Inside cold clouds of gas and dust where new stars ignite, astronomers have spotted unexpectedly delicate structures. The ALMA telescope, working in the millimeter range, has revealed more than 323 ultra-thin dust filaments within 121 massive clumps. These clumps are scattered across our Galaxy, from the center to the outskirts, at distances ranging from 0.5 to 12.7 kiloparsecs from the Milky Way's center.

A typical filament is only about 0.01 parsecs wide (roughly 2000 astronomical units, where one astronomical unit is the distance from Earth to the Sun). That's ten times narrower than the dust fibers previously found by the Herschel space telescope in nearby clouds. Yet the density of matter in these new filaments is astonishingly high: from $10^{23}$ to $10^{24}$ hydrogen molecules per square centimeter — orders of magnitude greater than in the broader fibers.

The discovery fills in the picture of star formation: first, broad fibers are found in clouds; within them, thinner fibers with coherent velocity; and now, ultra-thin filaments. It seems that as gas collapses, it fragments into ever finer structures, and it is in these dense threads that stars ultimately ignite.