Is it a flying elephant? A gingerbread man? When I was little, I used to search the clouds for amusing shapes as they drifted across the sky and imagine stories about their patterns. Now I’m a professional stargazer, and things haven’t changed much. These days I search for patterns in molecular clouds, the birthplaces of stars. The shapes I find in these stellar nurseries do more than stimulate my imagination—they also tell a very real story about when, where and how stars are born. For astronomers, understanding this story depends on our ability to identify and interpret the intricate forms we see in the clouds.
Observations reveal elaborate networks of material, including compact clumps of gas and long, skinny, noodlelike structures called filaments woven throughout. Far from being uniform and smooth like milk, molecular clouds are lumpy, more like chicken noodle soup. The gas and dust accumulate into a range of physical scales and are organized into increasingly dense formations. Their structure is hierarchical, like Russian nesting dolls, with smaller shapes enclosed within larger ones. Filaments are much denser than the diffuse gas that fills most of the volume of a cloud. And embedded within filaments are even smaller, denser knots of gas we call cores. These cores represent the final stage before a star is born.
The dynamics of molecular clouds are as complicated as their spatial structure. Stars, planets, and galaxies such as the Milky Way all spin around their axes in a fairly predictable manner. But the space between the stars—the interstellar medium, where molecular clouds reside—is a wild, chaotic frontier. The motions inside clouds are turbulent, with globs and eddies of gas swirling around like capricious fairies. Observations of both the dynamics and the spatial architecture of molecular clouds have enabled astronomers to paint a compelling, if incomplete, picture of how stars are born.
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