Neutron star collisions do not create the amount of chemical elements previously assumed, finds a new analysis of galaxy evolution. Research also reveals that current models cannot explain the amount of gold in the cosmos ̵
All the hydrogen in the universe – including every molecule of it on Earth – was created in the Big Bang, which also produced a lot of helium and lithium, but not much else. The rest of the naturally occurring elements are made up of nuclear processes that take place inside stars. Mass controls exactly which elements are forged, but they are all released in galaxies at the last moments of each star – explosive, in the case of really large, or as dense currents, similar to solar winds, for those in the same class as the sun.
“We can think of stars as giant pressure cookers where new elements are created,” explained co-author Associate Professor Karakas of Australia’s ARC Center of Excellence for All Sky Astrophysics in Three Dimensions (ASTRO 3-D).
“The reactions that get these elements also provide the energy that keeps the stars shining for billions of years. As the stars get older, they produce heavier and heavier elements when their interiors heat up.”
Half of all the elements heavier than iron – such as thorium and uranium – were considered to have been made when neutron stars, the super-dense remnants of burned-out suns, crashed into each other. Long theorized neutron star collisions were first confirmed in 2017. Now, however, new analysis by Karakas and other astronomers Chiaki Kobayashi and Maria Lugaro shows that the role of neutron stars may have been significantly overestimated – and that another stellar process is entirely responsible for making most of the heavy elements.
“Fusions of neutron stars did not produce enough heavy elements in the early life of the universe, and they still do not now, 14 billion years later,” Karakas said. “The universe did not make them fast enough to account for their presence in very old stars, and in general there are simply not enough collisions to account for the surface of these elements today.”
Instead, scientists found that heavy elements had to be created by a completely different kind of star phenomenon – unusual supernovae that collapse as they rotate at high speed and generate strong magnetic fields. The finding is one of several that emerge from their research, which has just been published in Astrophysical journal. Their study is the first time that the stars’ origin of all naturally occurring elements from carbon to uranium has been calculated based on the first principles.
The scientists say the new modeling will change the currently accepted model for how the universe evolved. “For example, we built this new model to explain all the elements at once and found enough silver, but not enough gold,” said co-author Associate Professor Kobayashi of the University of Hertfordshire in the UK.
“Silver is overproduced, but gold is underproduced in the model compared to observations. That means we may have to identify a new type of stellar explosion or nuclear reaction.” The study refines previous studies that calculate the relative roles of star mass, age, and arrangement in the production of elements. For example, scientists found that stars smaller than about eight times the mass of the sun produce carbon, nitrogen, and fluorine, as well as half of all elements heavier than iron. Massive stars over eight times the mass of the sun, which also explode as supernovae at the end of their lives, produce many of the elements from carbon to iron, including most of the oxygen and calcium needed for life.
“Apart from hydrogen, there is no single element that can only be formed by one type of star,” Kobayashi explained.
“Half of the carbon is produced from dying stars at low mass, but the other half comes from supernovae. And half of the iron comes from normal supernovae of massive stars, but the other half needs a different shape, known as Type Ia supernovae. These are produced in binary systems with low mass stars. “
Pairs of massive stars bound by gravity, on the other hand, can be converted into neutron stars. When these smash into each other, the shock produces some of the heaviest elements found in nature, including gold.
On the new modeling, however, the numbers simply do not add up.
“Even the most optimistic estimates of the collision frequency of the neutron star simply cannot account for the vast abundance of these elements in the universe,” Karakas said. “This was a surprise. It seems that rotating supernovae with strong magnetic fields are the real source of most of these elements.”
Co-author Dr. Maria Lugaro, who holds positions at the Hungarian Konkoly Observatory and Australia’s Monash University, believes the mystery of the missing gold can be solved quite soon. “New discoveries can be expected from nuclear facilities around the world, including Europe, the United States and Japan, which are currently targeting rare nuclei associated with neutron star fusions,” she said. “The properties of these nuclei are unknown, but they strongly control the production of the surfaces of the heavy element. The astrophysical problem with the missing gold can actually be solved by a nuclear physics experiment.”
The researchers admit that future research may find that neutron star collisions are more frequent than the evidence so far suggests, in which case their contribution to the elements that make up everything from cell phone screens to fuel to nuclear reactors may be revised upward again.
At the moment, however, they seem to be delivering much less money to their bangs.
Simulation of dwarf galaxy reveals different routes to strontium enrichment
Astrophysical journal (2020). DOI: 10.3847 / 1538-4357 / abae65
Provided by the ARC Center of Excellence for All Sky Astrophysics in 3D
Citation: Surprise elements: Neutron stars contribute a little, but something makes gold, research finds (2020, 15 September) retrieved 16 September 2020 from https://phys.org/news/2020-09-elements-neutron-stars-contribute- gold .html
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