<p>The reason that you know it is an emission spectrum, rather than an absorption spectrum, is that there are peaks rather than dips.</p>
<p>This is by <a href="https://www.atnf.csiro.au/outreach/education/senior/astrophysics/spectra_astro_types.html" rel="nofollow noreferrer">definition</a>: 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.</p>
<p>Some spectra are complicated and can have both! For example, quasars can have emission spectra with &quot;intervening&quot; absorption lines due to either intervening interstellar gas in our galaxy or due to intergalactic gas along the line of sight to the quasar.</p>
<p>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<span class="math-container">$\alpha$</span> 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 <a href="https://en.wikipedia.org/wiki/Wolf%E2%80%93Rayet_star#Classification" rel="nofollow noreferrer">the type of WR star</a> it is: nitrogen, carbon, or oxygen dominated, or some mixture.</p>
<p>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, <a href="https://britastro.org/node/19541" rel="nofollow noreferrer">WR 124</a>.</p>

CC BY-SA 4.0
2021-07-01T16:59:36.840Z
2021-07-01T19:34:59.040Z
2021-07-01T19:34:59.040Z
Daddy Kropotkin
<p><a href="https://i.sstatic.net/hnnO9.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/hnnO9.png" alt="enter image description here" /></a></p>
<p>Is it an Emission Spectra or an Absorption Spectra?</p>
<p>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?</p>
<p>PS: I do not study physics or astronomy or anything like that :)</p>

CC BY-SA 4.0
2021-07-01T16:00:23.330Z
Have you looked at [the Wikipedia article](https://en.wikipedia.org/wiki/WR_137) that uses the image? It should answer your question pretty directly.
HDE 226868
CC BY-SA 4.0
2021-07-01T16:16:21.947Z
@HDE226868 I was looking at the french version. Thank you.
evgeni fotia
CC BY-SA 4.0
2021-07-01T16:17:32.143Z
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.)
Peter Erwin
CC BY-SA 4.0
2021-07-01T16:52:57.130Z
These are emission lines. The units are Angstroms, not nm. Why do you think there is no carbon in a WR star?
ProfRob
CC BY-SA 4.0
2021-07-01T17:04:27.343Z
@ProfRob The unit mistake was my bad!
Daddy Kropotkin
<spectra>
What type of spectrum does the Wolf-Rayet star WR137 have?
evgeni fotia
# What type of spectrum does the Wolf-Rayet star WR137 have?

(Asked by: username_0 on 2021-07-01)

[![enter image description here](https://i.sstatic.net/hnnO9.png)](https://i.sstatic.net/hnnO9.png)

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 :)

**Tags:** spectra

> **Comments:**
> **username_1**: Have you looked at [the Wikipedia article](https://en.wikipedia.org/wiki/WR_137) that uses the image? It should answer your question pretty directly.  
> **username_0**: @username_1 I was looking at the french version. Thank you.  
> **username_2**: 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.)  
> **username_3**: These are emission lines. The units are Angstroms, not nm. Why do you think there is no carbon in a WR star?  
> **username_4**: @username_3 The unit mistake was my bad!  

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## 1 answers

### Answer by username_4 on 2021-07-01. Score: 2 | Accepted answer

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](https://www.atnf.csiro.au/outreach/education/senior/astrophysics/spectra_astro_types.html): 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](https://en.wikipedia.org/wiki/Wolf%E2%80%93Rayet_star#Classification) 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](https://britastro.org/node/19541).


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