<p>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.</p>

<p><a href="https://i.sstatic.net/CzNH1.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/CzNH1.gif" alt="Original HR diagram"></a></p>

<p>When the diagrams were constructed, it was <em>not at all clear</em> 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 <em>decreasing</em> temperature.</p>

<p>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. </p>

CC BY-SA 3.0
2015-10-28T11:23:39.510Z
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...
LocalFluff
CC BY-SA 3.0
2015-10-29T06:28:32.950Z
You can see the scan of Russell's 1914 diagram at NASA ADS [here](http://articles.adsabs.harvard.edu//full/1914PA.....22..275R/0000285.000.html).
Warrick
CC BY-SA 3.0
2015-10-28T10:27:55.783Z
2016-03-27T09:40:41.857Z
2016-03-27T09:40:41.857Z
ProfRob
<p>Originally, what was plotted was <em>luminosity against colour</em>, 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. </p>

<p><a href="http://www.stars-planets-galaxies-qa.com/stars" rel="nofollow noreferrer"><img src="https://i.sstatic.net/eHK0i.png" alt="stars-planets-galaxies-qa.com"></a></p>

<p>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.</p>

CC BY-SA 3.0
2015-10-28T08:40:17.577Z
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.
ProfRob
CC BY-SA 3.0
2015-10-28T10:22:30.310Z
As indeed the picture in your answer illustrates.
ProfRob
CC BY-SA 3.0
2015-10-27T23:17:18.683Z
2015-10-28T09:20:51.467Z
2015-10-28T09:20:51.467Z
James K
<p>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.</p>

<p>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.</p>

<observational-astronomy><history><luminosity><classification><hr-diagram>
Why does the Hertzsprung–Russell diagram's x-axis go from large temperatures to lower?
Whyka
# Why does the Hertzsprung–Russell diagram's x-axis go from large temperatures to lower?

(Asked by: username_0 on 2015-10-27)

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.

**Tags:** observational-astronomy, history, luminosity, classification, hr-diagram


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## 2 answers

### Answer by username_4 on 2015-10-27. Score: 7 | Accepted answer

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](https://i.sstatic.net/eHK0i.png)](http://www.stars-planets-galaxies-qa.com/stars)

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.

> **Comments:**
> **username_1**: 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.  
> **username_1**: As indeed the picture in your answer illustrates.  

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### Answer by username_1 on 2015-10-28. Score: 8

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](https://i.sstatic.net/CzNH1.gif)](https://i.sstatic.net/CzNH1.gif)

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.

> **Comments:**
> **username_2**: 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...  
> **username_3**: You can see the scan of Russell's 1914 diagram at NASA ADS [here](http://articles.adsabs.harvard.edu//full/1914PA.....22..275R/0000285.000.html).  

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