<p>Strictly speaking, neither.  The types and sub-types are qualitatively defined in terms of spectral features (the presence or absence of various lines) and the relationship to photosphere temperature is a derived and secondary property.</p>
<p>In practice the relationship is mixed. In the graph below the logarithm of temperatures (<a href="https://sites.uni.edu/morgans/astro/course/Notes/section2/spectraltemps.html" rel="nofollow noreferrer">source</a>) are plotted against spectral type.</p>
<p><a href="https://i.sstatic.net/3Gqcd3Bl.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/3Gqcd3Bl.png" alt="enter image description here" /></a></p>
<p>If there were a simple logarithmic relationship, you would expect a straight line.  That works reasonably well for the G, F and A stars, and also for the B stars (but at different parameters)  but the M and K stars have a linear interpolation (giving a curved section of the graph), and the O type stars are a little scattered too.</p>
<p>The evidence from these data is that the scaling of sub-types from 9-to-0 isn't done by simple mathematical scaling applied equally to all types, unlike (for example) the logarithmic magnitude or decibel scales.</p>

CC BY-SA 4.0
2024-05-19T17:12:35.810Z
2024-05-20T13:58:22.803Z
2024-05-20T13:58:22.803Z
James K
<p>Are spectral subclasses logarithmic? I can't seem to find any source on whether they're linear or logarithmic. For reference, by spectral subtypes I mean, for example, G2 vs G3 stars.</p>

<star><classification><stellar-classification>
Are spectral subtypes a logarithmic scale, or a linear one?
Astrovis
# Are spectral subtypes a logarithmic scale, or a linear one?

(Asked by: username_0 on 2024-05-19)

Are spectral subclasses logarithmic? I can't seem to find any source on whether they're linear or logarithmic. For reference, by spectral subtypes I mean, for example, G2 vs G3 stars.

**Tags:** star, classification, stellar-classification


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## 1 answers

### Answer by username_1 on 2024-05-19. Score: 11 | Accepted answer

Strictly speaking, neither. The types and sub-types are qualitatively defined in terms of spectral features (the presence or absence of various lines) and the relationship to photosphere temperature is a derived and secondary property.

In practice the relationship is mixed. In the graph below the logarithm of temperatures ([source](https://sites.uni.edu/morgans/astro/course/Notes/section2/spectraltemps.html)) are plotted against spectral type.

[![enter image description here](https://i.sstatic.net/3Gqcd3Bl.png)](https://i.sstatic.net/3Gqcd3Bl.png)

If there were a simple logarithmic relationship, you would expect a straight line. That works reasonably well for the G, F and A stars, and also for the B stars (but at different parameters) but the M and K stars have a linear interpolation (giving a curved section of the graph), and the O type stars are a little scattered too.

The evidence from these data is that the scaling of sub-types from 9-to-0 isn't done by simple mathematical scaling applied equally to all types, unlike (for example) the logarithmic magnitude or decibel scales.


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