Turbulence is one of the most ubiquitous processes in nature. Spend a little time studying weather patterns, fusion machines, aerodynamics, star and galaxy formation or almost any other process, and you will eventually come up against the inevitable consequences of chaotic fluid flow.

What makes this interesting for physicists is that turbulence in one system is exactly analogous to the turbulence in another. So the airflow around an insect’s wings is similar to the flow of honey around an orange. And the turbulence in an inertial confinement fusion machine is the same as the turbulence inside stars.

In recent years, aerodynamicists have discovered an entirely new phenomena of turbulence, called preferential concentration, that plays an important role in everything from aerosol production to crystallisation reactors. Last month, meteorologists showed how preferential concentration explains one of the last great mysteries of rain.

But the role of preferential concentration in star formation has never been studied in detail. Until now.

Today, Philip Hopkins from the California Institute of Technology in Pasadena says that preferential concentration must have a profound effect on the way stars form. His extraordinary conclusion is that preferential concentration means that some stars must be made entirely of metal.

First, some background. Any turbulent flow is made up of eddies of various sizes that rotate at different rates. When the flow contains particles, they are inevitably influenced by these vortices.

It’s easy to imagine that the influence of these eddies average out to create random motion. But in recent years, aerodynamicists have found something different.

They say that when the particles and the eddies both have the same scale, the particles become influenced by inertial forces in the rotating vertices. That causes the particles to be pushed out of the eddies, into the regions of low vorticity between them.

This turns out to be hugely important. It means that particles of a certain mass become concentrated in the regions between the vortices. And that has profound effects. For example, it concentrates nuclei in clouds, allowing raindrops to form quickly in a process that meteorologists had puzzled over until very recently.

It’s processes like these that got Hopkins thinking about star formation. If preferential concentration can have such important effects on Earth, why not also in space where turbulent gas clouds are common.

Turbulence is particularly important in the giant gas clouds in which stars form. The question that Hopkins has focused on is what role preferential concentration might play here.

His calculations have far-reaching effects. These gas clouds are the remnants of supernovas and so contain a wide range of newly created elements as well as the primordial light elements of hydrogen, helium and lithium.

Preferential concentration tends to force the heavier elements out of regions of high vorticity so that they become concentrated in the gaps between the eddies. This concentration of mass creates a stronger gravitational field which attracts more mass and so on. So in Hopkins’ new model of gas clouds, preferential concentration becomes an important trigger of star formation.

But here’s the thing. Because the heavy elements become concentrated separately from helium, hydrogen and lithium, some stars will form in these regions made entirely of heavy elements. Astrophysicists call the concentration of heavy elements in a star its metallicity. So the extraordinary consequence of preferential concentration is that some stars must be made entirely of metal. (In the parlance of astrophysicists, this means made of elements heavier than lithium.)

Hopkins goes on to calculate that perhaps one in 10,000 stars forms like this.

Nobody has ever seen a metal star. But Hopkins’ prediction is that they must exist out there somewhere. They may already have been observed and not recognised as such.

Either way, this work will send astronomers scurrying for their lens cloths and telescopes. If Hopkins is right, it’s only a matter of time before somebody claims the first observation of an entirely metal star.

Time to start observing!

Ref: arxiv.org/abs/1406.5509 : Some Stars are Totally Metal: A New Mechanism Driving Dust Across Star-Forming Clouds, and Consequences for Planets, Stars, and Galaxies