Exploring the Milky Way’s Spiral Arms

The Milky Way–our home galaxy–is a spiral galaxy, a classification I often describe as pinwheel-shaped.

The main difference between a spiral galaxy’s shape and a pinwheel’s shape is that spiral galaxies, like the Milky Way, only have two main arms. For the Milky Way, those are the Scutum-Centaurus arm and the Perseus arm. If you study the image above, you’ll notice that all the other arms are a bit wispier, and most branch off from the main arms.

There’s just one problem, though…

How do we even know that this image is an accurate depiction of our galaxy? How do we know that the Milky Way has spiral arms?

Several weeks ago, we explored the stars, dust, and gas that make up the disk component of the Milky Way.

We found that the disk component is mostly made up of young stars and the material needed to make them–dust and gas. Older stars are found in the central bulge and the spherical halo that surrounds the galaxy.

Clearly, the disk is home to the galaxy’s star formation. After all, that’s where the young stars are, and older stars are rare–they have had plenty of time to drift apart from the stellar associations of their birth and leave the disk entirely.

But if we study the disk itself in even more detail, we find patterns in where, specifically, these young stars are located.

Remember from much earlier posts that O and B class stars are the brightest and youngest stars. These stars have comparatively short lifespans of only a few million years.

That means that any O or B class star we observe in the sky can’t be any older than that–and it can’t have had enough time in its lifetime to drift away from the place of its birth. (You and I would have plenty of time to leave our hometown within a few million years, but distances in space are much greater–and velocities aren’t fast enough to be of much help!)

If O and B class stars haven’t had time to move away from their birthplaces, then they must still be there. And as we saw in my post on the structure of the Milky Way, many of those stars are found in stellar associations made up of their sibling stars.

By tracing those stellar associations, we can map out regions of star birth.

As it turns out, maps of O/B associations trace out patterns that look a whole lot like spiral arms.

But–I ask again–how do we know? It’s not like we can hold our phones out on a stick and take a selfie of our galaxy! How do we know that these arcs of stars are actually parts of spiral arms?

Well, we can compare these observations to other galaxies we observe. Note the galaxy in the left part of the image above. That’s an actual image of a galaxy–it’s not an artistic rendering of the Milky Way. The brightest stars–the O and B stars–are, in fact, found in the spiral arms.

But that’s not all that dominates the spiral arms.

Images of other galaxies, like that one, provide more clues to help us map the shape of our own galaxy. See the strong patterns of thick, brown dust that pretty much define the shapes of the arms?

Well…what if we could find similar patterns in our own galaxy?

As a matter of fact…we can.

In this image, we see concentrations of the molecule carbon monoxide. Carbon monoxide is found in giant molecular clouds, which are the sites of star formation in any galaxy. And when we see patterns of thick dust in images of galaxies, we are looking at molecular clouds.

The molecular clouds do, in fact, seem to be found in patterns that look like spiral arms, just like O/B associations. The evidence is mounting that our galaxy has spiral arms, too.

That’s not the only evidence that’s mounting, either. O/B stars are young stars. Molecular clouds are stellar nurseries. We’ve already observed that all the ingredients for star formation are found in the galactic disk. But, more specifically, it’s starting to look like the spiral arms–not the space in the disk between them–are involved in star formation.

And that’s not all.

Here are some particularly representative galaxies–including M51, the same deep-sky object we used for clues just a moment ago.

Galaxy NGC 1232 is particularly rich in the O/B stars we’ve already covered. But look again at M51.

Of course, its spiral arms are full of O/B associations and dark molecular clouds. Objects like these that help us trace the shapes of spiral arms are called spiral tracers. And M51 has a few other types of spiral tracers that can help us out.

Note the bright pink regions in the arms–these are emission nebulae. Also visible are open star clusters, groups of young stars that are a bit older than O/B associations. And it’s impossible to tell from a single still image, but there may also be variable stars.

All of these spiral tracers are observed in other galaxies as part of the spiral arms. Once again, that’s a clue that observations of these objects within our own galaxy can help us trace the Milky Way’s spiral arms.

In case you’re curious, the graph below is how astronomers use data from these objects. The data points (symbols explained by the key in the upper left) indicate specific spiral tracers. The gray lines indicate the approximate positions and shapes of the spiral arms themselves.

It’s no coincidence that these objects are all very young objects.

Emission nebulae are just gases in the interstellar medium that have been excited to produce their own light–by nearby O/B stars. They can’t exist without being very close to young stars.

As for variable stars, some of them are particularly young. Despite having nearly reached the end of their lifespan, they’ve only lived for a few million years–and thus are still very young objects.

Spiral arms are looking more and more like sites of star formation.

There’s one more way we can map the spiral arms of our galaxy: neutral hydrogen gas.

This neutral hydrogen is a lot harder to map. Hydrogen gas tends to be warmer than molecular clouds, and in physics, warmth is a result of more random motion of the particles and greater turbulence within the cloud. That makes observations turn out a bit fuzzy.

Still, though, this map reveals a vague pattern of spiral arms–or, at the very least, concentric circles. (Observations of the side of the galaxy opposite our sun are incomplete, due to the central bulge blocking our view.)

Hydrogen gas isn’t necessarily one of the young “objects” we’ve been using as spiral tracers. But since stars are made mostly of hydrogen, it is important for answering the side question we’ve been exploring: are spiral arms the primary sites of star formation?

Maps of hydrogen gas confirm that the gas is more concentrated in the spiral arms. So, at the very least, we know that the spiral arms contain abundant material to form all the young objects that are found within them.

Clearly, we live in a spiral galaxy–and the spiral arms contain many young objects and abundant star-forming material. But how the heck did those spiral arms form in the first place? And are they really the primary sites of star formation?

We’ll explore those questions in my next post.

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