If you have ever pushed your finger against the hole of a bicycle pump and felt the air grow warm as you compressed it, you already understand the physics at the heart of a new discovery about our own Galaxy. Because it turns out the Milky Way has a hot side and a cool side and the reason why comes down to exactly the same principle. Astronomers have known for some time that our Galaxy is surrounded by an enormous halo of hot gas. This vast, invisible sphere extends far beyond the familiar disc of stars we think of as the Milky Way, and is around two million degrees, several hundred times hotter than the visible surface of the Sun. What puzzled scientists was why one half of this halo appears to be warmer than the other. Data from the eROSITA X-ray observatory, released in 2024, showed that the southern half of the halo runs up to twelve percent hotter than the north. Nobody could quite explain why.…