What’s in a star? Well, if you’re a highly evolved specimen nearing the end of its life named HD 222925, that’s quite a lot, actually.
Scientists conducted an analysis of this dark object and identified 65 distinct 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 form during extremely energetic events such as supernovae or neutron star mergers, through a mechanism called the fast neutron capture process, the composition of this star could be a way to learn more about the formation of heavy elements.
“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 the elements listed in the lower two-thirds of the periodic table. We even detected gold,” said astronomer Ian Roederer of the University of Michigan.
“These elements were made by the process of fast neutron capture. That’s really the thing we’re trying to study: the physics to understand 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 nearly all matter.
The first stars formed when gravity pulled together clumps of hydrogen and helium. In the nuclear fusion furnaces of their cores, these stars transformed hydrogen into helium; then helium to carbon; and so on, fusing heavier and heavier elements as they run out of lighter elements until iron is produced.
Iron can fuse, but it consumes huge amounts of energy – more than such fusion produces – so an iron core is the end point. The core, which is no longer supported by the external pressure of fusion, collapses under the effect of gravity and the star explodes.
To create elements heavier than iron, the fast neutron capture process, or r-process, is needed. 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 a set of very high energy conditions to release them and add them to the nuclei of atoms,” Roederer said. “There aren’t many environments in which that can happen.”
This brings us back to HD 222925, located about 1,460 light years away, which is certainly a little weird. It has passed the red giant stage of its life, having run out of hydrogen to fuse with, and is now fusing helium in its core. It’s also called a “metal-poor” star, poor in heavier elements… but extremely rich in elements that can only be produced by the r-process.
Therefore, the r-process elements had somehow been distributed in the molecular cloud of hydrogen and helium from which HD 222925 formed, about 8.2 billion years ago. ‘years. This “somehow” must have been an explosion that shattered the r-process elements into space.
The next question is: what 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 narrow down precisely which ones it contains. It is a technique that relies on dividing the wavelength of light from a star into a spectrum of wavelengths.
Certain elements can enhance or attenuate specific wavelengths of light because 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 abundances. Of the 65 items the team identified in this way, 42 – nearly two-thirds – were r-process items.
These include gallium, selenium, cadmium, tungsten, platinum, gold, lead and uranium. Since HD 222925 exhibits no other oddities 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 whether the r-processes that produced these elements took place during a neutron star collision or a violent supernova, the level of detail we now have means that the star may be used as a kind of model to understand 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 physicist Anna Frebel of MIT.
“Any model that tries to understand what is happening with the r-process must be able to reproduce it.”
The search was accepted in The Astrophysical Journal Supplement Seriesand is available on arXiv.