What is that spiral arm in the centre of the Milky Way?
I bet you know pretty well we're living on a planet, the Earth,
orbiting a star, the Sun, orbiting around a galactic centre, the Milky Way galactic centre.
Our distance from it is about 8 kiloparsecs, a huuuge distance compared to the one we are
used to, but still a very...small one if you think about the large scale structure of the universe.
For example, the measuring unit for the distance between galaxies
is the Mega Parsec, which corresponds to ten to the sixth parsecs, or
ten to the third kiloparsec. 8 kpc corresponds to about 1/3
the distance from the centre of the Milky Way galaxy to its outer edges,
and the sun it’s located in a smaller spiral arm, between two large arms, called the Orion Arm.
In fact, the milky way is a spiral galaxy, named by its spiral structures that extend from
the centre into the galactic disc. The spiral arms are sites of ongoing star formation and
are brighter than the surrounding disc because of the young, hot OB stars that inhabit them.
A spiral arm orbit is pretty well defined, with some defined orientation,
that is often due to cinematic properties of the stars and gas populating it. In other words,
we can reasonably assume to know the path and orientation of a spiral arm in our galaxy.
A new discovery suggests that one of our galaxy’s spiral arms – located some 4,000
light-years from Earth – is jutting out at a weird angle. They’re calling it a “break”
in one of the Milky Way’s spiral arms. Follow me in this video to get to know
more about this unexpected break.
Imagine being alone in the middle of a forest. You're surrounded by all kinds of trees, and
you have to hurry up because the night is coming and it would be very dangerous
for you to stay there when the Sun will be gone. How could you tell where's a way out?
If there was a friend of yours watching you from a top view, he would have
perfect control of the territory, so you could ask him to guide you out of there.
But, suppose you have no friends. Unless you are very lucky, or unless you are Bear Grills,
you are probably gonna be stuck there. Because you simply can't see the forest for the trees,
and you have no chance for a top view. Well, this is pretty the same situation
that astronomers face when it's time to understand how our galaxy is.
Sometimes you can’t see the galaxy for the stars. Because we’re inside the Milky Way,
and we can’t get a complete view of the forest of stars surrounding us.
This is why is so difficult to understand its properties and features.
And when a team of astronomers start studying in detail some part of our galaxy, it means
that is probably time for new discoveries. That's why astronomers from all over the
world weren’t surprised to hear that we found a break in one of the Milky Way’s spiral arms.
But what is it? Well, this break, which
extends from the Sagittarius Arm of our Milky Way galaxy, has a strange orientation. Of course,
other pieces of spiral arms exist with very peculiar orientations, but this is the first major
structure identified with such a dramatic one. The scientists published their discovery on
July 21, 2021, in the peer-reviewed journal Astronomy and Astrophysics.
Let's try to understand where this break is located.
We could say that the splintered region of the Sagittarius spiral arm extends for some
3,000 light-years at a sharp angle from the main arm and that the tangent to the ‘start
of the Sagittarius’ spiral arm lies at a galactic latitude of about -17 degrees.
But we are not astronomers, and these numbers are not telling us much.
Maybe things are clearer if we talk in terms of constellations.
That region of the sky is the region we’re looking toward when we gaze at the constellations
Sagittarius and Serpens Cauda and the Milky Way that flows behind these stars.
Some of the stars and nebulae located in the spiral arm break
are the most famous in the heavens. They include the Eagle Nebula, which is known
for the Hubble Space Telescope’s Pillars of Creation photo. And it includes the Omega Nebula,
and the Trifid Nebula and Lagoon Nebula. These nebulae are often photographed together.
Here we are: now you get me!
Now that we know what the break is and where is located, let's see how scientists found it.
Of course, they needed a telescope or, better to say, they needed data from a telescope.
That's why they turned their attention to NASA's Spitzer Space Telescope.
Launched in 2003, NASA’s Spitzer Space Telescope was the fourth and final addition
to NASA’s Great Observatory program. Spitzer’s mission was to become NASA’s premier infrared
light observatory, offering astronomers the chance to study the universe in this critical part
of the electromagnetic spectrum of light with unprecedented clarity and sensitivity.
Thak to it, they analyzed a bunch of data regarding young stars that the telescope's
infrared vision could see hiding within these nebulae. They also used data from
ESA’s Gaia mission to measure precise distances to the stars.
Gaia is a very important instrument that will lead astronomers from all over the world to a
better understanding of the place we are living in and, more generally, our entire universe.
It is so precise that some astronomers started using its data before they were made official.
Alberto Krone-Martins, one of the authors of the study, from the University of California,
Irvine, said that distances are among the most difficult things to measure in astronomy.
It is only the recent, direct distance measurements from Gaia that make the
geometry of this new structure so apparent.
The combination of the two sets of data revealed that the splintered segment of the Sagittarius Arm
contains young stars moving at nearly the same velocity
and in the same direction through space. When they put the Gaia and Spitzer data
together and finally see this detailed, three-dimensional map, they could see that
there was quite a bit of complexity in this region that just hasn’t been apparent before.
The scientists do not yet fully understand why all that star material forms into these spiral arms.
While astronomers have discovered groups, local groups and clusters of galaxies
in an estimated number of more than 100 billion, they have not been able
to completely model and map our own Milky Way galaxy, for the reasons we mentioned above.
Okay, now that we know how this discovery was made, let's try to answer this question:
why is the break important?
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Here's where you'll learn something new today.
The arms of a spiral galaxy are measured in something called a pitch.
The pitch is said to be zero degrees if a spiral is so tightly wound that it forms a circle.
Instead, if a spiral arm opens up, the pitch goes up.
Now, a key property of spiral arms is indeed how tightly they wind around a galaxy, and
what their pitch is. Analyzing the milky
way galaxy's structure and putting all the physics we know in our best stellar models,
we found that the Sagittarius Arm should form a spiral that has a pitch angle
of about 12 degrees. But these were just models.
The problem, and the amazingness of this discovery, came when they found
the structure they examined really stands out at an angle of nearly 60 degrees.
It's a huge disagreement with models. It's a huge pitch. This spiral arm is very very loose.
Scientists have seen jutting breaks in spiral arms of other galaxies before, but
this is the first time such a structure has been identified in our own galaxy.
This is an important result, that breaks down our best models, and suggests us to revisit theories
and make new detailed measurements of the motion of individual stars.
The stars in the spiral arm break likely formed together. Forces within the Milky Way,
such as gravity and shear from the galaxy’s spin, likely influenced them.
This discovery acts as a reminder that there are many uncertainties about the
large-scale structure of the Milky Way, and we need to look at the details if we want
to understand that bigger picture, that is to say, we have to study every single tree
if we want to get out of the forest! Good luck! This structure is a small piece of the Milky Way,
but it could tell us something significant about the galaxy as a whole.
As you can see, once again reality surprises us. It is always way more complex than we think,
and the only thing we can do is keep observing it and try to understand a little bit more of it.
Piece by piece, tree by tree, star by star, we are building our picture of the universe.
Sometimes it is a blurred one, sometimes it gets clearer.
But the puzzle is a big one, the forest is a messy one, the galaxy is a peculiar one.
The only thing we can do, as always, is to keep it curious.
"This video ends here! Thanks for watching everyone! Are you in love with Milky Way Galaxy?
What's the weirdest thing you know about it? Is there anything more you want to hear? Let us know
in the comments below, be sure to subscribe, and I'll see you next time on the channel!"