The Most Massive Star

An important question in star formation is how massive can a star be? This is a very
tricky question, depending strongly on how the infalling material which forms the star interacts with the light emitted by it. Too much light (radiation pressure), it prevents from the material from falling onto the star. How the light interacts with the material depends on many factors: the chemical composition of the infalling material, the (unknown, honest) atomic properties of these atoms, etc. It was thought that the most massive star that could form today was 200 Solar Masses, but astronomers recently found something even more massive. Go here to read more about it.