# HR diagram x-axis (color) and redshift?

I've read several articles and watched some videos on the HR diagram, but no-one seems to mention the effect of redshift on the color of a star. Is there some calibration step that needs to be applied when looking up the Luminosity for a star that you've observed the color of?

For instance, if I observe a star a being a certain color then using the x-axis on the HR diagram I can find that observation's color and find a corresponding location in the HR diagram. However if the star I'm observing is redshifted the color I've observed is not the actual color of that star.

I'm super-new at reading the HR diagram but it really fascinates me and I cannot find why redshift isn't mentioned in relation to it.

Edit: it's not answered by the suggested Relationship between photometric colour and redshift as my question isn't so much about about spectroscopy or the types of redshift but simply if the HR diagram applies to far away stars or not.

• I think this might answer your question: Relationship between photometric colour and redshift Apr 28 at 12:44

But in order to distinguish and measure individual stars (so as to construct an H-R diagram), the galaxy has to be close enough -- the upper limit (for observations with the Hubble Space Telescope) is about 10 Mpc, or a redshift of $$V \sim 1000$$ km/s. For that kind of redshift, the K corrections will be $$\sim 0.01$$ magnitudes or smaller (you can test out different values using this online calculator -- note that it uses $$z = V/c$$ for redshift). This means the effect will be smaller than typical observational errors.
For stars within our own galaxy, the redshift (or blueshift!) effects will be due to relative velocities $$< 200$$ km/s or so, which means the effects will be really small.