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When you picture a galaxy, you might  just imagine a disk of rotating stars, but there’s more to it than that. Galaxies are also enveloped  in huge spheres of gas, ancient stars, and also dark matter, a mysterious, invisible something we can only detect  because of its gravitational pull.

And it turns out, that dark matter has  some weird effects on how galaxies move. Like, according to a paper  published online this spring in the. Monthly Notices of the Royal Astronomical Society, there’s a cluster of stars in the center  of our galaxy that has slowed down by at least 24% since the Milky Way formed.

And dark matter is to blame. This cluster is called the  Milky Way’s galactic bar. It’s a group of billions of stars  running through the galaxy’s center, and is the reason the Milky Way is  officially called a barred spiral galaxy.

In this study, the researchers were  specifically looking at a group of stars called the Hercules stream that have  a special relationship with the bar:. If the bar slows down, the  Hercules stream slows down as well, and the stars in the stream drift  away from the galaxy’s center. This happens because of two reasons:.

One is that these stars are in  corotation resonance with the bar, meaning that the speed they orbit  matches the speed the bar spins. So, their motions are basically linked. As for why the Hercules stream  would drift away when it slows down, that’s because of angular momentum.

This is how much momentum something has when it’s rotating or moving  in a circle, like in an orbit. And it depends on the object’s mass, velocity, and the radius of the circle it’s traveling on. But the key thing to know here  is that the momentum in a system has to stay the same unless  there’s some external force on it.

So, if an object gains momentum, another object in that system  has to lose the same amount. And if we’re dealing with just one  object, well, if that object speeds up, the radius of its orbit will shrink to compensate. Or, if the object slows down,  its orbit will get bigger.

So, if the galactic bar were to slow down,  so would the Hercules stream, and then, to keep angular momentum constant,  the star’s orbits would get wider. This is also true of a few  other stars farther away that are also in resonance with the bar. Now, the tricky bit is, this drifting  process would happen over billions of years.

So, how do we track a star’s motion  over that kind of time frame? Here, the authors used a totally new method:. They looked at the composition of  the stars in the Hercules stream.

Stars have different compositions  depending on when and where they formed. For instance, compared to the stars that  form far from the center of the galaxy, stars that form closer tend to be much  richer in elements heavier than helium, elements astronomers very broadly call “metals.” When the paper’s authors took  a look at the Hercules stream, they found that its stars are super metallic! They have way more metals in them than  they should, given their current positions.

So, they must have formed closer  to the center of the galaxy and moved outward as something slowed them down. The stars were also clearly arranged  from high to low metallicity, with the stars with the most metals  in the innermost part of the stream. And that would only happen from a slowdown from some central point of resonance: the galactic bar.

Working off those data, the authors  were able to get a hard number on just how much the galactic bar  has slowed down since formation, something astronomers had never done. And they concluded it’s slowed down at  least 24% over the galaxy’s lifetime. Now, the kicker is… If the  galactic bar is slowing down, its angular momentum would need  to be transferred somewhere to keep the system balanced.

But there was no obvious culprit, like  the bar dramatically changing shape. And that’s where dark matter comes in! If the bar’s momentum is being  transferred to dark matter, and the dark matter is moving differently to balance out the system, the physics work out!

This study teaches us more about  how dark matter shapes galaxies, and gives us a new method  for investigating it further. This is a big deal, because observing  the unobservable is pretty difficult! We really only had one way to  look at dark matter before, which was to look at the gravitational  effects it had on regular matter, things like how it held galaxies  together, or warped space.

But now, we can study how dark matter  influences objects’ motions, too, which will let us better quantify  how much dark matter exists, narrow in on what it’s made out of, and  understand the role it plays in the universe. And something else that helped us  understand things in the universe, like our Moon, was the lander Surveyor 1. The underdog spacecraft that  made the Apollo mission possible!

And if you want to have a smaller  version of it, well you’re in luck, because it’s our SciShow Space  Pin of the Month for June, and it’s available all month at DFTBA.com/SciShow. It’s only available in June, so you might  want to make your order soon because, in July, we’ll have a whole new pin for you. [♪ OUTRO].