A gas cloud the size of a small galaxy is falling into the Milky Way at hundreds of kilometers per second. Such clouds are called high-velocity clouds, and until now no stars had been found in them — only gas. Now, in one of them, named Complex H, astronomers have spotted a pair of young star clusters.

The clusters are 11.2 million years old — by stellar standards, that's infancy. Their metallicity (astronomers call all elements heavier than helium "metals") is very low: about 20 times lower than the Sun's. This suggests the stars were born from pristine gas that had not yet been enriched with heavy elements by previous generations of stars. The clusters themselves became an anchor: their light allowed astronomers to measure the cloud's distance precisely—13.8 kiloparsecs, or about 45,000 light-years.

According to calculations, the star clusters were born when two parts of the cloud collided with each other. The shock compressed the gas, triggering star formation. The same may be happening in other high-velocity clouds: we just hadn't noticed their stars before, because young stars quickly fly away (in less than 20 million years). And the carbon monoxide typically used to trace such clouds is too faint here to detect: metals are scarce, and the gas has already dispersed.

Now Complex H has become a living laboratory. By observing its stars, astronomers can understand what happens to gas before it merges into the Galactic disk and becomes raw material for new suns.