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 comes from stars. All stars are comprised primarily of hydrogen and helium. But they contain other elements, which astrophysicists refer to as a star’s metallicity. Our Sun has a high metallicity and contains 67 different elements, including about 2.5 trillion tons of gold.
Now astronomers have found a distant star that contains 65 elements, the most ever detected in another star. Gold is among them.
There’s a fairly bright star in our neighbourhood of the Milky Way named HD 222925. It’s close to the southern sky’s Tucana (Toucan) constellation. Astronomers are calling it the “gold standard” star because it’s their best opportunity to study how stars create some of the heavy elements in the Universe. That process is called the r-process, or rapid neutron capture process.
A new paper presents a chemical inventory for HD 222925 of all the elements produced by the r-process. The paper is “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.” It’s available online at the pre-press site arxiv.org and will be published in the Astrophysical Journal Supplement Series. The lead author is Ian Roederer, an astronomer at the University of Michigan.
HD 222295 is an r-process enhanced but metal-poor star. It has high metallicity, meaning it contains many elements other than hydrogen and helium, but not much of those elements by mass. It’s not the first one discovered. That distinction belongs to CS 22892–052, also known as Sneden’s star, after the scientist who first identified 53 chemical elements in it. But HD 222295 is much brighter in UV than Sneden’s star, making it much easier to observe spectroscopically. That’s how the researchers were able to identify 65 different elements.
“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, the most detected in a star other than the Sun, containing 67 different elements. Image Credit: The STScI Digitized Sky Survey
There are two types of neutron capture: the slow neutron capture process, or s-process, and the r-process. The s-process is reasonably well-understood, but scientists still have significant questions about the r-process. Astrophysicists have a good theoretical understanding of the r-process, but it wasn’t observed directly until 2019 when observers saw strontium in the wreckage of a kilonova explosion.
Rapid neutron capture allows an atomic nucleus to capture neutrons quicker than the neutrons can decay, creating heavy elements. The r-process begins with elements lighter than iron. In an environment with lots of neutrons and lots of energy, these lighter elements can capture neutrons since they’re neutral and have no charge. When an atom captures a neutron, it emits an electron,
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