What’s in a star? Well, if you’re a highly evolved specimen nearing the end of its life, called HD 222925, quite a lot actually.
Scientists have analyzed this fuzzy object and identified 65 individual elements. That’s the most elements ever found in a single object outside the solar system, and most of them are heavy elements from the bottom of the periodic table, which are rarely found in stars.
Since these elements can only form during extremely energetic events such as supernovas or neutron star mergers, through a mechanism called the fast neutron capture process, the composition of this star could provide a means of learning more about how heavy elements form. .
“To my knowledge, that’s 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 that line the lower two-thirds of the periodic table. We have even discovered gold,” said University of Michigan astronomer Ian Roederer.
“These elements were created by the rapid neutron capture process. That’s really what we’re trying to study: the physics to understand how, where and when those 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 virtually all matter.
The first stars formed when 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, fusing heavier and heavier elements together while the lighter ones run out until iron is produced.
Iron can melt, but it consumes enormous amounts of energy — more than that fusion produces — so an iron core is the end point. The core, no longer supported by the external fusion pressure, collapses under gravity and the star explodes.
To make elements heavier than iron, the fast neutron capture process, or r process, is required. True energetic explosions trigger a series of nuclear reactions in which atomic nuclei collide with neutrons to synthesize elements heavier than iron.
“You need a lot of neutrons that are free and a very high energy set of conditions to free them and add them to the nuclei of atoms,” Roederer said. “There aren’t many environments where that can happen.”
This brings us back to HD 222925, located about 1460 light-years away, which is certainly a bit of an odd one out. It has passed the red giant stage of its life, has run out of hydrogen to fuse, and is now melting helium in its core. It’s also what’s known as a “metal-poor” star, poor in heavier elements… but extremely enriched in elements that can only be produced by the r process.
Therefore, somehow r-process elements were scattered throughout the molecular cloud of hydrogen and helium from which HD 222925 was formed, about 8.2 billion years ago. That “somehow” must have been an explosion that spewed the r-process elements into space.
The next question is: which elements? And that’s 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 determine exactly which one it contains. This is a technique based on splitting the wavelength of a star’s light into a spectrum of wavelengths.
Certain elements can amplify or dim specific wavelengths of light as the atoms absorb and re-emit photons. Those emission and absorption features in the spectrum can then be analyzed and traced to the elements that produced them, and identify their abundances. Of the 65 elements the team identified in this way, 42 — nearly two-thirds — were r-process elements.
These include gallium, selenium, cadmium, tungsten, platinum, gold, lead and uranium. Since HD 222925 shows no other idiosyncrasy in its chemical composition, this means that we can consider it representative of the yields produced by the r process source.
While we don’t know whether the r-processes that produced these elements occurred in a neutron star collision or a violent supernova, the level of detail we have now means the star can be used as a sort of blueprint for understanding the output of the r process.
“We now know the detailed output by element 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 should be able to reproduce that.”
The study was accepted in The Astrophysical Journal Supplement Seriesand is available on arXiv.