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Usual methods of estimating stellar ages involve isochrone approximations. It can also help to estimate a star's radius by correlating its absolute magnitude with effective temperature and apparent magnitude. In the absence of these measurements or observations of tell-tale variability, how might you guess a star's age and evolutionary status?

Given a single, high-resolution $(R\gtrsim 50000)$ spectrum as your only data point, how easy is it to accurately infer the age and evolutionary status of a star? For example, how would the spectrum differ between a red dwarf and red giant, both with of $T_{\mathrm{eff}} = 4000~\mathrm{K}$? Or between two red dwarfs of ages $2~\mathrm{Gyr}$ and $8~\mathrm{Gyr}$?

A good answer could describe how surface gravity $(\log g)$ affects spectral lines (and how this relates to stellar mass and radius), what elements we might observe more strongly at different stages of evolution, and some observational results of gyrochronology.

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The point is not to differentiate between red dwarf and red gians (they are very different stars with very different spectrums) but to differentiate between a young red dwarf and an old one. –  Envite Nov 30 '13 at 14:07
Indeed that is true, but perhaps some of those differences are also manifested (albeit more subtly) in a comparison between a 2 Gyr and 8 Gyr old red dwarf - it's these more precise determinations that I am more interested in. Essentially, I am wondering how easy (or possible) it is to make a reasonably precise (say to 1 Gyr) age determination that does not rely on isochrones. –  Moriarty Nov 30 '13 at 14:30

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In short: you can not.

In length: best you can do is to match up your spectrum with a library of known spectra, and find the best match. But for these spectra to be useful you need to have determined their ages, masses, Y's (contents of Helium) and Z's (contents of metals, that is, evrything beyond Helium). And their ages come from... yes, isochrones, so you would be using isochrones indirectly.

So, in short again, yes, you can determine the mass, age and Y and Z of a star with its spectrum and without its own isochrone, maybe up to 5% of its main-sequence lifetime during main sequence status (e.g. 0.5 Gyr for a 10 Gyr main-secuence lifetime star like our Sun).

And yes again, this match-up of spectra gives additional info like surface gravity, which is not useful on its own but needs previous knowledge of mass and radius.

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