The Interstellar Medium, Part II: Dense Molecular Clouds

By the time collapse has taken place, often the center of the cloud has become very cold (~10 K) and quite dense (n > 100/cc). In this case the cloud has become opaque to starlight and is, hence, "self-shielding." This effect was dramatically revealed in this now-famous picture from the Hubble Space Telescope. One can almost see how the dark molecular clouds are shielding their interiors from the ablating starlight. This shielding allows inner parts of the cloud to collapse freely, unmolested by starlight or hot gas.

Because the collapse is fairly random, fragmentation occurs and local cores begin to form where condensation proceeds rapidly, visible as the knots in this close-up. Inside these cores, gravity takes over completely and pulls matter into protostars. As these cores collapse, their gravitational energy begins to be liberated as heat, creating the protostars, as seen here. In this photograph, three very young stars are starting to emerge from their dusty molecular womb.

Molecular clouds play a vital role in the universe--they make the stars. We are only now beginning to understand how they do so in detail, but recent observations with the Hubble Space Telescope leave little doubt of their importance.

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