One in every 10,000 stars may be composed of metal. It's likely the result of a phenomenon called "preferential concentration," which plays an important role in everything from aerosol production to the creation of raindrops.
The research that led to this conclusion, just published by CalTech astrophysicist Philip Hopkins, is based upon chaotic changes in the flow of fluids, more commonly known as turbulence. As the Physics arVix blog explains, any turbulent flow is made up of eddies of various sizes the rotate at different rates. (Think of each one as a tiny spinning vortex.) When this turbulent flow contains particles that are at the same scale as the eddies, they are pushed into regions between the vortices.
Put another way, turbulence can pluck out particles that share the same specific mass and concentrate them together.
So, what happens if we were to apply what we know about this phenomenon to the turbulence that occurs within giant gas clouds in space?
According to the Physics arXiv blog:
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.)
To date, nobody has observed a metal star. But, that might be because nobody had thought to look for one. Astronomers, start your engines!