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slcdmw01 t1_izdgn1f wrote

Most stars are created in open clusters. Open clusters are created from the collapse of molecular clouds. But what creates molecular clouds? Why are molecular clouds often hanging around ready for collapse? Are they just relatively dense parts of the interstellar medium? If so, what makes those parts dense?

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DoctorWho984 t1_izentek wrote

Molecular clouds are thought to be made from overdensities, or filamentary structures, in the interstellar medium where lots of atomic hydrogen is collected. We don't know precisely how these filamentary structures themselves are created, but the leading theory includes contributions from:

  1. Spiral density waves (the same waves that form galactic spiral arms) and other global galactic gravitational instabilities

  2. "Supershells", basically the compression of the ISM by stellar winds, supernovae, and other ionizing radiation.

  3. Turbulent compression, the stochastic movement of the ISM

We don't know the relative importance of each of these processes, but that's the general idea.

[Big ol' review of Star Formation for the brave of heart] (https://arxiv.org/abs/0707.3514)

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slcdmw01 t1_izeuhe7 wrote

Thanks, DoctorWho984! As an amateur astronomer, I am fond of observing open clusters. I haven't found comprehensive answers to this question before -- I learned a lot. And thanks for the Star Formation link.

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