Structure of the Milky Way

In the vast expanse of the cosmos, the Milky Way Galaxy is our home.

You’ve no doubt seen images of the Milky Way and similar galaxies elsewhere online. It’s a large, spiral galaxy, one of the most spectacular galactic shapes. That spiral shape is fairly iconic–and for years, that’s as far as I thought galaxy classification went.

Turns out, galaxies are way more diverse than just the main three classifications I knew about (spirals, ellipticals, and irregulars). The Milky Way is fully classified as an SBbc: a barred spiral galaxy with a medium-sized nucleus.

Spirals are also described as “grand design” (two distinct spiral arms) or “flocculent” (a sort of fluffy appearance); the Milky Way is somewhere in the middle.

But even those classifications and descriptions don’t fully describe our galaxy.

So what exactly is the structure of the galaxy we call home?

Perhaps the most obvious part of the galaxy’s structure is the disk. But there’s a bit more to the disk than you might think.

The disk of the Milky Way is home to its thick dust clouds, as well as open clusters and stellar associations of the galaxy’s youngest stars.

That’s because, as you might remember from previous posts, dust clouds are where stars are born.

When an interstellar cloud collapses to form stars, it produces a star cluster–a cluster of “sibling” stars that are fairly close together, gravitationally bound, and all close to the same age. Over time, the sibling stars drift farther apart from one another until the star cluster dissolves.

In the disk of our galaxy, we can find two different kinds of star clusters: very young stellar associations and somewhat older open star clusters.

Stellar associations are groups of sibling stars. These clusters contain around 10 to a few hundred stars, and the stars are very widely spread apart. That is, they’re not strongly bound together by gravity–but somehow, they still move together as they orbit the center of the galaxy.

H-R diagrams of stellar associations show that the stars are very young–generally the brilliant O and B class stars that only live for a few million years. That explains how these stars can still be clustered with their siblings: they just haven’t had time to drift apart.

On the other hand, open clusters are groups of about 100 to a few thousand stars, all clustered into a region about 25 pc (parsecs) in diameter.

These stars are older, usually around a few million to a few billion years old. They’re siblings, too, and it makes sense that they would still be so close together–they formed close enough together that the cluster is strongly bound by gravity.

And then, of course, the disk also contains scattered stars that are not part of star clusters. These are stars that have drifted apart from their sibling groups. Most often, they come from associations, but now and then, open clusters lose stars too.

The stars within the galaxy’s disk are all very young–such as our own sun, which is around 4.6 billion years old, and the galaxy’s youngest stars, the brilliantly bright O and B class stars.

So, what are the dimensions of the disk, anyway?

Earth maps have clear boundaries indicating borders such as country lines, but there’s nothing like that in space. In order to measure the disk, we need to rely upon the distribution of the objects within it.

As it turns out…the disk is way thinner than you might think.

Stars like the sun–several billions of years old–are found within 500 pc (parsecs) above and below the central plane. But younger stars, like the O and B class stars, don’t have time to migrate that far in their lifetime. They are only found within 50 pc above and below the plane.

The disk’s diameter, however, is roughly 25 kpc (kiloparsecs) across–around 100 times the diameter of the disk.

If the disk were the diameter of a pizza, it would be even thinner than the crust!

The disk, though, is just one component of the galaxy’s overall structure.

Here, you can see the disk we’re now familiar with, which contains the galaxy’s gas and dust, stars like the sun, and the young O and B class stars. But you can also see the two spherical components of the galaxy: the central bulge and the halo.

The central bulge makes up the center of the “pinwheel” shape that spiral galaxies are so famous for.

Its equator is hidden behind the dust clouds of the galactic disk, but it’s visible above and below the plane of the galaxy. And it’s a very chaotic place–something we’ll be coming back to in posts coming up. It’s extremely crowded, and unlike the disk component, it’s not home to any star formation. That’s because it has very little gas and dust to form stars.

With no star formation happening, the occupants of the central bulge are old, cool stars: red dwarfs, red giants, and white dwarfs.

That just leaves the final part of the spherical component: the galactic halo.

Okay, I admit it, I never can resist an opportunity to show off my favorite globular cluster, M13 (the Hercules Cluster).

Globular clusters–yes, the same star clusters that helped determine the size of the Milky Way–are denizens of the galaxy’s halo. Why? Well…we don’t quite know yet. But their presence in the halo does fit what we know about that part of the galaxy.

Notice above, how M13 is an extremely compact star cluster with an absolute crap ton of stars?

Specifically, it contains over 100,000 stars–right on par for a globular cluster. In general, globular clusters contain 50,000 to a million stars, squeezed into a sphere only a few tens of parsecs in diameter.

In other words, there’s a lot of gravity going on, and they’re very tightly bound.

A tightly bound cluster like that can survive for billions of years, and studies of globular clusters’ H-R diagrams confirm that they are around 11 billion years old.

Just like globular clusters, the lone stars that populate the galactic halo are old. That’s because there are no dense dust clouds to be found, so star formation can’t happen.

When I first started learning about galaxies, I had a vague idea that there were different types of shapes, but I had no idea that individual galaxies had so many components. And each component of a galaxy’s structure is full of clues to how and why galaxies form the way they do.

Next up, we’ll explore the mass of the Milky Way–just how much stuff it seems to contain.

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