When you think of a galaxy, you likely think of a spiral galaxy. More specifically, you likely imagine a spiral galaxy with two large sweeping arms of stars, such as the image of the Whirlpool Galaxy above. Such a galaxy is known as a grand design spiral, and while it’s an iconic style, it isn’t particularly common among galaxies.

While about 70% of the galaxies in our neck of the woods are spiral galaxies, only about 10% are grand design spirals. About 30% are known as flocculent spirals, with a patchy spiral structure, and the rest are multi-armed spirals. Our own Milky Way galaxy appears to be a multi-armed spiral.

While it’s tempting to think of such grand design spirals as simply being the result of stars orbiting the center of the galaxy. Since stars closer to the center should move a bit more quickly than stars farther away, over time the pattern of stars should twist into a spiral shape. If that were the case we would expect that all spiral galaxies rotate with their arms trailing the rotation.  While that’s true for most spiral galaxies, it isn’t true for all of them. There are spiral galaxies that rotate arm first, or “backwards” from what we’d expect.

It’s generally thought that most spirals develop their shape through density waves. Density waves are kind of like stellar traffic jams. As more dense regions of gas and dust build up in a region, they tend to slow down the motion of stars moving through them. Just as the pattern of a traffic jam can remain while individual cars can pass through the congestion, the spiral density wave pattern remains while individual stars pass into and out of a spiral arm. This idea is not only supported by computer simulations, but also by observing where star formation occurs in a galaxy. They tend to form along the edge of a spiral arm where stars enter it, which is where you would expect a compression of gas and dust to occur. The spiral structure of these density waves may occur early on in the formation of the galaxy, or they might be triggered by gravitational interactions with other galaxies.

What remains a mystery is how some galaxies become grand design spirals, while others are multi-armed or flocculent. It seems to be related to the amount of dust in a galaxy, but the relation is still unclear. Whatever the reason, that most rare of spiral galaxies will likely remain the most popular. We just seem to love galaxies with a grand design.

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When we look at galaxies in the universe, we see that many of them have sweeping spiral arms. These spiral galaxies are quite common, and even our own Milky Way galaxy is one of them. Just how these spiral galaxies form has long been an area of astronomical study. At first it would seem to have an easy solution: as stars orbit the galaxy, the faster stars nearer galactic center twist around the galaxy faster than more distant stars, which would produce a spiral shape over time. But we now know that isn’t the case. If that idea were true, then the rotation of all galaxies should have the spirals trailing, but we know that some galaxies rotate spiral first, as if the outer stars are moving faster than the inner stars. We also know from computer simulations that stellar motion would actually smear out the spirals fairly quickly, but know by observing distant spiral galaxies that their spiral shape is stable, barring collisions. So how do spiral galaxies form?

The most popular solution is that spirals form as a density wave effect. You can see a similar effect in traffic jams. Whenever there’s a fender-bender on the highway, there is inevitably a slowdown of traffic and a build-up of congestion. Individual cars slow, pass through the congestion and move on, but the traffic jam as a whole remains. Even hours after the accident has been cleared, the traffic jam can remain, which is why sometimes on the highway you find your journey slowed to a crawl for a bit and there’s no clear reason why.

A similar effect can occur with stars in a galaxy. A traffic jam (density wave) can cause a build-up of stars along a spiral. Individual stars move in and out of spiral arms, but the spiral shape of the galaxy remains stable. Since our Milky Way is a spiral galaxy, we should see the same effect. But demonstrating it can be difficult given that we’re in the midst of things and our view of the Milky Way can be limited by interstellar gas and dust. But now a new paper has presented strong evidence of the density wave effect in our own galaxy.

A map of Milky Way spiral arms for different chemical fingerprints. Credit: Jacques P Vallee

Good maps of our galaxy have been made several times before. It’s how we know that the Milky Way is not only a spiral galaxy, but a particular type known as a barred spiral. What makes this new work different is that the author mapped the spirals arms based upon different spectral signatures. So instead of simply mapping the location of “stuff” and how it’s distributed, the author mapped hydrogen, organic molecules such a carbon monoxide, cold and hot dust, and even where masers are produced. From these maps it is seen that the spiral arms of our galaxy have parallel layers of material within each spiral arm. The inner, middle, and outer sections of each spiral arm follows a similar pattern.

This is exactly what you’d expect if spiral arms are the product of density waves. It would be as if you noticed for every traffic jam you notice brake lights come on as cars enter the pattern, and their engines accelerate as they leave the pattern.

So it’s pretty clear from this work that density waves really do shape the spiral arms of a galaxy. Now we know that our Sun and other stars are simply riding the wave.

Paper: Jacques P. Vallée. Catalog of Observed Tangents to the Spiral Arms in the Milky Way Galaxy. ApJS 215 1 (2014)

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Even with the presence of dark matter, the stars closer to the center of a galaxy orbit faster than the stars farther away.  This raises a bit of a mystery. Many galaxies (including our own) have a spiral shape to them.  Since the central stars of a galaxy move faster than stars near the edge, the  spiral should gradually twist tighter and tighter until the spiral arms all blended together in a uniform disk.

If that were true, then we would expect to see very few spiral galaxies, and instead see lots of galaxies that look like uniform disks.  This isn’t what we see, in fact spiral galaxies are quite common.  There must be some mechanism that maintains a galaxy’s spiral shape.

This mechanism turns out to be an effect known as a density wave.  We also have such density waves here on Earth.  We call them traffic jams.  If you’ve ever been in a trafic jam you’ve noticed that while you slowly make your way through it the overall traffic pattern remains the same.  Usually when you’re caught in a traffic jam you eventually find the source (construction, minor accident, etc.) but sometimes you enter a traffic jam and go slowly through it without ever seeing a cause.  At some time earlier something started it, but now there is just the traffic.  The jam itself is now the cause of the jam.

A similar effect occurs with spiral arms in a galaxy.  Individual stars are not locked in a particular spiral arm, rather they move around the galactic center passing through one spiral arm after another.  But as a star moves toward a spiral arm, that arm’s mass gives it a little gravitational boost to edge them into the arm.  When a star begins to leave an arm, the gravitational pull of the arm slows it down just a bit.  As a result stars tend to cluster into the traffic jam of spiral arms, and the arm patterns sustain themselves even while individual stars move through them.

You can see this effect in the animation below.  The density waves of the spiral arms keep their shape, but you can see stars move in and out of them.

So the next time you’re caught in traffic, don’t get frustrated.  Take heart in the fact that you are participating in Earth’s rendition of the dance of the stars.

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