Scientists conducted an analysis of this opaque object, and identified 65 separate components. This is the largest number of elements found in a single body outside the solar system, and most of them are heavy elements from the bottom of the periodic table, rarely found in stars.
Since these elements can only form in highly energetic events such as supernovae or neutron star mergers, via a mechanism called the fast neutron capture process, the formation of this star could be a way to learn more about how heavy elements form.
“To my knowledge, this is a record of any object outside our solar system. What makes this star so unique is that it has a very high percentage of the elements listed along the bottom two-thirds of the periodic table. We’ve even discovered gold,” said astronomer Ian Roderer of the University of Michigan. .
“These elements are made through a process of rapid neutron capture. That’s what we’re really trying to study: the physics of understanding how, where and when these elements are made.”
The stars are the factories that produce most of the elements in the universe. In the early universe, hydrogen and helium – the two most abundant elements in the universe – made up pretty much all matter.
The first stars formed when gravity held clumps of hydrogen and helium together. In the nuclear fusion furnaces in their cores, these stars formed hydrogen into helium. Then helium into carbon. Thus, the heavier and heavier elements are combined as the lighter ones run out until iron is produced.
Iron can fuse, but it takes up huge amounts of energy – more than would be produced by that fusion – so the iron core is the end point. The core, no longer supported by the outward pressure of fusion, collapses under gravity, and the star explodes.
To create elements heavier than iron, a rapid neutron capture or r process is required. Really energetic explosions produce a chain of nuclear reactions in which atomic nuclei collide with neutrons to form elements heavier than iron.
“You need a lot of free neutrons and a very high energy set of conditions to liberate them and add them to the nuclei of atoms,” Roderer said. “There aren’t a lot of environments in which that could happen.”
This brings us back to HD 222925, which lies about 1,460 light-years away, and is definitely a bit of a weird ball. It has passed its red giant stage of life, having run out of hydrogen to fuse, and is now fusing helium in its core. It is also what is known as a “metal-poor” star, which is low in heavy elements…but so rich in elements that it can only be produced through the r-process.
Therefore, the elements of the r process were somehow distributed across the molecular cloud of hydrogen and helium from which HD 222925 was formed, about 8.2 billion years ago. This must “somehow” be an explosion that sprayed process r elements into space.
Next question: What are the elements? And this is where the HD 222925 comes in handy. We already knew that the star was rich in r-process elements. Roederer and his team used spectroscopy to narrow down exactly what it contained. This is a technique based on dividing the wavelength of light from a star into a spectrum of wavelengths.
Some elements can either enhance or darken certain wavelengths of light, as photons are absorbed and re-emitted by atoms. The emission and absorption features in the spectrum can then be analyzed and traced back to the elements that produced them, and their abundance determined. Of the 65 items the team identified in this way, 42—nearly two-thirds—were r process items.
They include gallium, selenium, cadmium, tungsten, platinum, gold, lead, and uranium. Since HD 222925 does not show any anomalies in its chemical composition, this means that we can consider it to be representative of the yields produced by the r process source.
Although we don’t know whether the r-processes that produced these elements occurred in a neutron star collision or violent supernova, the level of detail we now have means that the star can be used as a kind of blueprint to understand the output from the r-process.
“We now know the detailed outputs of each component of the r-process events that occurred in the early universe,” said physicist Anna Freibel of the Massachusetts Institute of Technology.
“Any model that attempts to understand what happens in process r should be able to reproduce that.”
The search has been accepted The Astrophysical Journal Supplement Seriesavailable at arXiv.