Scientists have discovered a surprising reason why one side of our galaxy is hotter than the other — and the explanation is something you recognize from everyday life.
A new study shows that the Milky Way is heated in the same way as a car engine, where gas is compressed like a piston and then heated as a result of the compression.
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The discovery helps explain a mystery that was first discovered in 2024 by the eROSITA space observatory. It revealed that the southern side of the Milky Way’s halo — the huge cloud of hot gas that surrounds the galaxy — is about 12 percent hotter than the northern side. Until now, scientists have not known why this temperature difference exists.
A close neighbour is pulling in our galaxy
The answer seems to lie in the influence of a nearby neighboring galaxy, called the Large Magellanic Cloud. This smaller galaxy orbits under the Milky Way and has enough gravity to pull on our galaxy as it passes.
The researchers found that this pull causes the Milky Way to move weakly towards the neighbouring galaxy. When 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. That explains why the southern half of the halo is hotter.
Advanced computer simulations reveal the mechanism
The idea comes from detailed computer simulations carried out by researchers at the University of Groningen and their international collaborators. These simulations modeled how the structure of the Milky Way changes over time under the influence of gravity from nearby galaxies.
The simulations showed that the galaxy’s disc is currently moving at about 40 kilometres per second towards the Large Magellanic Cloud — enough to create a pressure that heats the surrounding gas by as much as 13 to 20 percent.
A relatively new effect
The study, published in the Monthly Notices of the Royal Astronomical Society, also suggests that this effect has occurred relatively recently, on a cosmic scale. The temperature difference probably occurred during the last 100 million years.
The Milky Way’s halo itself is huge and extremely hot, with temperatures of around two million degrees Kelvin. Although the gas is very thin, it contains enormous amounts of matter — possibly more than the visible part of the galaxy where stars like our own Sun are located.
Understanding how this halo behaves is important because it serves as a source of material for future star formation.
Also explains other strange features
The simulations also help to explain other strange features of the galaxy. For example, astronomers have observed several “high-velocity clouds” — cooler regions of gas moving at unusually high 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.
“We weren’t even looking for this”
The interesting thing is that the researchers were not initially looking for this effect at all. Their previous simulations already contained this temperature difference before it was observed in real-world data – giving the researchers greater confidence in their results.
The discovery shows how giant forces can shape galaxies in ways that are both complex and surprisingly recognizable. Even something as huge as the Milky Way can behave a bit like an engine – where movement, pressure and heat work together to change its structure over time.
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