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Astronomers have theorized about an invisible star made up of theoretic particles in the past, but did we recently detect the gravitational waves of two of them colliding? Plus, extraterrestrial rocks from a decades-old mission keep yielding new discoveries!

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When you look into the night sky, you can see a whole galaxy of stars shining back at you. But what if there were other stars you couldn’t see, not because they were too faint or far away, but because they were invisible?

If such a thing exists, it might be a boson star — something theorized by astronomers, but not actually detected yet. Except, maybe one was? At least, that’s the argument in a paper published last week in Physical Review Letters.

If true—and that is a big if—it would change the search for dark matter and alter our understanding of physics. So, no pressure. The paper offers a new interpretation of a gravitational wave announced last fall.

Gravitational waves are ripples in spacetime that result from the universe’s most violent events. Like, in this case, the original researchers wrote that the wave seemed to have come from the merger of two black holes. But this new paper proposes something different.

See, there was something funny about the two supposed black holes in this collision: One was a good bit larger than physics suggests should be possible if it came from a collapsed star. Now, there are possible explanations for that. Like, maybe this black hole had already been through a collision.

But there’s room here for other ideas, too. And that is where boson stars come in. So, traditional stars are made of things like protons, neutrons, and electrons.

These particles are all lumped into a group called fermions, which is one of the two main categories of particles according to particle physics. The other category is bosons, and you may have heard of two of them. One is the Higgs boson, which is involved in giving matter its mass.

And the other is the photon, the particle of light. But it’s another boson that physicists are talking about here. It’s called the axion.

And some researchers think a bunch of them could create an object called a boson star. These stars wouldn’t interact directly with light, so they’d be invisible to us, making them kind of like black holes. But unlike black holes, they wouldn’t have an event horizon: a point past which nothing could escape their gravity.

So they’d be fascinating objects to study. There’s just one problem: Axions themselves are only theoretical. Let alone whole, star-like objects made of them.

So why investigate something with so many “ifs” and “maybes?” Well, some researchers think axions might be what makes up at least some of dark matter. Scientists aren’t sure what dark matter is, but they know it’s there because of its gravitational pull. In fact, calculations suggest there’s around five times more dark matter than the normal stuff, so understanding and identifying it would be huge.

And if it turns out axions are real and can also make boson stars — well, that’s something we’d want to know about, too. And that brings us back to this new paper. In it, the authors simulated the collision of two boson stars, in an attempt to figure out if that could have caused the gravitational wave detected last fall.

An event like this would be a lot less powerful than two black holes smashing together, but if it were a lot closer to Earth, the team found that it would make a gravitational wave really similar to what was observed. In fact, the wave from colliding boson stars fit the data just as well as two merging black holes. Which is kind of intriguing!

I mean, it’s a long way from proof of our first boson star, but it is a fascinating start. Now, while scientists keep investigating that, there’s plenty of other news — like the latest results from NASA’s Apollo missions to the Moon. Yeah, the missions might be a half a century old, but the rocks brought back by the astronauts are still a vital resource.

Like, in a paper published last week in Science Advances, a team found that the chemical signatures of key events in lunar history are still embedded in its rocks. In their study, the team looked at rocks returned by Apollos 15 and 17. Trapped inside the rocks are bits of magma called melt inclusions.

And they’re useful because they seal in gasses from the magma that otherwise would have leaked out millions or billions of years ago. In this case, the team specifically looked at two variations of sulfur in the magma: sulfur-32 and sulfur-34. After some measuring, they found that the inclusions had a range of sulfur compositions — showing that the rocks formed in different chemical environments.

By matching the sulfur data with known geologic processes, the researchers were able to figure out where they formed. For instance, when the Moon’s iron core formed and started pulling away from the material around it, it likely took more sulfur-34 than sulfur-32. So, samples with excess sulfur-32 probably formed deep inside the Moon during its very early history.

On the other hand, rocks with more sulfur-34 might have been on the surface as the Moon’s magma ocean cooled around 100 million years after its formation. As the ocean cooled, the heavier sulfur-34 would have been a bit more likely to condense out into the rock, leaving it over represented in the final sample. The researchers didn’t talk about this in their paper, but now that we have a sense of where these rocks came from, it seems like we should be able to learn more about what the Moon was like billions of years ago.

So, in essence, these rocks are time machines, revealing places and moments in history we will never be able to directly observe. And that’s kind of the core of astronomy. Whether it’s searching for stars we can never see using ripples in space, or studying rocks to probe places on the Moon we can never go, scientists have to get clever to understand the universe.

Thanks for watching this episode of SciShow Space News! If you want to learn more about the Apollo program, we did a whole documentary about it last summer. We explore the very big question “Was the Apollo program a bad idea?” And you can watch that after this. {♫Outro♫}.