[♪ INTRO]
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]