Scientists using the galactic equivalent of an x-ray machine have shed new light on the recipe for cosmic fertility.
Gather some space dust, add some gas, and stir until 5000 hydrogen molecules squeeze into a cubic centimetre, and voila - a star is born.
The recipe could help astronomers predict how many stars an interstellar gas cloud will produce, says Dr Christoph Federrath from Monash University.
Star creation is "the most important process going on in the galaxy", he says.
But astronomers have had trouble making predictions because, while they theorise in three dimensions, they've had to rely on distant two-dimensional cloud images.
A new technique pioneered by Dr Federrath and his colleagues creates, for the first time, a three-dimensional picture of how clouds work by adding some complex numerical modelling of gas movement.
By comparing their model to clouds known to have recently birthed stars, they came upon the magic recipe for star formation of 5000 hydrogen molecules per cubic centimetre.
When this tipping point is reached somewhere in a cloud, the area begins to collapse under its own gravity. It contracts until rising pressure and temperature eventually trigger nuclear fusion.
Young clouds generally have extremely low densities of about one particle per cubic centimetre, but over time, cosmic turbulence squeezes some areas together.
This happens rarely, Dr Federrath says. Only about one per cent of gas ever forms a star.
But by gauging the brightness of stars that lie on the other side of a galactic cloud and combining the data with the three-dimensional modelling, astronomers will be able to gauge a cloud's density.
The dimmer the star, the denser the cloud, and the more cosmically fertile it will be.
It's the same principle used in X-ray machines, which shoot radiation through bones to form a picture of their health.
The research, published on Friday in the journal Science, applied the new technique to relatively small clouds located about 1500 light years from Earth.
Dr Federrath says the next step is to apply the technique to much bigger, more distant, more fertile clouds.
"We could use it to make predictions for any cloud that we observe," he says.
