<p>Initially there is dense gas surrounded by an &quot;atmosphere&quot; of less dense gas. This atmosphere is being rapidly ejected by the nova, so you see the absorption spectrum of the thin outer parts of the nova, lit by the continuous spectrum of the dense central part.</p>
<p>As the nova proceeds, the gas thins, and now the central part no longer emits a continuous spectrum, but an emission spectrum.</p>

CC BY-SA 4.0
2021-06-13T07:36:49.213Z
2021-06-13T07:36:49.213Z
James K
<p>In the book &quot;<em>Horizons: exploring the universe-Cengage learning (2018)</em>&quot;, page 210, it states that:</p>
<blockquote>
<p>Observational evidence can tell you how nova explosions occur. As the explosion begins, spectra show blueshifted absorption indicating that the gas is dense and coming toward you at a few thousand kilometres per second. After a few days, the spectral lines change to emission lines, telling you the gas has thinned.</p>
</blockquote>
<p>But from what I learned, The [<a href="https://i.sstatic.net/ihpXB.png" rel="nofollow noreferrer">Kirchhoff’s Laws</a>] states that</p>
<blockquote>
<p>Law I: The Continuous Spectrum:A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus producea continuous spectrum.</p>
</blockquote>
<blockquote>
<p>Law II: The Emission Spectrum: A low-density gas excited to emit light will do so at specific wavelengths and thus produce an emission spectrum.</p>
</blockquote>
<blockquote>
<p>Law III: The Absorption SpectrumIf light comprising a continuous spectrum passes through a cool, low-density gas, the result will be an absorption spectrum.</p>
</blockquote>
<p>So both the emission and absorption spectrum only involve low-density gas, how did the conclusion <code>After a few days, the spectral lines change to emission lines, telling you the gas has thinned.</code> being drawn?</p>
<p>Image credit: Kirchhof laws from <a href="https://commons.wikimedia.org/wiki/File:Kirchhof_laws.svg" rel="nofollow noreferrer">Wikipedia -- Gustav Kirchhoff</a>.</p>

CC BY-SA 4.0
2021-06-13T09:30:37.500Z
Jack, it's good that you've accepted an answer, but it's much better if you wait at least 48 hours before doing so. It's much less likely that others will write an answer now that one has already been accepted, and perhaps others might have given different interpretations or used a different style, e.g. richer in detail. This also increases *your* choice for best answer. For further guidance, see this MetaSE post on [accepting an answer](https://meta.stackexchange.com/questions/5234/how-does-accepting-an-answer-work). :-)
Chappo Hasn't Forgotten
CC BY-SA 4.0
2021-06-13T09:39:14.380Z
@ChappoHasn'tForgottenMonica Thank you for letting me know. I will keep that in mind in the future.
Jack the Ranger
CC BY-SA 4.0
2021-06-13T16:38:41.107Z
Today I learned there's another set of Kirchoff's Laws besides the ones we electrical engineers are familiar with!
Hearth
CC BY-SA 4.0
2021-06-14T00:10:56.243Z
@Hearth There are total 5 (set of) laws under the name Kirchoff, kind of insane.
Jack the Ranger
<spectra>
Question regards to Kirchhoff’s Laws (of spectroscopy)
Jack the Ranger
# Question regards to Kirchhoff’s Laws (of spectroscopy)

(Asked by: username_0 on 2021-06-13)

In the book "*Horizons: exploring the universe-Cengage learning (2018)*", page 210, it states that:

> Observational evidence can tell you how nova explosions occur. As the explosion begins, spectra show blueshifted absorption indicating that the gas is dense and coming toward you at a few thousand kilometres per second. After a few days, the spectral lines change to emission lines, telling you the gas has thinned.

But from what I learned, The [[Kirchhoff’s Laws](https://i.sstatic.net/ihpXB.png)] states that

> Law I: The Continuous Spectrum:A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus producea continuous spectrum.

> Law II: The Emission Spectrum: A low-density gas excited to emit light will do so at specific wavelengths and thus produce an emission spectrum.

> Law III: The Absorption SpectrumIf light comprising a continuous spectrum passes through a cool, low-density gas, the result will be an absorption spectrum.

So both the emission and absorption spectrum only involve low-density gas, how did the conclusion `After a few days, the spectral lines change to emission lines, telling you the gas has thinned.` being drawn?

Image credit: Kirchhof laws from [Wikipedia -- Gustav Kirchhoff](https://commons.wikimedia.org/wiki/File:Kirchhof_laws.svg).

**Tags:** spectra
> **Comments:**
> **username_1**: Jack, it's good that you've accepted an answer, but it's much better if you wait at least 48 hours before doing so. It's much less likely that others will write an answer now that one has already been accepted, and perhaps others might have given different interpretations or used a different style, e.g. richer in detail. This also increases *your* choice for best answer. For further guidance, see this MetaSE post on [accepting an answer](https://meta.stackexchange.com/questions/5234/how-does-accepting-an-answer-work). :-)  
> **username_0**: @ChappoHasn'tForgottenMonica Thank you for letting me know. I will keep that in mind in the future.  
> **username_2**: Today I learned there's another set of Kirchoff's Laws besides the ones we electrical engineers are familiar with!  
> **username_0**: @Hearth There are total 5 (set of) laws under the name Kirchoff, kind of insane.  

---

## 1 answers

### Answer by username_3 on 2021-06-13. Score: 3 | Accepted answer

Initially there is dense gas surrounded by an "atmosphere" of less dense gas. This atmosphere is being rapidly ejected by the nova, so you see the absorption spectrum of the thin outer parts of the nova, lit by the continuous spectrum of the dense central part.

As the nova proceeds, the gas thins, and now the central part no longer emits a continuous spectrum, but an emission spectrum.


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