<p>This quantity is referred to as <em>star formation efficiency</em> (SFE) by astronomers who study star formation and galaxy evolution.   Estimates can vary but typically are around a few percent.  <a href="https://iopscience.iop.org/article/10.1086/316801/fulltext/" rel="nofollow noreferrer">In Sec. 4.1 of this paper</a> Inoue et al. review  some estimates from the literature.  Those numbers are for the central regions of spiral galaxies, and the rate <a href="https://iopscience.iop.org/article/10.1088/0004-6256/136/6/2782/meta" rel="nofollow noreferrer">may be lower in the outer regions</a> where the density is lower.</p>
<p>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.</p>
<p>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 <em>per year</em>, or alternatively, the inverse of that number gives you a timescale in years to deplete the gas entirely.</p>

CC BY-SA 4.0
2020-07-21T12:48:29.400Z
2020-07-21T12:48:29.400Z
Eric Jensen
<p><a href="https://en.wikipedia.org/wiki/Molecular_cloud#Star_formation" rel="nofollow noreferrer">Stars form within molecular clouds</a>. 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%?</p>

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

CC BY-SA 4.0
2019-11-21T16:35:28.747Z
Which question are you asking: the number of stars, or the portion of the cloud which gets collapsed?
Carl Witthoft
CC BY-SA 4.0
2019-11-22T01:42:49.647Z
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.
uhoh
CC BY-SA 4.0
2019-11-23T02:45:58.057Z
@uhoh I fixed your "molecular could" in title.
Wayfaring Stranger
CC BY-SA 4.0
2019-11-23T02:54:29.973Z
@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.
uhoh
<star-formation><interstellar-medium>
How much of a molecular cloud can end up as "starstuff"?
usernumber
# How much of a molecular cloud can end up as "starstuff"?

(Asked by: username_0 on 2019-11-21)

[Stars form within molecular clouds](https://en.wikipedia.org/wiki/Molecular_cloud#Star_formation). 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?

**Tags:** star-formation, interstellar-medium

> **Comments:**
> **username_1**: Which question are you asking: the number of stars, or the portion of the cloud which gets collapsed?  
> **username_2**: 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.  
> **username_3**: @username_2 I fixed your "molecular could" in title.  
> **username_2**: @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.  

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## 1 answers

### Answer by username_4 on 2020-07-21. Score: 3 | Accepted answer

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](https://iopscience.iop.org/article/10.1086/316801/fulltext/) 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](https://iopscience.iop.org/article/10.1088/0004-6256/136/6/2782/meta) 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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