# 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.

• Welcome to Astronomy SE! Could you please add some more details so you can receive better answers? Apr 28 '21 at 1:55
• I think this might answer your question: Relationship between photometric colour and redshift Apr 28 '21 at 12:44

## 1 Answer

The short answer is that the redshift of stars that are close enough to put into H-R diagrams (or "color magnitude diagrams", if we're being precise, given that you're talking about using colors) is so small that the effects are minimal.

The correction to color you're thinking of (called "K correction") depends on the redshift: the smaller the redshift, the smaller the effect.

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.

Put simply, galaxies that are far enough away to have large enough redshifts to worry about the effects on colors are too far away for you to observed individual stars and make H-R diagrams.

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.

• Thank you for answering this (and with way more precise terminology that I could think of, haha..) I was already beginning to think that individual stars wouldn't be visible in other galaxies due to the distance, but I wasn't entirely sure if redshift was significant enough for stars in our own milky way to affect the HR diagram. May 2 '21 at 22:07