Probably the most spectacular feature of our Milky Way galaxy is its spiral arms.

We can’t get a probe far enough out yet to take a galactic selfie, but astronomers are reasonably sure that we live in a spiral galaxy. Observations of other spiral galaxies offer clues to what kind of objects can help us trace out the shapes of spiral arms, called spiral tracers. Using those spiral tracers, we’ve been able to map out patterns within our own galaxy that appear to be spiral arms.

Over the years, astronomers have tested the spiral arm hypothesis against the evidence again and again, and there is now a great deal of confidence that the Milky Way is a spiral galaxy.

More than that–star formation, which we know is limited to the disk of the galaxy (rather than its central bulge or halo), appears to be specifically found in the spiral arms.

But why? And for that matter…what even are spiral arms?

It’s a good question, considering that galaxies are not solid objects. They’re made up of around 100 billion stars, all spread light-years apart from one another. So, why would the stars of a galaxy fall into such a distinct spiral pattern? What holds them together?

First, let’s come back to a fundamental fact: the galaxy rotates.

This rotation is the orbital motion of individual stars around the galactic core.

Galaxies don’t quite rotate as solid disks. Remember that they’re made up of around 100 billion stars, all spread light-years apart from one another. Those stars–and the gas and dust between them–follow their own, independent orbits. Over billions of years, the galaxy’s stars have drifted apart, met new neighbors, and drifted apart again countless times.

That means the spiral arms can’t possibly stay made up of the same stars throughout their lives.

And that rules out gravity. If streams of stars were gravitationally bound into the shapes of spiral arms, then the stars wouldn’t be drifting apart as they independently orbited the galaxy.

In fact–it would seem to imply that it would be impossible for a galaxy to retain a spiral shape.

But spiral galaxies are quite common in the universe.

Galaxies are far enough away from the Earth that when we observe them, we’re seeing them as they were billions of years ago, when the light we’re seeing first left their stars. But our own Milky Way clearly still has spiral arms. Somehow, these spiral structures manage to survive for billions of years.

Astronomers conclude that they are dynamically stable.

What does that mean, you ask?

Well, technically, dynamic stability is when a pattern’s appearance stays the same, even though the actual parts that make up the pattern are constantly changing.

I don’t know about you, but that definition sounds a bit wordy to me. Instead, let’s imagine a traffic jam.

Traffic jams are perfect examples of dynamic stability.

In this case, a slow-moving truck crawls along the highway. Faster-moving cars catch up to it, slow down as they get stuck behind it, and then speed back up as they pass on by. The cars involved in the traffic jam change over time. But, viewed from above, the traffic jam itself–the pattern of congestion–would be stable, consistently moving along the highway.

In a spiral arm, the slow-moving truck represents regions of compressed gases and dust that move slowly around the galaxy.

Generally, those compressed regions would be made up of giant molecular clouds (GMCs) and gas clouds.

Giant molecular clouds are, in plain English, space dust–specifically, very dense space dust. They also happen to be the defining features of many spiral arms, and in my last post, we used their carbon monoxide signature as a spiral tracer to trace out the shapes of the Milky Way’s spiral arms.

As dense, slow-moving dust clouds, GMCs make perfect candidates for the “truck” of the Milky Way’s traffic jams. It’s no wonder they’re so commonly found in spiral arms and make good spiral tracers.

Of course, other gas clouds make up the arms as well–but there’s no doubt that GMCs are very prominent features within the arms.

In the role of the passenger cars that overtake and pass the trucks, we have diffuse gas clouds and stars themselves. These objects orbit the galactic core a bit faster than the slow-moving arms. They overtake the arms from behind, pass through, and continue on their journeys around the galaxy.

However…they don’t just quietly pass through.

Seemingly an eternity ago, I described how stars are born when a shockwave triggers a giant molecular cloud to collapse. I also mentioned that one type of shockwave is, in fact, a galaxy’s spiral arm.

Consider that a shockwave is just a moving pattern of compressed particles–like the compressed clouds that make up spiral arms.

In my last post, we found that spiral tracers–the objects we can use to map the Milky Way’s arms–are all very young objects. We also began to wonder if spiral arms, and not the space in between them, are the primary places of star formation.

Indeed they are.

Fast-moving gas and dust overtakes the spiral arms from behind–and slams into the gas and dust that’s already there. The clouds are compressed, triggering star formation.

All sorts of stars begin to form, all across the spectral classes found on the H-R diagram. (The spectral classes are labeled here as O, B, A, F, G, K, and M, and correspond to the surface temperature of the star, as well as the color. Yes, the colors here are actually how the stars appear.)

Among the newborn stars produced within the spiral arm are massive, bright blue stars like Spica, medium-mass white stars like our own sun, and low-mass red dwarfs like Proxima Centauri. The full spectrum of stars along the main sequence are born.

(You might recall that the “main sequence” is a relationship between internal pressure and surface temperature that indicates a stable star.)

These stars are all born more or less simultaneously. They begin surrounded by their siblings, all of varying masses, luminosities, and surface temperatures. We can observe such brand-new stellar associations in star-forming regions with nebulae.

Here’s a few newborn associations from one of my all-time favorites, the Orion Nebula.

This star formation explains why spiral tracers exist–and why they’re all such young objects.

Medium-mass stars and the least massive stars have lifespans of billions of years. These stars will inherit the orbital motion of the fast-moving gas clouds that triggered their formation, and they will leave the stellar associations of their birth. They will pass through the spiral arm and circle the entire galaxy many times during their lives.

The most massive stars have lifespans of only a few million years. They will be left behind in the spiral arm; observations of “mature” stars will reveal stellar associations of only O and B stars, since their lower-mass siblings have left. These “O/B associations” will almost exclusively be found in spiral arms, making them excellent for mapping the arms.

This model of spiral arms as dynamically stable shockwaves is called the spiral density wave theory.

Alone, though, it doesn’t explain everything. You’ve probably noticed that the individual shapes and patterns of spiral galaxies are quite varied.

In the first image, we have NGC 3521, a galaxy that definitely has spiral arms–but they look quite a bit blurred together and “fluffy.” Astronomers call these spirals flocculent, meaning “woolly.”

Then in the second image, we have NGC 1300, a galaxy with two quite prominent spiral arms! Galaxies like these are called grand-design spirals. Which, by the way, doesn’t actually have anything to do with intelligent design. Their dramatic shapes just look pretty, well, grand.

The spiral density wave theory isn’t enough to explain such broad variation among spiral galaxies. But it does explain the primary method of star formation, and that can possibly explain flocculent and grand-design galaxies.

Ages ago, I covered contagious star formation, which is when star birth triggers more star birth.

This happens when winds and shockwaves from stars trigger nearby regions of dust clouds to collapse and begin forming even more stars.

It can be a result of powerful winds from newborn massive stars–or it can be a result of a supernova shockwave from massive stars at the end of their lives.

And that’s exactly the kind of thing that can happen to star-forming regions of spiral arms.

As a fast-moving cloud passes through a spiral arm and triggers star formation, the initial star formation will become “contagious.” Shockwaves and winds will trigger star formation in nearby regions.

But these regions don’t keep pace with the spiral arm. They’ll inherit the orbital motion of their own, separate clouds.

The resulting region of star formation will get stretched out–and it’ll develop into a branch or “spur” off of the main spiral arm.

Presumably, flocculent galaxies are brimming with this self-sustaining star formation, creating so many new branches and spurs that we can barely make out the shapes of the original two arms. And apparently, grand-design galaxies are quite poor in it–few star-forming regions develop into new spurs.

Our Milky Way seems to be somewhere midway between these two extremes.

We’ve now fully explored the disk component of our galaxy, including the spiral arms that are home to its star formation. Next up, we’ll explore the galactic core–including the mysterious gravitational presence known as Sagittarius A*!