Astronomers Find a Star That Contains 65 Different Elements
Universe

Astronomers Find a Star That Contains 65 Different Elements

Have you ever held a chunk of gold in your hand? Not a little piece of jewelry, but an ounce or more? If you have, you can almost immediately understand what drives humans to want to possess it and know where it comes from.

We know that gold is formed by stars. All stars are largely composed of hydrogen and helium. They do, however, include additional elements, which astronomers refer to as a star’s metallicity. Our Sun is highly metallic, containing 67 distinct elements, including around 2.5 trillion tons of gold.

Astronomers have discovered a distant star with 65 elements, the most ever detected in another star. Gold is one of them.

HD 222925 is a somewhat bright star in our Milky Way neighborhood. It’s close to the Tucana (Toucan) constellation in the southern sky. Astronomers refer to it as the “gold standard” star because it provides the best opportunity to investigate how stars produce some of the Universe’s heaviest elements. This is known as the r-process, or rapid neutron capture process.

A recent publication gives a chemical inventory of all the elements created by the r-process for HD 222925. “The R-Process Alliance: A Nearly Complete R-Process Abundance Template Derived from Ultraviolet Spectroscopy of the R-Process-Enhanced Metal-Poor Star HD 222925,” the report says. It is now available online at arxiv.org and will be published in the Astrophysical Journal Supplement Series. Ian Roederer, an astronomer at the University of Michigan, is the study’s principal author.

HD 222295 is an r-process enhanced but metal-poor star.  It has a high metallicity, which means it contains numerous elements other than hydrogen and helium, but not a lot of them. It’s not the first one found. This honor goes to CS 22892–052, commonly known as Sneden’s star after the scientist who discovered 53 chemical elements in it. However, HD 222295 is significantly brighter in UV than Sneden’s star, making spectroscopy much easier. That’s how the researchers identified 65 distinct components.

“To the best of my knowledge, that’s a record for any object beyond our Solar System. And what makes this star so unique is that it has a very high relative proportion of the elements listed along the bottom two-thirds of the periodic table. We even detected gold,” Roederer said in a press release. “These elements were made by the rapid neutron capture process. That’s really the thing we’re trying to study: the physics in understanding how, where and when those elements were made.”

The star HD 222925 is a ninth-magnitude star located toward the southern constellation Tucana. Astronomers detected 65 different chemical elements in the star, which is the most detected in a star other than the Sun, which contains 67 different elements. Image Credit: The STScI Digitized Sky Survey
The star HD 222925 is a ninth-magnitude star located toward the southern constellation Tucana. Astronomers detected 65 different chemical elements in the star, the most detected in a star other than the Sun, containing 67 different elements. Image Credit: The STScI Digitized Sky Survey

The slow neutron capture process, or s-process, and the rapid neutron capture process, or r-process, are the two forms of neutron capture. The s-process is very well understood, but the r-process remains a mystery to scientists. Although astronomers have a strong theoretical knowledge of the r-process, it was not directly observed until 2019 when scientists discovered strontium in the wreckage of a kilonova explosion.

Rapid neutron capture permits an atomic nucleus to capture neutrons faster than they can decay, resulting in the formation of heavy elements. The r-process begins with lighter elements than iron. Because they are neutral and have no charge, these lighter elements can catch neutrons in an environment with a lot of neutrons and a lot of energy. An atom emits an electron when it captures a neutron, transforming the neutron into another proton. The atomic number rises, and the lighter element becomes heavier.

Because the astrophysical sites that promote the r-process are rare, these heavier metals, including rich gold, are rarely identified in stars. “You need lots of neutrons that are free and a very high energy set of conditions to liberate them and add them to the nuclei of atoms,” Roederer said. “There aren’t very many environments in which that can happen—two, maybe.”

This rarity makes the r-process difficult to investigate, and it is also what makes heavier elements, such as gold, rare. It is what distinguishes HD 222295 as the gold standard.

This periodic table from the study shows the elements examined in HD 222925. Elements with no long-lived isotopes are indicated using a light gray font. Helium was previously detected in near-infrared spectra. Image Credit: Roederer et al. 2022.
This periodic table from the study shows the elements examined in HD 222925. Elements with no long-lived isotopes are indicated using a light gray font. Helium was previously detected in near-infrared spectra. Image Credit: Roederer et al. 2022.

One of the conditions that promotes the r-process is neutron star mergers and the subsequent kilonova explosions. The other is the explosion of huge stars known as supernovae. Understanding the r-process requires determining the astrophysical settings that allow it to occur. Astrophysicists are now interested in studying the process in greater depth.

“That’s an important step forward: recognizing where the r-process can occur. But it’s a much bigger step to say, ‘What did that event actually do? What was produced there?’” Roederer said. “That’s where our study comes in.”

The heavy elements in HD 222295 were not created. They were created earlier in the Universe and then spread into space by supernovae or kilonovae. They were then picked up by another generation of star formation, this time by HD 222295.

“We now know the detailed element-by-element output of some r-process event that happened early in the universe,” said study co-author Anna Frebel. Frebel is a physics professor at MIT. “Any model that tries to understand what’s going on with the r-process has to be able to reproduce that,” she said.

Scientists are aware that the r-process is one of the primary means by which stars and their remnants produce heavier elements with atomic numbers larger than 30. Recent observations verified the existence of the r-process in neutron star mergers and the resulting kilonova explosions. However, there are several long-standing unanswered mysteries, such as which elements it creates and in what quantities.

In 2019, a team of European researchers found signatures of strontium formed in a neutron-star merger. This artist’s impression shows two tiny but very dense neutron stars at the point at which they merge and explode as a kilonova. In the foreground, we see a representation of freshly created strontium. Image Credit: ESO/L. Calçada/M. Kornmesser
In 2019, European researchers found signatures of strontium formed in a neutron-star merger. This artist’s impression shows two tiny but dense neutron stars when they merge and explode as a kilonova. In the foreground, we see a representation of freshly created strontium. Image Credit: ESO/L. Calçada/M. Kornmesser

These problems led to the formation of the R-Process Alliance, a group of scientists seeking answers. Some of the authors of this new study are Alliance members. This is the second study released by Alliance members focusing on HD 222295. The researchers believe HD 222295 is part of a group of stars that developed in an r-process-enriched environment. The metallicity of the star is higher than that of most known stars enhanced by the r-process. This shows that it was enriched by several supernovae. HD 222295 did not develop as part of our galaxy, but was caught by it at some point in the past.

“HD 222925 exhibits no remarkable characteristics in its chemical abundance pattern, other than the overall enhancement of r-process elements,” the authors write. “Thus, it may be considered as reflecting the yields of the dominant r-process source(s) in the early universe.”

Now that astrophysicists have discovered a bright star with r-process components, it can serve as a proxy for what supernovae and kilonovae create. Researchers must build models of the r-process that generates the heavy elements within these events, and such models must have the same signature as HD 222295. As a result, it is known as the gold standard.

Gold has always maintained a special fascination for mankind. It is unique among the elements and appears frequently in mythology around the world. In ancient Greece, the Gods dressed in gold and golden apples conferred mortality on those who ate them if they could get past the dragon that guarded them. In Hindu mythology, gold is a source of power and has the ability to transfer heavenly consciousness. It is also the soul of the universe.

The ancients treasured gold, but they couldn't have guessed at its origins. King Tut's mask (l) and his inner coffin. The inner coffin is solid gold and weighs almost 243 pounds. Image Credits: Mask: By Roland Unger - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=48168958. Inner Coffin: Egyptian Museum, Cairo.
The ancients treasured gold, but they couldn’t have guessed at its origins. These are images of King Tut’s mask (l) and his inner coffin (r). The inner coffin is solid gold and weighs almost 243 pounds. Image Credits: Mask: By Roland Unger – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=48168958. Inner Coffin: Egyptian Museum, Cairo.

Those beliefs are wiped away now, lost to time.  But the science that will take their place is much more fascinating. The ancients could never have predicted that science would replace their mythologies and that stars would burst, creating gold and other metals. They could never have imagined massive mountain-top telescopes peering great distances into space. They could never have thought that we would be able to split apart the light of a star and determine that the star contains gold.

And they had no idea that our own Sun contained 2.5 trillion tons of gold.

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