<p>In the absence of emission lines from ionized gas, the (optical) spectrum of a galaxy will be the sum of the spectra of individual stars. These spectra will have absorption lines for hydrogen (H-alpha, H-beta, H-gamma, H-delta, etc.); the absorption lines are strongest for A stars and weaker for both earlier (O, B) and later (F, G, K, M) stars.</p>

<p>If ionized gas <em>is</em> present (as is typical for spiral and irregular galaxies), then there will be hydrogen lines in emission. (This is normally due to O and B stars, which ionize gas with their strong UV emission; you've misread what the Wikipedia article actually says about A stars.) The emission is strongest for H-alpha, weaker for H-beta, and weaker still for the subsequent Balmer lines. This means that emission from H-alpha and (usually) H-beta overwhelms the stellar H-alpha and H-beta absorption, but H-gamma, H-delta, etc. can often still be seen in absorption because the emission from those lines is too weak to "fill in" the absorption lines.</p>

<p>Elliptical galaxies usually have only F or G and later stars present; they usually lack both the gas <em>and</em> the UV-emitting O and B stars necessary to ionize the gas, so you see only the stellar absorption lines.</p>

<p>The Ca II H and K lines <em>can</em> be seen in emission from individual M stars, due to emission from their chromospheres being strong enough to overwhelm the absorption in the photospheric spectrum (e.g., <a href="https://arxiv.org/abs/astro-ph/0602293" rel="nofollow noreferrer">https://arxiv.org/abs/astro-ph/0602293</a>).</p>

CC BY-SA 3.0
2018-01-15T15:28:36.120Z
2018-01-15T15:28:36.120Z
Peter Erwin
<p>I've heard that $\mathrm{H\beta}$ and $\mathrm{H\delta}$ lines in galaxy spectra might be either emission lines or absorption lines, is that correct?</p>

<p>From <a href="https://en.wikipedia.org/wiki/Balmer_series#Role_in_astronomy" rel="nofollow noreferrer">Wikipedia</a>, Balmer lines are usually emission lines in the spectra of spiral and irregular galaxies probably due to the presence of type A stars. If ellipticals lack hot (O, B and A type?) stars, and are gas poor(?), then is it the remaining stars doing the absorbing?</p>

<p>Also, are the CaII-H and -K doublet ever emission lines?</p>

CC BY-SA 3.0
2018-01-15T11:50:31.813Z
For H lines, isn't that there is enough hydrogen out and around?
Alchimista
<galaxy><spectra>
In galaxy optical spectra, why are Hbeta and Hdelta sometimes absorption lines?
Chris
# In galaxy optical spectra, why are Hbeta and Hdelta sometimes absorption lines?

(Asked by: username_0 on 2018-01-15)

I've heard that $\mathrm{H\beta}$ and $\mathrm{H\delta}$ lines in galaxy spectra might be either emission lines or absorption lines, is that correct?

From [Wikipedia](https://en.wikipedia.org/wiki/Balmer_series#Role_in_astronomy), Balmer lines are usually emission lines in the spectra of spiral and irregular galaxies probably due to the presence of type A stars. If ellipticals lack hot (O, B and A type?) stars, and are gas poor(?), then is it the remaining stars doing the absorbing?

Also, are the CaII-H and -K doublet ever emission lines?

**Tags:** galaxy, spectra
> **Comments:**
> **username_1**: For H lines, isn't that there is enough hydrogen out and around?  

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## 1 answers

### Answer by username_2 on 2018-01-15. Score: 2

In the absence of emission lines from ionized gas, the (optical) spectrum of a galaxy will be the sum of the spectra of individual stars. These spectra will have absorption lines for hydrogen (H-alpha, H-beta, H-gamma, H-delta, etc.); the absorption lines are strongest for A stars and weaker for both earlier (O, B) and later (F, G, K, M) stars.

If ionized gas *is* present (as is typical for spiral and irregular galaxies), then there will be hydrogen lines in emission. (This is normally due to O and B stars, which ionize gas with their strong UV emission; you've misread what the Wikipedia article actually says about A stars.) The emission is strongest for H-alpha, weaker for H-beta, and weaker still for the subsequent Balmer lines. This means that emission from H-alpha and (usually) H-beta overwhelms the stellar H-alpha and H-beta absorption, but H-gamma, H-delta, etc. can often still be seen in absorption because the emission from those lines is too weak to "fill in" the absorption lines.

Elliptical galaxies usually have only F or G and later stars present; they usually lack both the gas *and* the UV-emitting O and B stars necessary to ionize the gas, so you see only the stellar absorption lines.

The Ca II H and K lines *can* be seen in emission from individual M stars, due to emission from their chromospheres being strong enough to overwhelm the absorption in the photospheric spectrum (e.g., <https://arxiv.org/abs/astro-ph/0602293>).


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