Internal structure of a cold dark molecular cloud inferred from the extinction of background starlight

Stars and planets form within dark molecular clouds, but little is understo od about the internal structure of these clouds, and consequently about the initial conditions that give rise to star and planet formation. The clouds are primarily composed of molecular hydrogen, which is virtually inaccessi ble to direct observation. But the clouds also contain dust, which is well mixed with the gas and which has well understood effects on the transmissio n of light. Here we use sensitive near-infrared measurements of the light f rom background stars as it is absorbed and scattered by trace amounts of du st to probe the internal structure of the dark cloud Barnard 68 with unprec edented detail. We rnd the cloud's density structure to be very well descri bed by the equations for a pressure-confined, self-gravitating isothermal s phere that is critically stable according to the Bonnor-Ebert criteria(1,2) . As a result we can precisely specify the physical conditions inside a dar k cloud on the verge of collapse to form a star.