How big can stars be? people have been wrestling with this question for a long time is there a cap on the mass of stars? could a star ever form that was 1000 time the mass of the sun? here is another paper that attempts to derive an observational probable upper bound on star mass http://arxiv.org/abs/astro-ph/0503193 the paper has been accepted by the journal Nature and will appear this month. I think Nature is pretty selective so maybe it worth tagging. exerpt: "Theory provides little guide in determining the most massive star that can form. Pulsational instabilities were once thought to destroy stars more massive than 95 Msolar, however, these pulsations may be damped. Radiation pressure, and/or ionizing flux, inhibit accretion for stellar masses greater than 60 Msolar, but direct collisions of protostellar clumps may overcome these effects. While stellar evolution models have been computed for massive stars covering a large range in mass, up to 1000 Msolar, no such stars have ever been observed. Indeed, some of the most massive candidates have proven to be systems of multiple stars..." this guy has an argument why it is very unlikely that there are any stars bigger than 150 solar mass.