepted answer

You can only quantify it with a full radiative transfer calculation and a model atmosphere. You can argue it from an order of magnitude that the lower levels of visible transitions in helium are about 20 eV above the ground state, compared with 10.2 eV for the hydrogen Balmer lines.

In other words, for visible hydrogen absorption lines you need to have a population of atoms excited to a level 10.2 eV above the ground state. For helium, that excitation energy needs to be about 20 eV.

Since $k\_BT$ in the solar photosphere is about 0.5 eV, this corresponds to a decrease of about another 20 e-foldings in the Boltzmann factor; $e^{-20} = 2 \times 10^{-9}$ and so the number density of He atoms in the appropriate lower levels will be *about* that factor less than the number of hydrogen atoms in the $n=2$ state.

This is crude because helium is a bit more complicated than hydrogen - an ion to consider, and also it is ten times less abundant than hydrogen (by number). But you might expect atomic helium lines to become prominent at about twice the solar temperature (B type stars), since the transition cross-sections will be similar.


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