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Recent Podcast
A Tour of GW170817
A Tour of GW170817
Astronomers have used NASA's Chandra X-ray Observatory to make the first X-ray detection of a gravitational wave source. (2017-10-16)

Spirals in Nature

Narrator (April Hobart, CXC): Hurricanes are extreme weather events that can affect millions of people. Most of the concern surrounding hurricanes involves the experience of a hurricane from below – and for good reason given how much damage they can cause. But it is also very interesting to consider hurricanes from the other direction, that is, how they appear from the air. Looking at a hurricane from this point of view, we can see that the storm is, in fact, a giant spiral shape. And, it turns out, this spiral shape appears in many different objects of various sizes and scales across the Universe.

Let's compare hurricanes with two other spiral-shaped objects that are very different: water going down a drain and a spiral galaxy. The common thread for all of these three things is angular momentum, a physical principle that remains constant with time for a spinning object and applies over all scales.

When water is pulled down toward a drain by gravity, it spins faster near the drain so that the angular momentum is conserved. Think of a skater bringing in their arms to rotate more rapidly. This increase in speed helps generate the spiral structure in the flow. The direction of the spin is mainly determined by the shape of the bowl and the initial direction of the water.

Now let’s look at hurricanes. Hurricanes are driven by powerful uplift from warm, humid air and the very low pressures in the atmosphere this creates. As a storm forms and intensifies, winds are pulled inward and pick up speed because of the conservation of angular momentum. The initial rotation of a hurricane is determined by what is known as the Coriolis Effect . Because the Earth rotates about an axis, different points on the Earth's surface will rotate at different speeds. Points on the equator go the fastest and points at the pole are stationary. This difference in speed imparts a rotation to air flowing into regions of low pressure, like those of a hurricane. Because of this Coriolis Effect, hurricanes rotate in different directions depending on what hemisphere you are in.

Finally, let’s look at spiral shapes in galaxies. The formation of a so-called spiral galaxy involves the collapse of an enormous cloud of gas. To conserve angular momentum, a rotating disk results. The source of the familiar spiral pattern is thought to be density waves that pass over the disk. As the density wave passes over a region, the gas is condensed, resulting in the formation of bright stars in a spiral pattern.

So the bottom line is that all three objects – water going down a drain, a hurricane moving up a coast, and the spiral pattern of a galaxy – are connected. All three objects involve spin, and therefore the conservation of angular momentum plays a role in each case. But there are crucial differences in the details of the physics. This is another example of why it’s interesting to look at science here, there, and everywhere.

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