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In my textbook, the H–R diagram’s y-axis is $\log(L/L_{\odot})$ and values are higher as we go higher on the scale, but the x-axis is $\log(T_\text{eff})$ and gets smaller as we proceed to the right.

This confuses me. Why not let the x-axis go from lower values to higher values? This will also help visualising the “linear” relation between the luminosity and the temperature of a star in the main sequence.

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The original Hertzsprung-Russell diagrams constructed by Henry Russell and Eijnar Hertzsprung consisted of absolute magnitude on the y-axis and a spectral type or an indicator of spectral type on the x-axis. Below you can see an original HR diagram produced by Russell in 1913.

Original HR diagram

When the diagrams were constructed, it was not at all clear what the sequence of spectral types or spectral type indicators actually meant. It turned out of course that the sequence (in modern day parlance O,B,A,F,G,K,M) actually corresponds to decreasing temperature.

Astronomers have simply stuck with this convention to the present day, there is no particular reason for that. Most HR diagrams are now plotted with temperature (decreasing) along the x-axis, although that is not what the original HR diagram was.

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  • $\begingroup$ It's one of those charming reminders of the brilliant history of astronomy. Since it isn't final yet, new letters were added just a few years ago, one might as well wait with ordering and naming it logically until one day everything is known... $\endgroup$
    – LocalFluff
    Oct 28, 2015 at 11:23
  • $\begingroup$ You can see the scan of Russell's 1914 diagram at NASA ADS here. $\endgroup$
    – Warrick
    Oct 29, 2015 at 6:28
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Originally, what was plotted was luminosity against colour, and by colour I mean the wavelength of the peak intensity. Short wavelengths on the left, and long on the right, as you would expect.

stars-planets-galaxies-qa.com

Now since stars emit (nearly) black body radiation, there is a close relationship between colour and temperature. I suspect that the reason that the x axis isn't inverted when temperature is used is just inertia:- We have always plotted out H-Z diagrams like this and we will keep doing so.

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    $\begingroup$ Originally what was plotted was absolute magnitude versus spectral type. If colour is on the x-axis then the diagram is known as a colour magnitude diagram. $\endgroup$
    – ProfRob
    Oct 28, 2015 at 8:40
  • $\begingroup$ As indeed the picture in your answer illustrates. $\endgroup$
    – ProfRob
    Oct 28, 2015 at 10:22

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