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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:

(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.

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    $\begingroup$ 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 $\endgroup$
    – astrosnapper
    Commented Aug 31, 2021 at 23:16

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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.

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  • $\begingroup$ It does, thank you :). Silly question, but I'm, assuming superscript minus would also exist and denote lower luminosity? $\endgroup$
    – aggregate1166877
    Commented Sep 2, 2021 at 1:11
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    $\begingroup$ @aggregate1166877 So far through my astronomy experience I haven't seen such a thing. $\endgroup$
    – WarpPrime
    Commented Sep 2, 2021 at 12:48
  • $\begingroup$ Awesome, thank you @fasterthanlight $\endgroup$
    – aggregate1166877
    Commented Sep 2, 2021 at 22:10

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