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Stars form within molecular clouds. These clouds can be up to 6 millions solar masses. When the cloud collapses into stars, is it possible to know a rough figure for how much of this material actually turns into stars? 1%? 99%?

Do larger clouds tend to be more mass-efficient in star formation than smaller ones?

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    $\begingroup$ Which question are you asking: the number of stars, or the portion of the cloud which gets collapsed? $\endgroup$ – Carl Witthoft Nov 21 '19 at 16:35
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    $\begingroup$ I edited your title to match the body of your question. There are two close votes so I thought an edit wold be more expedient than the close-reopen cycle which usually takes a few days and the effort of about ten users. Feel free to edit further. $\endgroup$ – uhoh Nov 22 '19 at 1:42
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    $\begingroup$ @uhoh I fixed your "molecular could" in title. $\endgroup$ – Wayfaring Stranger Nov 23 '19 at 2:45
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    $\begingroup$ @WayfaringStranger I see that, thanks! I'm going to pretend that I'd meant to write 'How much of a molecular cloud could end up as “starstuff”?' and then deleted one of them by accident. $\endgroup$ – uhoh Nov 23 '19 at 2:54
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This quantity is referred to as star formation efficiency (SFE) by astronomers who study star formation and galaxy evolution. Estimates can vary but typically are around a few percent. In Sec. 4.1 of this paper Inoue et al. review some estimates from the literature. Those numbers are for the central regions of spiral galaxies, and the rate may be lower in the outer regions where the density is lower.

Note that even if the rate is a few percent in a given cloud for a given episode of star formation, the rest of that gas mass will be dispersed into the interstellar medium and eventually find its way back into another cloud to form stars later - star formation is ongoing in spiral galaxies.

Note that you will often see SFE expressed as star formation rate (solar masses per year converted to stars) divided by total gas mass. That has units of (mass/yr) / mass, which reduces to units of 1/yr. You can think of that as the fraction of gas that forms into stars per year, or alternatively, the inverse of that number gives you a timescale in years to deplete the gas entirely.

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