Two 11-second movies shows a computational simulation of a collision of two converging streams of interstellar gas, leading to collapse and formation of a star cluster at the center. In both movies, the numbers rapidly increasing shows the passage of time in millions of years; left panel shows the density of interstellar gas (yellow and red are densest) and right panel shows red and blue “tracer dyes” added to watch how the gas mixes during the collapse. Face-on view (upper pair in the stills) shows the plane where the two gas streams meet while the edge-on view (lower pair in the stills) shows a cross section through the two streams. Circles outlined in black are stars; stars are shown as white in the left panel, and in the right panel their color reflects the amount of the two tracer dyes in each star. The simulation reveals that gas streams are thoroughly homogenized within a very short time of converging, well before stars begin forming. Credit: Mark Krumholz/University of California, Santa Cruz