Star formation is vital to galaxy evolution and the production of planets, both inhabited and uninhabited. And yet, this messy process is still not fully understood by astronomers. But one clue has been unearthed by astronomers studying the most nearby gas clouds in the galaxy.

There is a fairly long history of trying to determine how efficient gas clouds are at making stars. It begins with Maarten Schmidt who hypothesized that star formation rate (SFR) was related to the surface density of atomic hydrogen gas in the cloud.

This relation was refined with time, including by Robert Kennicutt in 1998, leading to the Schmidt-Kennicutt relation that is familiar to most astronomers. However, it has been recently argued that since molecular gas is involved in star formation, that should define the relation.

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As astronomers looked into this, they started discovering that some of the most massive and spatially extended clouds nearby to us were not forming stars at the larger rates of some less massive, smaller clouds. So what really does drive a molecular cloud to start producing stars?

Charles Lada, Marco Lombardi, and João Alves set out to survey 11 local molecular clouds within 1630 light years of the sun. They looked at the most recently formed and forming stars in the clouds from previously published data. They also accessed public data from the Spitzer Space Telescope, making this yet another astronomy project where new gems were found in old data.

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The optical and infrared data also measured the extinction in these molecular gas clouds. This means that they measured how much starlight was blocked or reduced by the interstellar “fog,” essentially telling them the surface density.

They discovered that star formation is pretty well non-existent until the surface density reaches a certain threshold in parts of the cloud. Above that threshold, the SFR increases linearly with that surface density. So, by not collapsing into denser regions, these massive nebulae, like the 100,000 solar mass California Nebula shown above, don’t necessarily start making baby stars and planets.

Remember, “dense” in space is not “dense” as we are used to. Assuming I did did all my unit conversions correctly, the threshold surface density is approximately 2 grams per centimeter-squared. For comparison, a typical piece of paper is 80 grams per centimeter-squared. Pretty sparse, but enough to start the seeds of light and life in the galaxy.

Image: An optical image of the California Nebula, which really does look like California to me! Credit: Caltech, Palomar Observatory, Digitized Sky Survey

This research is published by the Astrophysical Journal, and the preprint is available on