<p>You're right, it is used to separate stellar components in a multiple star system. However, sometimes a notation like <span class="math-container">$\text{A0IV+G}$</span> means that the spectral type of the primary is known but little is known about the companion, so a general spectral type is assigned based on its luminosity.</p>
<p>On the other hand, a superscript plus <sup>+</sup> typically means a higher luminosity than typical stars of its luminosity class, but standard classification is <span class="math-container">$Xa$</span>, <span class="math-container">$X$</span>, and <span class="math-container">$Xb$</span>, with <span class="math-container">$a$</span> being on the brighter end and <span class="math-container">$b$</span> on the dimmer end. The most common place is hypergiants, where you will find a luminosity class of either <span class="math-container">$0$</span> or <span class="math-container">$\text{Ia}^+$</span>.</p>
<p>For example, a star system noted as <span class="math-container">$\text{K0-1III+A4V+A}$</span> states that there is a K-type giant star which variates between <span class="math-container">$K0$</span> and <span class="math-container">$K1$</span> spectral type, and two companion stars of spectral type <span class="math-container">$\text{A4V}$</span> and <span class="math-container">$A$</span>, the latter having unknown stellar properties. I hope this helps relieve the ambiguity of such notations.</p>

CC BY-SA 4.0
2021-09-02T01:11:24.600Z
It does, thank you :). Silly question, but I'm, assuming superscript minus would also exist and denote lower luminosity?
aggregate1166877
CC BY-SA 4.0
2021-09-02T12:48:41.097Z
@aggregate1166877 So far through my astronomy experience I haven't seen such a thing.
WarpPrime
CC BY-SA 4.0
2021-09-02T22:10:57.343Z
Awesome, thank you @fasterthanlight
aggregate1166877
CC BY-SA 4.0
2021-09-01T14:03:46.120Z
2021-09-01T14:03:46.120Z
WarpPrime
<p>I've noticed some stellar classifications might look like this:</p>
<ul>
<li><code>A0IV+G:</code>  (HD 70003)</li>
<li><code>F3IV+F0IV</code>  (HD 82434)</li>
<li><code>K0-1III+A4V+A</code>  (HD 39547)</li>
<li><code>G7IIIa+F9IV</code>  (HD 130458)</li>
<li><code>S6+/1e</code>  (HD 187796)</li>
</ul>
<p>I assume it means it's a binary (or triple, etc) star system, but cannot find any literature to back this up.</p>
<p>Resources I've looked at:</p>
<ul>
<li><a href="https://courses.lumenlearning.com/astronomy/chapter/the-spectra-of-stars-and-brown-dwarfs/" rel="nofollow noreferrer">The Spectra of Stars (and Brown Dwarfs)</a></li>
<li><a href="https://en.wikipedia.org/wiki/Stellar_classification" rel="nofollow noreferrer">Stellar Classification</a></li>
<li><a href="https://www.enchantedlearning.com/subjects/astronomy/stars/startypes.shtml" rel="nofollow noreferrer">Star Type</a></li>
<li><a href="https://astronomy.swin.edu.au/cosmos/s/Spectral+Type" rel="nofollow noreferrer">Spectral Type</a></li>
<li><a href="https://www.atnf.csiro.au/outreach/education/senior/astrophysics/spectral_class.html" rel="nofollow noreferrer">Spectral Class</a></li>
<li><a href="https://astronomy.swin.edu.au/cosmos/m/morgan-keenan+luminosity+class" rel="nofollow noreferrer">Morgan-Keenan Luminosity Class</a></li>
<li><a href="https://www.astro.princeton.edu/%7Eburrows/classes/204/stellar.atmospheres.HR.pdf" rel="nofollow noreferrer">The classification of stellar spectra</a></li>
<li><a href="http://www.star.ucl.ac.uk/%7Epac/spectral_classification.html" rel="nofollow noreferrer">The Classification of Stellar Spectra</a></li>
<li><a href="https://academic.oup.com/mnras/article/325/1/343/959384?login=true" rel="nofollow noreferrer">Studies of multiple stellar systems</a></li>
</ul>
<p>(There are more, but this list is getting a bit ridiculous.)</p>
<br>
What does the plus actually mean?
<br><br>
<p><strong>EDIT:</strong> doing further research, it would appear as though you get multiple types of plusses, too: <br>
<code>⁺</code> (superscript) - Increase in strength / magnitude? <br>
<code>+</code> (regular) - A sibling star?</p>
<p>Where it potentially gets a bit confusing is that downloadable star catalogs appear to present both as regular <code>+</code>, meaning you'd need to read it contextually.</p>

CC BY-SA 4.0
2021-08-31T23:16:58.087Z
You are correct: the '+' is used to separate multiple components in a binary system. Offten the secondary star is much less luminous and harder to see in the combined spectra and so it can be hard to make more than general statements about the spectral class (e.g. the '+G' in your first example). I haven't found a definitive reference for this either
astrosnapper
<luminosity><star-systems><spectral-type>
What does a + (plus) mean in stellar classification?
aggregate1166877
# What does a + (plus) mean in stellar classification?

(Asked by: username_0 on 2021-08-31)

I've noticed some stellar classifications might look like this:

* `A0IV+G:` (HD 70003)
* `F3IV+F0IV` (HD 82434)
* `K0-1III+A4V+A` (HD 39547)
* `G7IIIa+F9IV` (HD 130458)
* `S6+/1e` (HD 187796)

I assume it means it's a binary (or triple, etc) star system, but cannot find any literature to back this up.

Resources I've looked at:

* [The Spectra of Stars (and Brown Dwarfs)](https://courses.lumenlearning.com/astronomy/chapter/the-spectra-of-stars-and-brown-dwarfs/)
* [Stellar Classification](https://en.wikipedia.org/wiki/Stellar_classification)
* [Star Type](https://www.enchantedlearning.com/subjects/astronomy/stars/startypes.shtml)
* [Spectral Type](https://astronomy.swin.edu.au/cosmos/s/Spectral+Type)
* [Spectral Class](https://www.atnf.csiro.au/outreach/education/senior/astrophysics/spectral_class.html)
* [Morgan-Keenan Luminosity Class](https://astronomy.swin.edu.au/cosmos/m/morgan-keenan+luminosity+class)
* [The classification of stellar spectra](https://www.astro.princeton.edu/%7Eburrows/classes/204/stellar.atmospheres.HR.pdf)
* [The Classification of Stellar Spectra](http://www.star.ucl.ac.uk/%7Epac/spectral_classification.html)
* [Studies of multiple stellar systems](https://academic.oup.com/mnras/article/325/1/343/959384?login=true)

(There are more, but this list is getting a bit ridiculous.)

  
What does the plus actually mean?
  
  

**EDIT:** doing further research, it would appear as though you get multiple types of plusses, too:   
`⁺` (superscript) - Increase in strength / magnitude?   
`+` (regular) - A sibling star?

Where it potentially gets a bit confusing is that downloadable star catalogs appear to present both as regular `+`, meaning you'd need to read it contextually.

**Tags:** luminosity, star-systems, spectral-type

> **Comments:**
> **username_1**: You are correct: the '+' is used to separate multiple components in a binary system. Offten the secondary star is much less luminous and harder to see in the combined spectra and so it can be hard to make more than general statements about the spectral class (e.g. the '+G' in your first example). I haven't found a definitive reference for this either  

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## 1 answers

### Answer by username_2 on 2021-09-01. Score: 1 | Accepted answer

You're right, it is used to separate stellar components in a multiple star system. However, sometimes a notation like $\text{A0IV+G}$ means that the spectral type of the primary is known but little is known about the companion, so a general spectral type is assigned based on its luminosity.

On the other hand, a superscript plus + typically means a higher luminosity than typical stars of its luminosity class, but standard classification is $Xa$, $X$, and $Xb$, with $a$ being on the brighter end and $b$ on the dimmer end. The most common place is hypergiants, where you will find a luminosity class of either $0$ or $\text{Ia}^+$.

For example, a star system noted as $\text{K0-1III+A4V+A}$ states that there is a K-type giant star which variates between $K0$ and $K1$ spectral type, and two companion stars of spectral type $\text{A4V}$ and $A$, the latter having unknown stellar properties. I hope this helps relieve the ambiguity of such notations.

> **Comments:**
> **username_0**: It does, thank you :). Silly question, but I'm, assuming superscript minus would also exist and denote lower luminosity?  
> **username_2**: @username_0 So far through my astronomy experience I haven't seen such a thing.  
> **username_0**: Awesome, thank you @fasterthanlight  

---

