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.


1 Answer 1


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) are plotted against spectral type.

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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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