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Is it an Emission Spectra or an Absorption Spectra?

I think that it's an absorption spectrum, but then why is the light intensity curve focused on elements that should not be in the star in this case, like the carbon at position 4658?

PS: I do not study physics or astronomy or anything like that :)

  • 2
    $\begingroup$ Have you looked at the Wikipedia article that uses the image? It should answer your question pretty directly. $\endgroup$
    – HDE 226868
    Jul 1, 2021 at 16:00
  • 1
    $\begingroup$ @HDE226868 I was looking at the french version. Thank you. $\endgroup$ Jul 1, 2021 at 16:16
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    $\begingroup$ The caption to the figure says, "Spectrum of WR 137 showing the prominent emission lines..." -- which should be a clue that it's an emission spectrum. (Which it is.) $\endgroup$ Jul 1, 2021 at 16:17
  • $\begingroup$ These are emission lines. The units are Angstroms, not nm. Why do you think there is no carbon in a WR star? $\endgroup$
    – ProfRob
    Jul 1, 2021 at 16:52
  • $\begingroup$ @ProfRob The unit mistake was my bad! $\endgroup$ Jul 1, 2021 at 17:04

1 Answer 1


The reason that you know it is an emission spectrum, rather than an absorption spectrum, is that there are peaks rather than dips.

This is by definition: absorption lines are where light was absorbed by the atom, so you see a dip in the spectrum; emission lines are where the atom released a photon (whose energy corresponds to the energy difference of its quantum transition), so you see spikes/peaks in the spectrum.

Some spectra are complicated and can have both! For example, quasars can have emission spectra with "intervening" absorption lines due to either intervening interstellar gas in our galaxy or due to intergalactic gas along the line of sight to the quasar.

Wolf-Rayet stars are a complicated phenomenon. They are a class of stars that are broadly characterized as having little hydrogen in their spectra (i.e., the small emission line for H$\alpha$ in the OP's figure), and are dominated by strong emission lines corresponding to helium, nitrogen, carbon, and oxygen. The other trace elements in their spectra determine the type of WR star it is: nitrogen, carbon, or oxygen dominated, or some mixture.

Some Wolf-Rayet stars experience very intense stellar wind mass-loss, and hence are surrounded by a nebula of gas being expelled from the stellar surface. This nebula can cause absorption lines in Wolf-Rayet spectra, similar to the case of quasars. For example, WR 124.


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