This record-breaking Gold Standard star is unlike anything we’ve seen before

This record-breaking Gold Standard star is unlike anything we’ve seen before

What’s in a star? Well, if you’re an advanced specimen called HD 222925 nearing the end of its life, quite a lot actually.

Scientists conducted an analysis of this faint object and identified 65 separate elements. These are the most elements ever found in a single object outside the solar system, and most of them are heavy elements from the end of the periodic table that are rarely found in stars.

Because these elements can only form during extremely energetic events like supernovae or neutron star mergers via a mechanism called fast neutron capture, this star’s composition could provide a means to learn more about how heavy elements are formed.

“To the best of my knowledge, this is a record for any object outside our solar system. And what makes this star so unique is that it has a very high relative proportion of the elements listed in the bottom two-thirds of the periodic table. We even discovered gold,” said University of Michigan astronomer Ian Roederer.

“These elements were made by the fast neutron capture process. That’s really what we’re trying to study: the physics of understanding how, where and when these elements were made.”

Stars are the factories that produce most of the elements in the universe. In the early Universe, hydrogen and helium — still the two most abundant elements in the cosmos — made up pretty much all matter.

The first stars formed as gravity pulled clumps of this hydrogen and helium together. In the fusion furnaces of their cores, these stars forged hydrogen into helium; then helium in carbon; and so on, merging heavier and heavier elements as they run out of the lighter ones, until iron is produced.

Iron can fuse, but it consumes huge amounts of energy – more than such a fusion produces – so an iron core is the endpoint. The core, no longer supported by the outward pressure of fusion, collapses under gravity and the star explodes.

To create elements heavier than iron, the fast neutron capture process or r-process is required. Truly energetic explosions produce a series of nuclear reactions in which atomic nuclei collide with neutrons to synthesize elements heavier than iron.

“You need a lot of free neutrons and very high-energy conditions to release them and add them to the atomic nuclei,” Roederer said. “There aren’t many environments where that can happen.”

That brings us back to HD 222925, which is about 1,460 light-years away, which is certainly a bit strange. It has passed the red giant stage of its lifetime, having run out of hydrogen to fuse, and is now fusing helium in its core. It is also a so-called “metal-poor” star, poor in heavier elements…but extremely enriched in elements that can only be made by the r-process.

Thus, r-process elements were somehow distributed in the hydrogen-helium molecular cloud that formed HD 222925 about 8.2 billion years ago. That “somehow” must have been an explosion that threw the R-process elements into space.

The next question is: Which elements? And this is where HD 222925 comes in handy. We already knew that the star was rich in R-process elements. Roederer and his team used spectral analysis to narrow down exactly which ones are in it. This is a technique based on splitting the wavelength of light from a star into a spectrum of wavelengths.

Certain elements can either amplify or attenuate certain wavelengths of light as the atoms absorb and re-emit photons. These emission and absorption features in the spectrum can then be analyzed and traced back to the elements that produced them and identify their abundance. Of the 65 items the team identified in this way, 42—almost two-thirds—were R-process items.

These include gallium, selenium, cadmium, tungsten, platinum, gold, lead and uranium. Since HD 222925 has no other distinctive features in its chemical composition, this means that we can consider it representative of the yields produced by the r-Process source.

Although we don’t know if the r processes that produced these elements occurred in a neutron star collision or a violent supernova, the level of detail we now have means the star can be used as a kind of blueprint for understanding the output of the r -process.

“We now know the detailed element-by-element output of an r-process event that occurred early in the Universe,” said MIT physicist Anna Frebel.

“Any model trying to understand what’s going on with the r-process has to be able to reproduce that.”

The research was accepted The Astrophysical Journal Supplement Seriesand is available on arXiv.

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