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# Herzsprung-Russell diagram

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Can anyone explain why the Herzsprung-Russell diagram is always shown with the x-axis going in reverse - i.e. decreasing as you go along it rather than increasing as is normal in just about every other such graph or diagram ever conceived?

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Possibly relation between temperature and "color" or dominant wavelength?

"Normal" wavelength graph goes from shorter wavelengths to longer wavelengths (smaller number on the left and larger on the right as you would expect). Such graph places blue on left part and red on right part (450nm vs 650nm). Keeping "color" ordering, high temperatures produce bluish stars while lower temperature red ones - so blue left, red on the right.

This is just a guess of course, but only thing that I can come up with that remotely makes sense

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from what i can remember its a function of age/time. hotter stars are short lived so dont make it to the right of the chart

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Because it follows the traditional classification of spectral types, Oh Be A Fine Girl Kiss Me (with the mnemonic).

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Moving to the right along the x-axis usually goes with an increase in the relevant quantity, but mathematically there is no reason why this should always be the case.  For the HR diagram, it is just a convention that temperature drops towards the right.

Spectral class (OBAFGKM - roughly blue/white/yellow/red) is plotted left to right, and blue stars are young and hot, red ones are older and cooler, so I guess (and that's all this is, as I'm no expert) that broadly speaking it is time that increases (in some complicated way) as you move along the x-axis.

Just a suggestion!

Doug.

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If you plot colour index (B-V) on the X axis  it runs the right way

Robin

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Apparently Russell actually reversed the X and Y axes in his original plot

Edited by robin_astro

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