The glowing hydrogen clouds in other galaxies are like a sponge: the gas itself takes up only a small fraction of the volume, and the rest is nearly empty space. For a long time, astronomers could only guess how 'holey' these nebulae are. Now a team of scientists has proposed a way to measure it.

They took images of two nearby spiral galaxies — NGC 2403 and NGC 628—and using computer vision carefully picked out hundreds of glowing regions. For each region, they measured brightness and size, then calculated two densities. The first is the average over the whole volume, as if all the gas were spread out evenly. The second is the density within individual clumps where the gas actually collects. The difference between them shows what fraction of the space is occupied by matter.

It turned out that in the clumps the density does not exceed 300 particles per cubic centimeter: this is billions of times more rarefied than air at the Earth's surface. And the average density over the whole cloud is another 10–100 times lower. That means gas fills only from 0.01% to 10% of the nebula's volume. The rest is emptiness.

Moreover, both galaxies behave the same way: the larger the nebula, the lower its average density, but only up to a size of about 50 light-years. Beyond that, this relationship disappears. The scientists also noticed that the average density weakly depends on the distance from the galaxy's center, and the density in clumps does not depend on it at all.

When comparing these data with measurements of other galaxies, interesting patterns emerge. The average density of nebulae is related to their typical size, to the fraction of large regions in the galaxy, and to how intensely stars are born in the galaxy. So far these are only hints, but if they are confirmed on a larger number of objects, they will help understand how massive star clusters form and explain why in very distant galaxies nebulae appear denser than those near us.