The most massive known stars are Wolf-Rayet stars. However, as Wolf-Rayet stars do not appear to be the first stage of any star's lifecycle, I infer that whatever these Wolf-Rayet stars used to be must have been even more massive. My guess is that the most massive stars are Wolf-Rayet because of some combination of 1) their luminosity and/or 2) the potential short life of prior stages of the star's evolution. Assuming this is correct, to what group would we expect the most massive stars to belong (even if we don't know of specific examples)?
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$\begingroup$ I wasn't sure if you were asking which subgroup of evolved massive stars had the highest masses when they were born? $\endgroup$– ProfRobMar 17, 2015 at 7:45
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$\begingroup$ @Rob, yes, that plus telling me the name for the first (most massive) stage of such a star's evolution would answer the question $\endgroup$– kuzzoorooMar 17, 2015 at 11:47
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2 Answers
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Since you phrase your question "What type does theory predict?", I guess the answer must be the so-called Population III stars, which are thought to be the first generation of stars, born from zero-metallicity gas. With no metals, it is difficult for the gas to cool. The mass of a star is given by the Jeans mass of the collapsing cloud, which is proportional to $T^{3/2}/\rho^{1/2}$. This temperature dependence means that the cooler the cloud can be, the smaller clumps the cloud is able to fragment into. Hence, if it's difficult to cool, the cloud has a large Jeans mass, i.e. only large cloud collapse and form stars (this explanation is rather simplified and doesn't take into account stuff like shocks, turbulence, etc., but captures the basic physics).
Moreover, without metals to act as absorbers, the radiation may escape the star more easily, i.e. without interacting with the stellar atmosphere, so mass losses may be less significant and the star is expected to maintain its mass.
Pop III stars are expected to have masses of several hundred to 1000 $M_\odot$. With such large masses, they burn the fuel fast ($\sim$$10^6$ yr), but their spectra are extremely hard, and they should be able to ionize helium, so one way to get a hint of the existence of these guys would be by detection of the He $\lambda$1640 Å line.
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There is no particular type. Yes Wolf Rayets were once more massive and have suffered extensive mass loss. Other examples of evolved massive stars are very luminous red supergiants and luminous blue variables (LBVs). The evolutionary tracks through these processes are somewhat poorly understood, as are the exact magnitudes and durations of the various mass loss phases.
However, they all started out as massive main sequence stars with very short hydrogen burning lifetimes. There are a number of examples of O3V main sequence stars in the Galaxy which probably have masses of around 100 Suns.
Hence the most massive stage of a massive star's life is (briefly) as a main sequence star of type early-O.
