<p>H$\delta$ absorption is formed when hydrogen in the level $n=2$ is excited to $n=6$.</p>

<p>To get strong H$\delta$ absorption lines you need large amounts of hydrogen in the first excited state $n=2$ and a radiation field that contains large numbers of photons with an energy equal to the difference between the $n=6$ and $n=2$ states.</p>

<p>These requirements are satisfied in stars with photospheres between about 8,000K and 15,000K, and we call these early-F and A-type stars.</p>

<p>At cooler temperatures most of the hydrogen is in the ground state, there are few photons with sufficient energy (3eV) to be absorbed and the H$\delta$ line weakens. At much higher temperatures (in O-stars for example) then none of the hydrogen is in the $n=2$ level, and in fact most of it is ionised.</p>

CC BY-SA 4.0
2020-11-29T00:37:14.917Z
different but related: [What exactly are “H30α and He30α images”? (ALMA)](https://astronomy.stackexchange.com/q/39583/7982)
uhoh
CC BY-SA 4.0
2018-08-07T16:12:08.547Z
2018-08-07T22:59:50.830Z
2018-08-07T22:59:50.830Z
ProfRob
<p>I understand factually that H<sub>$\delta$</sub> lines are most prominent in type A stars and less so in more extreme types of stars on the H-R diagram. However I was wondering the reason for why they are not prominent in more extreme type stars such as type O or type F. For type F stars I believe (this may not be correct) none of the hydrogen is in the 4th excited state (or very few are) so there aren't many H<sub>$\delta$</sub> balmer lines. However I am very unsure of why the EW H<sub>$\delta$</sub> is low in type O stars. Can someone please explain this, Thanks!</p>

<star><observational-astronomy><spectroscopy><stellar-evolution>
Why is $H_\delta$ prominent in type A stars?
QuantumPanda
# Why is $H_\delta$ prominent in type A stars?

(Asked by: username_0 on 2018-08-07)

I understand factually that H$\delta$ lines are most prominent in type A stars and less so in more extreme types of stars on the H-R diagram. However I was wondering the reason for why they are not prominent in more extreme type stars such as type O or type F. For type F stars I believe (this may not be correct) none of the hydrogen is in the 4th excited state (or very few are) so there aren't many H$\delta$ balmer lines. However I am very unsure of why the EW H$\delta$ is low in type O stars. Can someone please explain this, Thanks!

**Tags:** star, observational-astronomy, spectroscopy, stellar-evolution

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## 1 answers

### Answer by username_1 on 2018-08-07. Score: 12 | Accepted answer

H$\delta$ absorption is formed when hydrogen in the level $n=2$ is excited to $n=6$.

To get strong H$\delta$ absorption lines you need large amounts of hydrogen in the first excited state $n=2$ and a radiation field that contains large numbers of photons with an energy equal to the difference between the $n=6$ and $n=2$ states.

These requirements are satisfied in stars with photospheres between about 8,000K and 15,000K, and we call these early-F and A-type stars.

At cooler temperatures most of the hydrogen is in the ground state, there are few photons with sufficient energy (3eV) to be absorbed and the H$\delta$ line weakens. At much higher temperatures (in O-stars for example) then none of the hydrogen is in the $n=2$ level, and in fact most of it is ionised.

> **Comments:**
> **username_2**: different but related: [What exactly are “H30α and He30α images”? (ALMA)](https://astronomy.stackexchange.com/q/39583/7982)  

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