Scientists have discovered a surprising reason why one side of our galaxy is warmer than the other, and the explanation is something you might recognise from everyday life.
A new study shows that the Milky Way is being heated in a way similar to how a car engine works, with gas being compressed like a piston and warming up as a result.
The finding helps explain a mystery first noticed in 2024 by the space observatory eROSITA.
It revealed that the southern side of the Milky Way’s halo, the vast cloud of hot gas surrounding the galaxy, is about 12 percent hotter than the northern side. Until now, scientists did not know why this temperature difference existed.
The answer appears to lie in the influence of a nearby companion galaxy called the Large Magellanic Cloud.
This smaller galaxy orbits below the Milky Way and has enough gravity to pull on our galaxy as it passes by. Researchers found that this pull causes the Milky Way itself to move slightly toward the companion galaxy.
As the Milky Way shifts, it pushes into the gas in its own halo. This movement compresses the gas on the southern side, much like a piston compresses air inside an engine.
When gas is compressed, it heats up, which explains why the southern half of the halo is warmer.
This idea comes from detailed computer simulations carried out by scientists at the University of Groningen and their international collaborators. These simulations modeled how the Milky Way’s structure changes over time under the gravitational influence of nearby galaxies.
They showed that the galaxy’s disc is currently moving at about 40 kilometres per second toward the Large Magellanic Cloud, creating enough pressure to heat the surrounding gas by as much as 13 to 20 percent.
The study, published in Monthly Notices of the Royal Astronomical Society, also suggests that this effect developed relatively recently, in cosmic terms. The temperature difference likely formed within the last 100 million years.
The Milky Way’s halo itself is enormous and extremely hot, with temperatures around 2 million degrees Kelvin. Even though the gas is very thin, it contains a huge amount of matter, possibly more than the visible part of the galaxy where stars like our Sun are located. Understanding how this halo behaves is important because it acts as a source of material for future star formation.
The simulations also help explain other odd features of the galaxy. For example, astronomers have noticed more “high-velocity clouds,” which are cooler gas regions moving at unusual speeds, on the northern side of the Milky Way. The study suggests that the lower pressure there makes it easier for these clouds to form and survive.
Interestingly, the researchers were not originally looking for this effect. Their earlier simulations already contained this temperature difference before it was observed in real data, which gives scientists greater confidence in their results.
This discovery shows how large-scale forces can shape galaxies in ways that are both complex and surprisingly familiar. Even something as vast as the Milky Way can behave a bit like an engine, with motion, pressure, and heat working together to change its structure over time.
