(ORDO NEWS) — What is in a star? Well, if you’re a highly evolved individual called HD 222925 in the twilight of your life, then quite a lot.
Scientists analyzed this dim object and identified 65 individual elements. This is the largest number of 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, rarely found in stars.
Since these elements can only be formed in extremely energetic events such as supernovae or neutron star mergers, through a mechanism called fast neutron capture, the composition of this star could be a vehicle to learn more about how heavy elements are formed.
“To my knowledge, this is a record for any object outside our solar system. And what makes this star so unique is the very high relative proportion of elements listed in the bottom two-thirds of the periodic table. We even found gold,” says astronomer Yang. Roederer from the University of Michigan.
“These elements were produced by the fast neutron capture process. That’s really what we’re trying to study: physics in understanding how, where and when these elements were produced.”
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 space – made up virtually all matter.
The first stars were formed as a result of the gravitational attraction of bunches of hydrogen and helium. In the fusion furnaces of their cores, these stars converted hydrogen into helium, then helium into carbon, and so on, fusing heavier elements as the lighter ones ran out, until iron was formed.
Iron can be melted, but this consumes a huge amount of energy – more than is released during this melting – so the iron core is the end point. The core, no longer supported by the external pressure of fusion, collapses under the influence of gravity, and the star explodes.
To create elements heavier than iron, a fast neutron capture process, or r-process, is needed. Really energetic explosions set off a series of nuclear reactions in which atomic nuclei collide with neutrons to fuse elements heavier than iron.
“You need a lot of free neutrons and a very high-energy set of conditions for them to be released and attached to the nuclei of atoms,” Roderer said. “There are not many conditions in which this can happen.”
This brings us back to HD 222925, about 1460 light-years away, which is definitely a little odd. It has already passed the stage of a red giant, it has run out of hydrogen for fusion, and now helium fusion is taking place in its core. It is also a so-called “metal-poor” star, low in heavy elements… but extremely enriched in elements that can only be produced in the r-process.
Therefore, elements of the r-process were somehow distributed throughout the molecular cloud of hydrogen and helium from which HD 222925 formed, approximately 8.2 billion years ago. This “somehow” was supposed to be an explosion that spewed elements of the r-process into space.
The next question is: what elements? This is where HD 222925 comes in handy. We already knew that the star is rich in r-process elements. Roederer and his team used spectral analysis to determine exactly what elements it contained. This method is based on dividing the wavelength of light from a star into a spectrum of wavelengths.
Certain elements can either amplify or attenuate certain wavelengths of light as 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 quantified. 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. Because HD 222925 shows no other oddities in its chemistry, this means we can consider it representative of a crop derived from an r-process source.
Although we do not know whether the r-process that produced these elements occurred in a neutron star collision or in a violent supernova, the level of detail we now have means that the star can be used as a sort of blueprint for understanding the r output. -process.
“Now we know the detailed element by element result of some r-process event that occurred early in the universe,” said physicist Anna Fröbel of the Massachusetts Institute of Technology.
“Any model that tries to understand what’s going on with the r-process should be able to reproduce it.”
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