Scientists Search for Origins of 'Star-Stuff' We Are Made From

Two teams of scientists may have found a way to discover where the elements that compose stars, planets, and even us, came from.

While physicists have known for some time that the lighter elements of the universe are created by nuclear processes inside stars, and that they then spread throughout galaxies when these stars go supernova at the end of their lives, the origins of heavier elements have remained mysterious.

These nuclear fusion processes are only suitable for explaining the creation of elements as heavy as iron.

During the 2021 Fall Meeting of the APS Division of Nuclear Physics, taking place from October 10 to 14, two separate teams of researchers aim to unveil new measurements that could explain the birth of half the universe's elements.

When Carl Sagan said "The cosmos is within us. We are made of star-stuff. We are a way for the universe to know itself," the American astronomer, planetary scientist, cosmologist, astrophysicist, astrobiologist and author was not using hyperbole.

Every element that makes up our bodies was created in the nuclear furnaces that are stars, with lighter elements like hydrogen and helium fused into the carbon that makes up our cells, the oxygen we breathe, and the calcium in our teeth.

When these stars exhausted their fuel, the end of their lives was signaled by massive supernovas that spread these elements through their galaxies, and into the wider universe.

The consequence of this is, a carbon atom in the tip of your finger, and one in your nose may now be separated by no more than a foot or two, but they could have once been found billions of light-years apart, at opposite sides of the universe. They will come together, just once, to make up the conglomeration of atoms that is "you."

While each subsequent generation of stars is made up of material created by its predecessors in nuclear processes, thus becoming composed of heavier and heavier elements, there is a limit to the atomic mass of an element that the usual nuclear processes of a star can create.

Stars can only force together enough light elements to create iron. That means, that as of now, we aren't quite sure how elements heavier than iron are made. That's around half of the elements in the periodic table.

Supernova
A composite of images taken by the Hubble Space Telescope shows a supernova that exploded 8,000 years ago. Explosions like this seeded the universe with elements created by stars, but the origins of heavy elements are difficult to explain. HHT/NASA/ESA

One theory suggests elements heavier than iron form when atoms are bombarded by fast-moving neutrons, neutral particles normally found inside the atomic nucleus with positively charged protons. The atom absorbs these neutrons, changing the number of protons it contains turning it into a heavier element.

For this mechanism of heavy element synthesis—called the r-process—to occur there must be an environment filled with an excess of free neutrons, moving at incredible speeds.

That's why NASA Hubble Fellow at the Carnegie Observatories researcher, Erika Holmbeck, and her team's line of inquiry focused on stellar remnants that are left behind by the gravitational collapse of stars that have exhausted their fuel, neutron stars.

Objects that just so happen to take their name from the excess of free neutrons in the dense matter that forms their cores.

"The densest form of luminous matter in the universe exists in neutron stars: the final stopping point in the lives of certain stars much more massive than the sun," Holmbeck said.

This team used the Neutron star Interior Composition Explorer (NICER) aboard the International Space Station to study these stellar remnants and the heavy-element abundance of other stars.

The abundance of neutrons suggests that the cores of neutron stars could be the ideal place for neutron capture, or the r-process, to occur. Heavy element generation through the r-process could be especially favored in situations where binary neutrons stars spiral together and merge.

Bringing the investigation down to earth, the lab-based researchers investigated arrangements of atoms called isomers. When atoms are stacked and ordered in different ways it can give an element a different property.

An example of this is diamond and graphene, both are made up of carbon, but are different isomers, giving them different appearances and very different properties.

The team focused on a type of isomer called an "astromer"—an arrangement of atoms that can last for an unusually long time in the hottest regions of space. The team believed that these astromers might react differently than usual arrangements of atomic nuclei, like those found here on Earth, and could play an important role in the r-process.

Despite taking very different approaches to the problem of heavy element synthesis and the r-process, both the laboratory team and the group that used astronomical observations found decent agreement between their results.

In fact, both studies' combined results may have actually provided astronomers with a new equation state to describe what is occurring beneath the surface of neutron stars.

"Although this approach is drastically different from other methods, we surprisingly find agreement with both NICER measurements and theory calculations about the structure of these exotic stars," Holmbeck concluded. "The results also simultaneously explain the origin of the heaviest elements found in our solar system."

Neutron Star
A NASA illustration of a neutron star. A team of scientists has researched the possibility that the universe's heavy elements could be created in the neutron rich cores of these stellar remnants. Dana Berry/NASA