<p>I remember that there are five types of <a href="https://en.wikipedia.org/wiki/Nebula" rel="nofollow noreferrer">nebulae</a>:</p>
<ol>
<li><a href="https://en.wikipedia.org/wiki/Planetary_nebula" rel="nofollow noreferrer">Planetary nebulae</a> have not much to do with planets, but are <a href="https://en.wikipedia.org/wiki/Emission_nebula" rel="nofollow noreferrer">emission nebula</a> <em>consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives</em>.</li>
<li><a href="https://en.wikipedia.org/wiki/H_II_region" rel="nofollow noreferrer">H II regions</a> are emission nebulae in which interstellar atomic hydrogen is ionized.</li>
<li><a href="https://en.wikipedia.org/wiki/Reflection_nebula" rel="nofollow noreferrer">Reflection nebulae</a> are <em>&quot;clouds of interstellar dust which might reflect the light of a nearby star(s)&quot;</em>.</li>
<li><a href="https://en.wikipedia.org/wiki/Dark_nebula" rel="nofollow noreferrer">Dark nebulae</a> are <em>interstellar clouds that is so dense that it obscures the visible wavelengths of light from objects behind it</em>.</li>
<li><a href="https://en.wikipedia.org/wiki/Supernova_remnant" rel="nofollow noreferrer">Supernova remnants</a> are just that - <em>the leftovers of a <a href="https://en.wikipedia.org/wiki/Supernova" rel="nofollow noreferrer">supernova</a></em></li>
</ol>
<p>As <a href="https://en.wikipedia.org/wiki/Donald_Knuth" rel="nofollow noreferrer">Donald Knuth</a> once <a href="https://en.wikiquote.org/wiki/Donald_Knuth" rel="nofollow noreferrer">said</a></p>
<blockquote>
<p>Science is what we understand well enough to explain to a computer. Art is everything else we do.</p>
</blockquote>
<p>I fully understand the definitions of the different types of nebula, but <strong>I struggle to write a computer-readible definition for each of nebulae types</strong>. What I mean: Given a deep-sky image with pre-identified nebulae zones, I do not know how a nebula type identification algorithm would look like. Do different types of spectra uniquely define each nebula type? That would work for the emission nebulae at least, I guess.</p>
<h3>References</h3>
<p>The answer is possibly given by Steven R. Coe's article <a href="https://link.springer.com/chapter/10.1007/978-3-319-22530-2_11" rel="nofollow noreferrer">What Are All These Different Types of Nebulae, and What Details Can I See in Them with My Telescope?</a> which hidden behind a paywall and not accessible for me.</p>

CC BY-SA 4.0
2021-01-16T21:45:20.017Z
Knuth doesn't seem to be aware of computer vision in that quotation... We can't even tell a computer what a cat is, in terms that allow it to tell from an image whether there is one on there or not - not without resorting to machine learning (which is what you do when you've given up on finding a definition that can be programmed).
Kristoffer Sjöö
CC BY-SA 4.0
2021-01-16T22:26:34.370Z
Well, I purposedly did not mention AI and the fact that this is the usual approach of doing such classifications. Assuming that some machine learning algorithm already attributed a type to a given nebula zone on an image, one could in a 2nd step plot a state space spanned by different variables (e.g. mean wavelength, elements identified in the spectrum etc.) and analyze whether one find some semi-hard criteria.
B--rian
CC BY-SA 4.0
2021-01-16T22:34:50.243Z
Differently put: Adanced [feature selection](https://en.m.wikipedia.org/wiki/Feature_selection) methods would allow you to determine the hard criteria I am after.
B--rian
CC BY-SA 4.0
2021-01-16T22:36:07.713Z
@uhoh If I may bother you once again for your input? You helped me more than once to make my questions more accessible.
B--rian
CC BY-SA 4.0
2021-01-16T22:49:09.333Z
Planetary nebulae emit light in the doubly ionized oxygen (O III) line. I know this is a way to tell them apart from stars when they have a very small angular diameter. Not sure about the other types of nebulae—maybe H-alpha for H II regions?
Pierre Paquette
CC BY-SA 4.0
2021-01-17T00:51:49.003Z
@B--rian Sorry it looked really badly :-) I also voted the post up. My goal was actually [this](https://astronomy.stackexchange.com/review/suggested-edits/24736) tag wiki edit suggestion review: OP stated that the "untagged" tag is mod-only, I also tested it (if I can retag it as "untagged", then I had to roll it back and then reject the edit suggestion, however an error message said that yes it is really mod-only).
peterh
<spectra><nebula><algorithm>
How to algorithmically distinguish between different types of nebulae?
B--rian
# How to algorithmically distinguish between different types of nebulae?

(Asked by: username_0 on 2021-01-16)

I remember that there are five types of [nebulae](https://en.wikipedia.org/wiki/Nebula):

1. [Planetary nebulae](https://en.wikipedia.org/wiki/Planetary_nebula) have not much to do with planets, but are [emission nebula](https://en.wikipedia.org/wiki/Emission_nebula) *consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives*.
2. [H II regions](https://en.wikipedia.org/wiki/H_II_region) are emission nebulae in which interstellar atomic hydrogen is ionized.
3. [Reflection nebulae](https://en.wikipedia.org/wiki/Reflection_nebula) are *"clouds of interstellar dust which might reflect the light of a nearby star(s)"*.
4. [Dark nebulae](https://en.wikipedia.org/wiki/Dark_nebula) are *interstellar clouds that is so dense that it obscures the visible wavelengths of light from objects behind it*.
5. [Supernova remnants](https://en.wikipedia.org/wiki/Supernova_remnant) are just that - *the leftovers of a [supernova](https://en.wikipedia.org/wiki/Supernova)*

As [Donald Knuth](https://en.wikipedia.org/wiki/Donald_Knuth) once [said](https://en.wikiquote.org/wiki/Donald_Knuth)

> Science is what we understand well enough to explain to a computer. Art is everything else we do.

I fully understand the definitions of the different types of nebula, but **I struggle to write a computer-readible definition for each of nebulae types**. What I mean: Given a deep-sky image with pre-identified nebulae zones, I do not know how a nebula type identification algorithm would look like. Do different types of spectra uniquely define each nebula type? That would work for the emission nebulae at least, I guess.

### References

The answer is possibly given by Steven R. Coe's article [What Are All These Different Types of Nebulae, and What Details Can I See in Them with My Telescope?](https://link.springer.com/chapter/10.1007/978-3-319-22530-2_11) which hidden behind a paywall and not accessible for me.

**Tags:** spectra, nebula, algorithm
> **Comments:**
> **username_1**: Knuth doesn't seem to be aware of computer vision in that quotation... We can't even tell a computer what a cat is, in terms that allow it to tell from an image whether there is one on there or not - not without resorting to machine learning (which is what you do when you've given up on finding a definition that can be programmed).  
> **username_0**: Well, I purposedly did not mention AI and the fact that this is the usual approach of doing such classifications. Assuming that some machine learning algorithm already attributed a type to a given nebula zone on an image, one could in a 2nd step plot a state space spanned by different variables (e.g. mean wavelength, elements identified in the spectrum etc.) and analyze whether one find some semi-hard criteria.  
> **username_0**: Differently put: Adanced [feature selection](https://en.m.wikipedia.org/wiki/Feature_selection) methods would allow you to determine the hard criteria I am after.  
> **username_0**: @uhoh If I may bother you once again for your input? You helped me more than once to make my questions more accessible.  
> **username_2**: Planetary nebulae emit light in the doubly ionized oxygen (O III) line. I know this is a way to tell them apart from stars when they have a very small angular diameter. Not sure about the other types of nebulae—maybe H-alpha for H II regions?  
> **username_3**: @B--rian Sorry it looked really badly :-) I also voted the post up. My goal was actually [this](https://astronomy.stackexchange.com/review/suggested-edits/24736) tag wiki edit suggestion review: OP stated that the "untagged" tag is mod-only, I also tested it (if I can retag it as "untagged", then I had to roll it back and then reject the edit suggestion, however an error message said that yes it is really mod-only).  

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## 0 answers

