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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] 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.

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  • $\begingroup$ 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. :-) $\endgroup$
    – Chappo Hasn't Forgotten
    Commented Jun 13, 2021 at 9:30
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    $\begingroup$ @ChappoHasn'tForgottenMonica Thank you for letting me know. I will keep that in mind in the future. $\endgroup$
    – Jack the Ranger
    Commented Jun 13, 2021 at 9:39
  • $\begingroup$ Today I learned there's another set of Kirchoff's Laws besides the ones we electrical engineers are familiar with! $\endgroup$
    – Hearth
    Commented Jun 13, 2021 at 16:38
  • $\begingroup$ @Hearth There are total 5 (set of) laws under the name Kirchoff, kind of insane. $\endgroup$
    – Jack the Ranger
    Commented Jun 14, 2021 at 0:10

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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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