Listen to this Podcast from Scientific American. How big can a star get? Based on observations, astronomers think there's a limit of about 150 times the mass of the sun for the vast majority of stars.

Listen to a discussion about stars that may have with masses of up to 600 suns according to a study submitted to the Monthly Notices of the Royal Astronomical Society. [Tony Pan and Abraham Loeb,"Identifying Stars of Mass 150 Msun from Their Eclipse by a Binary Companion"