Record-Breaking Star Is A New “Gold Standard” For The Milky Way

Star HD 222925 is a cool, unassuming star in the southern constellation of Tucana. It might appear to be just another of the 100 billion stars in the Milky Way, but new analysis has revealed it’s very unique. It has the widest range of elements found in a star beyond the solar system.

The researchers found 65 different elements present in the star and 42 of them are heavy metals such as selenium, silver, tellurium, platinum, gold and thorium. These elements are rarely found in stars, and astronomers still don’t have an accurate model of how they formed. But, as noted in The Astrophysical Journal Supplement Series, the existence of HD 222925 may well change that.

“To the best of my knowledge, this is 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 elements listed in the lower two-thirds of the periodic table. We even detected gold,” University of Michigan astronomer Ian Roederer, who led the study, said in a statement. “These elements were made by the fast neutron capture process. That’s really the thing we’re trying to study: physics to understand how, where and when these things were made.

Heavy elements are created by the so-called “r-process”, or fast neutron capture process. It is a set of nuclear reactions responsible for creating about half of the naturally occurring elements in the periodic table that are heavier than iron. Iron is the heaviest element produced by standard nuclear fusion.

The r-process requires a lot of energy and it has been proposed that certain types of supernovae can provide this type of energy, but models have always lacked something. In 2017, the first direct observation of a neutron star collision suggested that these cataclysmic events may also create r-process elements.

And that’s where HD 222925 is key. Its abundance of heavy elements can be considered a gold standard. The correct model, favoring one scenario (a massive supernova), the other (a neutron star merger), or a combination of the two, should be able to explain what astronomers observed in that particular star.

“We now know the detailed element-by-element output of an r-process event that occurred early in the universe,” added co-author Professor Anna Frebel of the Massachusetts Institute of Technology. “Any model that tries to understand what is happening with the r-process must be able to reproduce it.”

This work is a solid step towards finally understanding how and where the r-process occurs.

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