When a massive star explodes, its remnants fly apart like fragments of fireworks. But how do we know how far away such an explosion occurred? Astronomers have found a way: they look at the gas clouds that these remnants collide with.

The idea is simple. Supernova remnants are often surrounded by clouds of cold molecular gas. These clouds move at a certain speed, which can be measured from the emission of carbon monoxide (CO). If a supernova remnant is interacting with such a cloud, then the distance to the cloud is the distance to the remnant. To find the distance to the cloud, scientists use dust extinction maps: as we look farther away, dust dims the stars, and at certain distances, you see 'steps'—sharp jumps in extinction. If such a step coincides with the position of the cloud, the distance is determined.

Using this method, they measured distances to four supernova remnants: G93.7−0.2, G109.1−1.0, G156.2+5.7, and G166.0+4.3. The values range from 0.6 to 3.4 kiloparsecs (one kiloparsec is about 3260 light-years). For comparison, the nearest star to us, Proxima Centauri, is about 1.3 parsecs away, so these remnants are truly far. The accuracy of the new method is 4 to 8 percent, which is noticeably better than previous estimates.

This approach helps us better understand how supernova remnants interact with surrounding gas and how they influence the birth of new stars. After all, shock waves from explosions compress clouds and can trigger their collapse, leading to the formation of new stars.