slide1 ==> 4.1: The Orion Nebula, a region of active star formation during the past few Myr
slide2 ==> 4.2: Stages of star formation: (a) collapse of cores, (b) protostar & accretion disk, (c) outflow jet, (d) star and disk
slide3 ==> 4.3: Four young stars surrounded by dark accretion disks visible against the background Orion Nebula (NASA)
slide4 ==> 4.4: A "test body" moves through a cloud of "targets" and collides with those within the tube it sweeps out
slide5 ==> 4.5: A "test body" may collide with a larger body even if its impact parameter a is larger than the radius R