Where Do Heavy Metals Come From? (Not Ozzy Osborne!)

T​he University of Guelph is a short drive from us. In fact, we can phone them without paying Long Distance. We mention this because a researcher who joined that institution this month has made a breakthrough finding. He has shed new light on the origin of the heavy metals we find in the Earth’s crust.

We know that all the elements in the universe come from star stuff. Stars like our sun routinely convert hydrogen into helium, for example. But those are the two lightest elements. Stars generate heavier elements, from beryllium up to iron though the same kind of fusion process deep in their cores.

At the end of a star’s life, its core turns to iron. It becomes so dense that it explodes. This explosion generates about half of all the heavier elements we find throughout the universe, including here on earth. Everything heavier than iron, from gold up to uranium comes from these explosions.

University of Guelph physics professor Dr. Daniel Siegel has written a research paper along with colleagues from Columbia University. The journal Nature published it last week. In it, he provides a new explanation of how the heavy metals formed. Until now, scientists believed that the heaviest elements came from collisions between neutron stars, or from collisions between neutron stars and black holes. Dr. Siegel believed that himself. Yet, when his team tried to study these collisions they came up with a better explanation.

Based on the team’s work, they now think that eighty percent of heavy metals form from a special kind of supernova explosion. A supernova happens when a massive star reaches the end of its life and vanishes in an indescribably powerful explosion. From time to time we can see a supernova here on earth. It seems to us that a bright new star has appeared all at once in the night sky. We now realize that it’s an aging star that has died. The bright object fades from view over the weeks or months that follow.

Scientists have found an often neglected kind of supernova called a collapsar. Collapsar is short for “collapsed star”. Collapsars result from old, massive stars that are in the neighborhood of thirty times the mass of our sun. Because of their enormous mass, collapsars end up forming a black hole when they die.

Today’s supercomputers allowed the research team to simulate how a collapsar behaves. Their computer model shows collapsars spinning at high speeds and throwing off the elements we consider heavy metals. The most interesting part is that the quantity and arrangement of these elements are consistent with what we see in our own solar system.

This empirical evidence aligns with the view that this is the source of the majority of the heavy metals we mine here on earth. This includes gold, platinum, uranium and plutonium. It also includes rare earth elements like neodymium that go into our smart phones and tablets.

Collapsars are rare because most stars are not massive enough to form them. When they do happen, they make up for their scarcity by throwing off enormous volumes of heavy elements. Incidentally, they also give off intense flashes of gamma rays.

The team needs to confirm their model definitively with more empirical data. The plan is to use the new James Webb Space Telescope to do this. It will launch in 2021 and will be equipped with infrared instruments. The team expects it to be able to detect the kind of radiation given off by heavy elements form a collapsar. This should work even if the collapsar happens in another galaxy.

If confirmed, this study will do more than explain where our heavy metals come from. It will also help to explain the formation of our galaxy itself. Cosmologists use heavy elements to trace the larger questions in their field around the origin of the universe.

We didn’t discuss the kind of music we hear from Metallica or Black Sabbath in this post. Even so, we think this research team’s discovery rocks!

Daniel M. Siegel, Jennifer Barnes, Brian D. Metzger. Collapsars as a major source of r-process elements. Nature, 2019; 569 (7755): 241 DOI: 10.1038/s41586-019-1136-0

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