American Scientist

Astronomy can seem like a remote science, its events unfurling over billions of years, across distances that make our planet appear to be little more than a mote. Yet physics is still physics wherever you are in the universe, and many of the processes that shape stars and galaxies are no different from those that govern our everyday experience. This connection is what fascinates me about fluid dynamics in particular: The lessons we learn on small scales transfer to flows that span large, even cosmic scales. These phenomena are all identical in a mathematically precise way.

For instance, consider the puzzle of how sunlight manages to escape from the Sun. Solar energy arises from nuclear fusion reactions in the core, but that energy is buried hundreds of thousands of kilometers beneath the surface, and most of the Sun’s overlying gas is nearly opaque; it hinders light from passing through, like a blanket thrown over a flashlight. Clearly the Sun does shine—but how? For the answer, you can simply go to your kitchen, fill a kettle, and flip on a burner.

Heating water on the stove creates convection currents. Hot water rises to the surface due to its lower density. There, it cools and sinks, to be replaced by freshly heated water. Rather than one big, swirling current, many smaller circulation cells form, resembling compartments in an egg carton. They’re hard to see in water, but a spoonful of mica powder added to vegetable oil highlights the convection cells beautifully.

Such convection cells form in our Sun and other stars. Deep inside the Sun’s core, the photons made by nuclear fusion initially ricochet from one atom to the next, getting absorbed and reemitted at every encounter. But they gradually move outward and reach the Sun’s convective zone, which lies about 200,000 kilometers below its surface. There, the photons are conveyed upward by the rising motion of gases. Looking at the Sun’s surface with solar telescopes, we see distinctive regions known as granules— the tops of the convection cells.

I’m a tea drinker, but my husband brews coffee at breakfast and adds creamer. If you do this in a transparent mug and let it sit without stirring, you can watch another fluid dynamical show with astronomical parallels.

Because the creamer is denser than the coffee, it sinks, forming fingerlike protrusions of falling creamer and rising coffee. In space, this phenomenon—known as the Rayleigh-Taylor instability—shows up when denser regions of gas and dust are pushed into less dense ones. It plays an important role in causing massive stars to explode. As such a star nears the end of its life, it runs out of hydrogen to fuel its nuclear reactions and begins fusing heavier elements instead. This process builds up onionlike layers in the dying star, stacked with the heaviest elements in the inner layers and progressively lighter ones farther out. Once the star’s core is full of iron and nickel, fusion ends and the core collapses, triggering a supernova.

Many of the processes that shape stars and galaxies also govern our everyday experience.

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As the explosive wave moves outward, it drives heavier layers into lighter ones, forming Rayleigh-Taylor fingers. Although the explosion is over in a matter of hours, Rayleigh-Taylor fingers stretch into the gas surrounding the stellar debris for years to come. We see these intricate structures in supernova remnants such as the Crab Nebula —the aftermath of a stellar explosion that was seen on Earth in the year 1054.

My bike ride to work takes me alongside a nearby pond. On a breezy day, I notice little waves rippling across the water. As the wind blows across the surface, friction between the air and water builds up these waves. In the ocean, this effect, known as the Kelvin-Helmholtz instability, leads to swells that are meters high. If you keep an eye on the sky, you can sometimes catch short-lived Kelvin-Helmholtz clouds as beautiful lines of curling waves. And you can also see them if you look even farther out.

The Kelvin-Helmholtz instability occurs across the cosmos wherever two layers of fluid—liquid or gas—rub against one another. For example, Jupiter is ringed by vast bands of clouds along whose borders you can see trails of swirling vortices, thousands of kilometers across, born from such an instability. And they are puny compared with their relatives in the Perseus cluster of galaxies, 240 million light years away.

Astronomers think the Perseus cluster was set in motion billions of years ago when another galaxy cluster grazed it, causing its cold gases to slosh into a spiral surrounded by hotter gases. This motion formed a Kelvin-Helmholtz instability, visible in x-ray images as a concave feature that astronomers call the “bay.” It is about 160,000 light-years wide, making it 10²² times larger than those pond ripples, but it is otherwise very much the same.

My bike ride complete, it’s time to get to work. In my job, I contemplate mostly commonplace examples of fluids: the swirling chaos of turbulence, the flow of molasses, and the sharp kick of a shock wave in air. As I work, shock waves not unlike the ones I study surge through interstellar gas and dust, leaving behind the turbulent whirls and eddies that seed new star systems.