Stars are born in massive clouds of cold gas and dust called nebulas. Something as seemingly insignificant as the gravitational tug of a passing star can create an instability in these "stellar nurseries" triggering a gravitational collapse leading to clumping of material under its own gravity. As the clumps condense, they heat up, eventually igniting thermonuclear fusion in the core and a star is born. Massive stars forming this way pull in large amounts of material in the process.
