I love it when I read a journal paper and learn something. 

Of course, I always learn something, but in general it’s something specific to the research. The scientists look to answer a question and then find support or arguments against their idea, so you learn what they concluded.

But in this case it’s more of a general concept I had no idea about. Even better, it’s really cool.

I’ve written many times about stellar streams on The Old Blog™; these are long, thin noodley collections of stars stretching along thousands of light-years, all moving together with similar orbits around the galaxy. They’re the remnants of globular clusters or even dwarf galaxies that have been torn apart by the Milky Way. The gravity of our big galaxy yanks stars out of the more compact object, eventually pulling it apart like taffy. What’s left is a stream of stars orbiting the galaxy.

The stars can be identified by their similar orbits (as seen by the Gaia spacecraft, which can measure the very tiny motions of stars in the sky as they move around the galaxy), as well as other things like their ages and composition.

The galaxy is riddled with such streams; a team of astronomers recently found a dozen streams within a small volume of space centered on the Sun and roughly 800 light-years on a side; compare that to the size of the whole galaxy, which is 120,000 light-years across!

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Another nearby stream is OCSN-49, discovered in 2023. It has 257 member stars that lie on an arc about 700 light-years from Earth. That arc is long; it is an incredible 40° across on the sky: when one end of it is halfway up the sky the other end is just barely rising above the horizon! In physical terms, that means the arc is at least 500 light-years long.

A team of astronomers examined four of its stars, sampled across the arc, taking spectra of them — breaking up their incoming light into wavelengths, which can yield information on things like their elemental composition, rotation, and more [link to journal paper]. They found the stars all had roughly the same age, about 400 – 600 million years. Interestingly, though, they used the motions of the stars to run the clock backwards in time, and found that all the stars would have been all together in one spot in space about 83 million years ago.

That means that these stars were all part of a cluster (called an open cluster) for a few hundred million years, and then something happened to pretty rapidly tear the cluster apart. Simply traveling through the galaxy will do that, but it takes a long time, but it’s clear whatever happened here was fairly sudden.

The scientists posit that, 80+ million years ago, the cluster encountered a giant molecular cloud. This is a massive and dense cloud of cold hydrogen and dust. There are a lot of them in the galaxy, like the immense Orion Molecular Cloud, which is hundreds of light-years across and contains hundreds of thousands of times the Sun’s mass.

They’re so big that any star orbiting the galaxy is likely to hit one sooner or later, and it looks like that’s what happened to OCSN-49. The gravity of the cloud is strong, and as the cluster approached it, any star on the outskirts would eventually feel more gravity toward the cloud than toward the center of the cluster, and would get stripped away. This is called the tidal force, and it has profound effects on objects all over the galaxy (including Earth and the Moon). Over time, the entire cluster was pulled apart, shredded by the far larger and more massive cloud it had the misfortune to run into.

And that’s the thing I learned: a molecular cloud can rip apart a star cluster! That’s amazing. I see how it can happen, and the math isn’t that complex, so I understand the physics of it. It just never occurred to me before. It’s just a bit odd to think something so large and diffuse can destroy something much more compact and dense. It feels like seeing a car getting torn apart when it drives into a fog. But that analogy fails due to scale; a molecular cloud is an immense object with a lot of gravity. Also star clusters aren’t as cohesive as a lump of metal, so cars are pretty solid comparatively. But that’s how my analog-making mind works sometimes, and it can steer me wrong me as it did here.

By the way, the Sun was almost certainly born in an open cluster, perhaps one that was similar to OCSN-49, but now wanders the galaxy on its own. Did our stellar nursery dissolve over hundreds of millions of years, or did it run headlong into a vast and dark cloud that rapidly tore it to bits? I’m not sure it’s possible to know. But other astronomers have calculated that the Sun will pass through such a cloud, on average, every 625 million years, so there have been plenty of chances.

And that’s also something I didn’t know. Bonus learning. I’m really glad I read this paper!

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