<p><a href="https://i.sstatic.net/SONX0.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/SONX0.png" alt="STAR CLASSIFICATION DIAGRAM"></a></p>

<p>In this diagram the $x$-axis is spectral type for stars and the $y$-axis is B-V colour.</p>

<p>Taking <strong>Series 1</strong> as the Main Sequence, <strong>Series 2</strong> as Giants and <strong>Series 3</strong> as Super-giants. Why the spectral class for all these stars differ from the other ones?</p>

<p>If we were to say differences, for instance, how would one express that information?</p>

CC BY-SA 3.0
2015-11-19T17:52:39.313Z
What is measured on the vertical axis?
James K
CC BY-SA 3.0
2015-11-20T00:46:33.757Z
Hi there.The vertical axis has the star colour B - V.
J. Astro
CC BY-SA 3.0
2015-11-20T07:56:09.950Z
This question doesn't strike me as clear, but I think what's being asked is "Why do stars of the same spectral type have different colours at different brightnesses?" In his case, for example, a K5 dwarf has B-V=0.8ish, vs a K5 giant with B-V=1.1ish. (Based on the plot above. No idea where it comes from.)
Warrick
CC BY-SA 3.0
2015-11-20T23:49:46.967Z
This is clearly ***not*** the Hertzsprung Russell diagram http://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram
Eubie Drew
CC BY-SA 3.0
2015-11-20T23:54:26.590Z
Okay, the vertical axis is the B-V colour index, as explained here http://en.wikipedia.org/wiki/Color_index
Eubie Drew
CC BY-SA 3.0
2015-11-21T00:22:48.320Z
The horizontal axis, spectral type, is explained in excruciating detail here http://en.wikipedia.org/wiki/Stellar_classification#Spectral_types but none of the links I have so far provided actually answer your Q, they just make your Q clearer.
Eubie Drew
CC BY-SA 3.0
2015-11-21T01:28:47.577Z
I believe the answer is buried here: http://www.handprint.com/ASTRO/specclass.html
I'll be back in a few days to explain.
Eubie Drew
<spectra><observational-astronomy><classification><main-sequence><magnitude>
Why does spectral class and U-V colour correlate slightly differently for main sequence, giant and supergiant stars?
J. Astro
# Why does spectral class and U-V colour correlate slightly differently for main sequence, giant and supergiant stars?

(Asked by: username_0 on 2015-11-19)

[![STAR CLASSIFICATION DIAGRAM](https://i.sstatic.net/SONX0.png)](https://i.sstatic.net/SONX0.png)

In this diagram the $x$-axis is spectral type for stars and the $y$-axis is B-V colour.

Taking **Series 1** as the Main Sequence, **Series 2** as Giants and **Series 3** as Super-giants. Why the spectral class for all these stars differ from the other ones?

If we were to say differences, for instance, how would one express that information?

**Tags:** spectra, observational-astronomy, classification, main-sequence, magnitude

> **Comments:**
> **username_1**: What is measured on the vertical axis?  
> **username_0**: Hi there.The vertical axis has the star colour B - V.  
> **username_2**: This question doesn't strike me as clear, but I think what's being asked is "Why do stars of the same spectral type have different colours at different brightnesses?" In his case, for example, a K5 dwarf has B-V=0.8ish, vs a K5 giant with B-V=1.1ish. (Based on the plot above. No idea where it comes from.)  
> **username_3**: This is clearly ***not*** the Hertzsprung Russell diagram http://en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram  
> **username_3**: Okay, the vertical axis is the B-V colour index, as explained here http://en.wikipedia.org/wiki/Color_index  
> **username_3**: The horizontal axis, spectral type, is explained in excruciating detail here http://en.wikipedia.org/wiki/Stellar_classification#Spectral_types but none of the links I have so far provided actually answer your Q, they just make your Q clearer.  
> **username_3**: I believe the answer is buried here: http://www.handprint.com/ASTRO/specclass.html I'll be back in a few days to explain.  

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## 0 answers

