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SciShow Space describes a new phenomenon that might be out there: Stars made entirely out of metal. But it’s not quite what it sounds like!

Hosted by: Caitlin Hofmeister

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The sun is made up of about 75% Hydrogen and 24% Helium. That other 1 percent? It's a mix of heavier elements that astronomers, in a rare instance of scientists calling things not quite what they actually are, lump together under the term "metals". Even though they're not all metallic!

Okay, a lot of that 1% of the sun does include things like Iron, and Titanium, and Gold, but to astronomers "metals" also include things like Carbon, Oxygen, and Silicon. That's partly because inside a star it's so hot that the properties we use to determine metals on Earth, like conducting electricity, just don't apply. And since a big chunk of elements in the periodic table are metals, at least on Earth, astronomers just use "metal" to describe everything except Hydrogen and Helium. Weird, I know.

But some stars have way less of what astronomers call "metal" than the sun does, and still others have a lot more. So much more that a new theory predicts that some stars may be totally metal. The weird thing is that according to the theory these all metal stars might form in the same way as something I'm sure we're all familiar with: rain. According to theoretical astrophysicist Philip Hopkins at Cal Tech, both stars and raindrops form through the swirling, whirling effects of turbulence.

If you're like me your encounters with turbulence are probably limited to bumpy airplane rides. But turbulence shows up all over nature. In 2013, scientists discovered that turbulence is actually what causes raindrops to form and fall from the skies. It turns out that in big clouds of particles: of dust, water vapor, or nearly anything else, turbulence helps kick heavier particles out of the mix, and concentrates all of them in one place, an effect called preferential concentration. And since water droplets contain billions upon billions of water molecules, they're a lot heavier than gases like Oxygen and Nitrogen. So water droplets quickly collect and form raindrops.

Now, one of the great things about physics is the same laws often apply everywhere, even if you're talking about things on vastly different scales. So even though the clouds in space where stars form are more than 1027 times more massive than a typical cloud in our sky, the effective turbulence is pretty much the same. In interstellar clouds, much of the heavy elements like Carbon and Oxygen are locked up in tiny dust grains that are less than 1/10,000th of a centimeter wide. And because of things like supernova explosions and stellar winds, these interstellar clouds are really turbulent. So they're probably also experiencing preferential concentration, which would cause the heavy dust particles to clump together.

According to Hopkins' new theory, the turbulence in these clouds may create concentrations of dust, and the elements trapped in those particles, right where stars are actually forming. His calculations suggest that on rare occasions this concentration could become so extreme that some stars are made almost entirely of metal. But again we're talking astronomy metal here: a star that doesn't have any Hydrogen or Helium but that is completely made up of Carbon and Oxygen is considered to be "totally metal".

Since these stars would be so different from normal stars which start off made almost entirely of Hydrogen, nobody's run the calculations to see how these weird metal stars would grow and change over time. Not to mention how different planets might be if they form around metal stars. If they can even form around them at all. But if these metal stars still have mass, they could potentially still undergo nuclear fusion; it's just that instead of fusing Hydrogen and Helium like normal stars do, these metal stars could squeeze Helium atoms onto Oxygen atoms and create Neon. So they might not literally be metallic, but a Neon star sounds pretty cool too. 

Some astronomers think that a metal star might resemble the core of an old massive star: extremely hot and blue but relatively faint, because they'd also be small and dense. Hopkins predicts that the effect of preferential concentration could cause about 1 in 10,000 stars to be "totally metal". That means if we look at a young cluster of stars, like the one at the heart of the Orion Nebula, one of the thousands of stars that we see could be, for all astronomical purposes, completely made of metal. And it's possible that we've actually seen these weird stars before, but just didn't know what they were. Some star clusters contain anonymously faint blue stars that we haven't been able to explain. So it's possible that they could be some of these all-metal stars. 

Thanks for joining me for this episode of SciShow Space. If you want to learn how you can help us keep exploring the universe together, go to And don't forget to go to and subscribe. 

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