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The spiral of the Milky Way is not as smooth as we once thought because an arm not so far from home appears to be broken! And we may have discovered the answer to why a local asteroid puts on the appearance of a comet.

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[♪ INTRO].

Since ancient times, humans have looked at the night sky and found a giant band of stars and dust stretching across the heavens: the Milky Way, our galaxy. We eventually learned that the Milky Way is one galaxy of many.

But in some ways, it’s among the harder ones to study because we’re sitting inside it. Sometimes, though, scientists find something that sheds light on what the Milky Way is like and why. For instance, last month in the journal Astronomy and Astrophysics, astronomers revealed a new feature in our galactic home that could help reveal why it’s shaped the way it is.

Now, because we’re inside the Milky Way, we don’t actually know what the entire galaxy looks like, at least, not exactly. Space is so big that sending a spacecraft far enough to snap a photo of the whole thing would take a hundred thousand years… minimum. Still, astronomers can make good estimates.

By measuring exactly where each star is in the night sky and then pairing that with how far the star is from Earth, they’ve made approximate maps of the galaxy. And these maps have revealed that the Milky Way has massive, spiral-shaped arms, a lot like some other galaxies we’ve seen. Today, scientists are cataloging even more stars, gathering more data to refine their picture of the galaxy’s structure.

And that is where this new paper comes in. This team took data collected by NASA’s now-retired Spitzer Space Telescope, along with the European Space Agency’s Gaia mission. And they calculated the locations of newborn stars inside gas-filled stellar nurseries in the Sagittarius Arm.

That’s one of the arms next to ours, one closer to the center of the Milky Way. The team chose to study young stars because they serve as a link between the structure of stars, and the structure of the dust they form from. Young stars are also thought to form in alignment with the arm around them.

Except, about 4000 light-years from our solar system, the team found some serious outliers. Instead of following the spiral, a collection of 25 stellar nurseries is sticking out of the Sagittarius Arm like a broken limb, one more than 3000 light-years long. In the past, similar features had been seen in distant spiral galaxies, but this is the first time we’ve found a major structure like this in our galaxy, because, you know, being stuck inside the thing you’re trying to look at makes it harder to see the details.

This structure hints at the Milky Way’s true appearance, that it isn’t as smooth as some researchers might have thought, and that it shares more features with galaxies far, far away. Also, now that they’re studying these stars that formed at roughly the same time and in roughly the same place, astronomers can go even deeper:. They could start to figure out what properties the galaxy had to have to make so many stars break off like this, something that could ultimately tell us more about how galaxies evolve.

But those will be questions for another study. Meanwhile, astronomers may have figured out why a local asteroid has been acting like a comet. Discovered in 1983, the asteroid Phaethon is a little special among space rocks.

For one, it gets much closer to the Sun than many other asteroids. Its orbit takes it closer to our star than Mercury’s gets, during which time temperatures on the surface climb to over 750 degrees Celsius. This means that if Phaethon started with any volatile molecules, like water or carbon dioxide, they were lost to space long ago, making this asteroid a dead, ice-less rock.

Phaethon is also the only asteroid that provides one of Earth’s annual meteor showers: the Geminids, in December. The others are created by debris shed by comets. And also like a comet, Phaethon gets brighter as it gets closer to the Sun.

For comets, that’s because their ice vaporizes, kicking up dust from the surface that scatters sunlight. But Phaethon doesn’t have any ice… so what’s going on? Well, one team of astronomers thinks the Sun might be vaporizing something else: sodium.

Their study was published last week in the Planetary Science Journal. In it, they performed tests on meteorite samples, not pieces from Phaethon, but samples that were decently similar and that they had a large enough quantity of. In the tests, they heated up small pieces of rock to the scorching temperatures Phaethon can experience.

And the results showed sodium gas escaping, while everything else stayed in place. On Phaethon, that gas would scatter sunlight, explaining why the asteroid appears to get brighter. But it would also explain the Geminid meteor shower, specifically, why these meteors have less sodium than others:.

If Phaethon has been losing sodium every time it approaches the Sun, any sodium on the surface would have depleted a long time ago. The rocks beneath the surface, though, well, if they’re hot enough, those rocks could still release sodium up through the cracks in the asteroid’s crust with just enough force to push low-sodium rocks into space. Then, eventually, those rocks would fall to Earth as meteors.

Now, Phaethon is about five kilometers across, its gravity isn’t very strong. So, it wouldn’t need big sodium bombs or anything to make this happen:. One of the scientists said it would be more like a steady fizz.

That said, the samples in this study weren’t a true match for Phaethon’s composition, so to confirm these results, we’ll have to wait for JAXA’s Destiny+ mission to Phaethon. It is scheduled to launch in 2024. These results add to the discussions scientists are having about the fuzzy boundary between asteroids and comets, and how nature defies the strict categories humans love to sort things into.

But one thing I definitely know fits into a very specific category: it’s our pin of the month. And this month’s pin is gonna bring the sunshine right into your pocket by showcasing the foldable panels of the Discovery shuttle. We’ve made this beautiful little pin, a tiny shuttle with its lengthy solar panels under the blazing sun, and it’s available all month at

But it’s only available in August, so make sure you order yours soon. Because in September, we’re gonna have a whole new pin for you. [♪ OUTRO].