Today in the Milky Way, star formation typically occurs in low-mass environments. Young star clusters (e.g., younger than 10 million years), in fact, typically have a mass of a few hundred solar masses. Nevertheless, our Galaxy hosts a few very massive star-forming regions that can produce tens to hundreds of thousands of stars, including some of the most massive stars
OB associations are objects of great importance, since they contain a rich population of OB stars. Despite their paucity, OB stars have a strong impact on the evolution of their parental clouds, nearby stars, and the whole Galaxy. For instance, their intense UV emission affects the evolution of the parental cloud and the star formation process in it, sometimes triggering
During their short existence lasting only few million years, massive stars (more massive than 8 solar masses with spectral type O and B-early) strongly affect the surrounding environment thanks to their intense UV radiation and stellar winds. A single O star, in fact, can loose about 10-6 solar masses of gas in a wind reaching velocities of 1600–2500 km s−1 (Stevens & Hartwell, 2003).
