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While looking at a list of nearby stars, I noticed that the spectral classes were not always consistent compared to the masses (and radii) of the main sequence stars. Especially the comparison between Tau Ceti and Epsilon Indi A was striking for me. The mass of the two stars is almost the same (0.783±0.012 and 0,778 ± 0,039) and the radius only slightly different (0.793±0.004 and 0,707 ± 0,035), but there is a concise difference in their temperature (~700+ K) and therefore their spectral class (G8V and K5V).

After an extensive search on the internet I could not find a satisfying answer, as to what, apart from the mass and the radius, can influence the temperature of a star so strongly.

To be sure, I looked up the properties of the stars in other sources and the values remained the same. What could be responsible for this difference, if one does not assume that it is about measuring errors.

https://en.wikipedia.org/wiki/Tau_Ceti, https://en.wikipedia.org/wiki/Epsilon_Indi

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  • $\begingroup$ Very good question. I recon one can find an answer in terms of age and metallicity. But I don't have and find quickly good sources which thoroughly outline their influence on stellar properties. $\endgroup$
    – planetmaker
    Jun 28, 2022 at 12:45

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Metallicity (or the fraction of things other than Hydrogen or helium)

If you look on the Wikipedia page it points out that Tau ceti is metal deficient ([Fe/H] = -0.55) and Epsilon Indi has [Fe/H] = -0.13 which is still metal poor compared to the Sun which is [Fe/H] = 0.0, but not as metal poor as Tau ceti.

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  • $\begingroup$ Would there be a possibility that you have a source at which I can look into the subject a little further? I had looked into metallicity before, but I didn't know about such strong effects on temperature, as my sources were rather rudimentary (mainly difference between Pop. I and II and so on). After all, a difference of 700K is nothing small in the cooler spectral types. $\endgroup$
    – vinyazu
    Jun 28, 2022 at 13:51
  • $\begingroup$ Indeed, metal-poor stars are hotter and more luminous at the same mass. $\endgroup$
    – ProfRob
    Jun 28, 2022 at 14:59

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