I'm reading some older materials which refer to something called the "k-effect". This was described as "hot stars in the Sun’s neighborhood are moving away from us in all directions, while cooler stars do not".

I tried looking in Google, but the only near hit I got was a 1955 article that was comparing the values of redshift in the local arm with the sag arm and showing the same K value for both. Laking the historical context though, I'm not clear if he is concluding that K is basically zero (2.5 kps), or that it is a widespread constant.

Does anyone recall this anomaly and perhaps have later papers on the topic?


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The k-effect (or Trumpler effect) is an anomalous, additional redshift identified in the spectra of hot O/B and other luminous stars (Arp 1992). It is discussed in the article "Redshifts of unknown origin" by Burbidge 1996 and so it was a live issue then.

Essentially what you can determine, by looking at the spectra of hot stars in clusters - where the assumption is that the hot stars don't have any systematic velocity with respect to cooler stars - is that there is an additional redshift of order 10-20 km/s that is too large just to be explained by General Relativity. Note that this is miniscule compared to the redshifts measured at cosmological distances but possibly significant when examining the dynamics of stellar populations.

The atmospheres of hot and luminous stars are complicated - featuring massive winds and outflows with velocities of hundreds of km/s. Interpretation of the spectra and line of sight velocities is hazardous.

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    $\begingroup$ Thanks, that link is just what I was looking for. And the last bit-- the same issue that Dravins, Lindegren & Nordlund noted for the sun while explaining the anomalous redshift of the solar limbs, so likely the same explanation applies to both. $\endgroup$ Jul 17, 2023 at 15:05

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