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So, if you search for Henyey tracks on wikipedia (I know, the shame of it!) you will come across this statement:

The Henyey track is characterized by a slow collapse in near hydrostatic equilibrium. They are approaching the main sequence almost horizontally in the Hertzsprung–Russell diagram (i.e. the luminosity remains almost constant).

Ignoring the bad grammar, I take umbrage with the following:

the luminosity remains almost constant.

I would say that the apparent horizontal track associated with the Henyey track is more due to the fact that in a typical HR diagram the temperature axis ranges over about one order of magnitude, while the luminosity axis may vary over five or more orders of magnitude.

So, to frame this as a question, would you agree or disagree with the statement on the site or with what I have stated (i.e., that the Henyey track is an apparent horizontal track caused by the nuance in the magnitude scales of luminosity in comparison with the temperature)?

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  • $\begingroup$ I dislike both - sorry. Your wording is unreadable (to me) and the original wording is incorrect. (Well you wanted honesty.) What's wrong with "The luminosity remains relatively constant"? $\endgroup$
    – Andy
    Feb 24 '16 at 14:07
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    $\begingroup$ That's wrong tho. It doesn't remain constant or relatively constant. It's lost in the scaling. $\endgroup$ Feb 24 '16 at 14:13
  • $\begingroup$ Fair enough - my use of "relatively" is obviously a lot looser. Actually it might be better posting this on the Wikipedia "Talk" page if you intend to edit, to see what the opinion there is. $\endgroup$
    – Andy
    Feb 24 '16 at 14:20
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    $\begingroup$ Would it be fair to say "the luminosity remains almost constant relative to the Hayashi tracks"? $\endgroup$
    – Dean
    Feb 24 '16 at 14:23
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The wording provides a contrast with the Hayashi track phase that immediately precedes it, where the luminosity decreases by orders of magnitude with little change in effective temperature.

You are correct though, the Henyey track for low-mass stars is not horizontal in the HR diagram, however it is a critical point that the luminosity does not change very much (less than a factor of 3) as the effective temperature increases, since it implies that the star slowly shrinks, and by the virial theorem, the core temperature gradually rises and is ultimately able to sustain nuclear fusion.

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