There’s a wall of gas out there twice the length of the entire Milky Way galaxy and five times hotter than the core of the Sun.

Thankfully, it’s not headed right for us. It’s about two hundred and forty million light-years away, in the Perseus galaxy cluster.

For a while, astronomers didn’t know what this wall of gas was doing there. But in a new study published in the Monthly Notices of the Royal Astronomical Society, a group of researchers discovered that this wall-looking thing is actually a gigantic wave. And it started rippling because a smaller galaxy cluster ripped through Perseus two and a half billion years ago.

Galaxy clusters are some of the largest structures in the universe, often with more than a thousand galaxies in them. But most of a cluster’s mass isn’t in the galaxies themselves — it’s in the light-years-big clouds of superheated gas sitting between the galaxies. The Perseus cluster has relatively cool gas in the center surrounded by swirling masses of much warmer gas.

Back in 2006, astronomers studying Perseus’s gas clouds with the Chandra X-Ray space telescope noticed what looked like a wall of gas that was much hotter and denser than anything nearby. It’s around two hundred thousand light-years long, with temperatures as high as eighty million degrees Celsius. At first, researchers thought it might be caused by a black hole swallowing up the gas behind the wall.

But other telescopes didn’t see any signs of black hole activity. So then they checked to see if the extra heat and density could be from the gas cloud sloshing around naturally. And yes, sloshing is the technical term for it.

But that didn’t really fit the observations, either. Now it seems like we finally have an answer. In this new paper, a team of astronomers used computer simulations to show that what looked like a wall is actually more of a /wave/, caused by the gravitational tug of a smaller galaxy cluster passing near Perseus’s center a couple billion years ago.

The simulations showed that the smaller cluster’s gravity would’ve started the colder and warmer gases swirling together. After about 2.3 billion years, the swirling gas started sort of tripping over itself, heating and bunching up in the process. And whenever two fluids with different densities or temperatures or velocities rub against each other, it makes waves — whether it’s two types of gas swirling together or wind over the ocean.

So this wall we’ve known about for a decade is actually a type of wave. The team also thinks that at least two other clusters, Centaurus and Abell 1795, have waves that formed in the same way, but they’re not as easy to see as the one in Perseus. Matching simulations with observations like these helps astronomers figure out which factors are most important when gigantic structures like galaxy clusters interact with each other.

For example, they know from the simulations that if Perseus’s magnetic fields were a lot stronger, the wave would have never formed. And now that we know what to look for, it’ll be easier to find evidence of galaxy clusters ramming into each other in other places in the universe. Anyone else up for some extreme galactic surfing?

Or maybe you’re more interested in visiting a Moon base. Because yeah, that’s a thing the European Space Agency wants to build. And now they have a 3D printer that can build bricks out of just Moon dust.

The hard part about building on the Moon -- well, okay, there are /lots/ of hard parts. There’s a ton of solar radiation, there’s practically no atmosphere, Moon dust — aka regolith — can damage your lungs... The list is pretty long.

But /one/ reason there’s still no Moon base is that launching building materials like bricks all the way to the Moon takes a /lot/ of expensive rocket power. So sending astronauts up with bags of bricks and mortar isn’t exactly feasible. Instead, the ESA wants to invent 3D printers that can use the regolith itself as a building material.

In theory, basically all you’d need to do is send up a bunch of those printers and let them turn lunar soil into all the bricks you need. The ESA’s newest printer is at the DLR German Aerospace Center in Cologne, and last week, it printed its first bricks out of volcanic soil that’s processed in a way that makes it a good substitute for the Moon’s regolith. If you think the bricks look kinda like some soil smushed together, well, that’s pretty much what they are.

The brick printer focuses sunlight to melt layers of regolith into bricks that stay solid on their /own/, without the need for any sort of extra material from Earth. The process isn’t totally practical yet: the printers make bricks a 0.1-millimeter-thick layer at a time, so it can take as much as five hours to build a hand-sized brick. They still have to test the strength of the bricks, and a related project called RegoLight is still working on making the process better for the Moon’s harsh conditions.

But this is early proof that we can build a Moon base out of the Moon itself. So maybe someday the ESA will have a team of robotic regolith masons ready to build humanity’s first extraterrestrial outpost. Thanks for watching this episode of SciShow Space News, and thanks especially to our patrons on Patreon who help make this show possible.

If you want to help us keep making episodes like this, you can go to patreon.com/scishow. And don’t forget to go to youtube.com/scishowspace and subscribe!