11-second movie 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. Edge-on view shows a cross section through the two streams as they meet. Numbers rapidly increasing at upper left 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. 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