by The secrets of supernova star explosions may be hidden in the dust scattered across the moon – and a team of scientists from the China Institute of Atomic Energy (CIAE) has devised a new way to unlock those clues of stellar death.
The research could help scientists get a clearer picture of how stars die and provide material for the next generation of stars, planets, moons, and sometimes even life – at least, when it comes to Earth.
The technique relies on improved detection of rare iron isotopes found in infinite amounts within lunar dust. This form of iron was created millions of years ago in the cores of previous generations of massive stars. When these stars lost their “tug of war” battles with gravity that went on for millions (or billions) of years, ending their lives in supernova explosions, the isotopes would have been released and spread throughout the cosmos – including, according to scientists. , on the moon.
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“Our team agreed that the only way to accurately track historical supernova events was to push the limits of what our equipment could do,” team leader and CIAE researcher Bing Guo said in a statement.
How supernovae contribute to cosmic recycling
When the first generation of stars was born between 200 million and 400 million years after the Big Bang, the universe was populated mostly by hydrogen, with sprinklings of helium. At this time, there were very few atoms of elements heavier than this, which astronomers (somewhat confusingly) call “metals.”
That means the first stars, paradoxically known as Population III stars, were composed of hydrogen, a little helium, and hardly any metals. As these stars lived, the nuclear fusion processes in their cores, by which they turned hydrogen into helium, allowed them to shine brightly in the cosmos. This fusion process also provided the outward radiation pressure that prevented the inward force of their own gravity from collapsing.
However, this meant that when the hydrogen ran out in the cores of these stars, the balancing act between radiation pressure and gravity ended, with the latter being the clear winner. Thus, the cores of these stars collapsed and their outer layers, where nuclear fusion was still occurring, were blown away.
For stars with solar masses around them, their stellar cores result in white dwarf stars, surrounded by a diffuse and cooled cloud of single-stellar material. That is not the fate of stars with at least eight times the mass of the Sun, however.
When these massive stars collapse, the pressure generated in their cores triggers the nuclear fusion of helium with other heavier elements. For the most massive stars, this process is repeated until the core is filled with iron, the heaviest element that can forge a star.
After that, the core of a massive star will collapse again, triggering a supernova explosion. That explosion, in turn, releases all the elements that the star has generated throughout its life and scatters them throughout the surrounding galaxy. The star then becomes a dense stellar remnant — a neutron star or, in the case of total gravity, a black hole.
That is not the end of the elements that the star has created during her life, however. These materials find their way to interstellar clouds of gas and dust, which can eventually collapse into stars and birth planets.
That is how the subsequent stellar generations become more and more “metal rich” as time goes on. All the scattered matter is also integrated into young planets orbiting these stars and any life forms that may exist on those worlds. So when scientists say “you’re star stuff,” it’s more than just lip service; it is a fact.
The lunar dust tracking team is interested in a tracer of this cosmic recycling process that is not an element created during the star’s lifetime but a rare isotope created during a supernova.
Iron-60: From supernova to the moon
Atoms are made up of three particles: in the atomic nucleus, positively charged protons and neutral neutrons live, and this nucleus “orbits”, negatively charged electrons zip around.
Elements are defined by the number of protons within their atomic nucleus. Therefore, an atom with six protons in its nucleus is always carbon. Add another proton, and it becomes a nitrogen atom. However, elements have more flexibility in the number of neutrons within their nucleus.
A carbon atom can have six protons and six neutrons, or six protons and seven neutrons, or six protons and eight neutrons. These different variations of atoms of the same element are called “isotopes” of that element. A carbon atom with six protons and six neutrons is called “carbon-12” and the carbon isotope “carbon-14” is a carbon atom with six protons and seven neutrons.
Some of these isotopes, especially the heavier ones, are unstable and undergo a process known as radioactive decay. The time it takes half a given amount of a radioactive isotope to decay is called a “half-life”.
When a supernova erupts, in a few seconds, they release as much energy as it will take to radiate the sun in billions of years. This provides the necessary conditions for the creation of heavy radioactive isotopes. The team is trying to improve ways to hunt for a radioactive isotope of iron called “iron-60” in lunar dust.
Iron-60 has an atomic nucleus containing 26 protons and 34 neutrons, and its half-life is about 2.3 million years. Although a supernova can create iron-60 in amounts equal to about 10 times the mass of Earth, however, the production of this isotope within the solar system is negligible. Scientists predict that a supernova occurs three times every 100 years in the entire Milky Way, and that “nearby” star explosions occur more frequently, once every a million years.
Finding iron-60 on Earth, or the moon, is a good indicator that a supernova has erupted relatively close to the solar system — say, within about 100 light years — in the recent history of our 4.6 billion-year-old planet. , says the research team.
However, due to the scarcity of iron-60 and the effect of other more common interfering elements, it is extremely challenging to detect its low abundance for a low-sensitivity spectrometer. To combat this, Guo and his colleagues made adjustments to CIAE’s HI-13 tandem accelerator facility. This involved adding a “Wein filter”, a device that can be used to select charged particles traveling at specific speeds, to conduct “accelerator mass spectrometry” (AMS).
The team found that AMS was able to detect iron-60 in simulated samples with a sensitivity far beyond what can be achieved with the technology typically used for these studies.
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The CIAE team believe it is now possible to push the detection sensitivities of their AMS system even further, a development that could greatly improve our understanding of stars that died in supernova explosions, so that we could survive.
“The installation of the Wien filter could be a game changer for us,” Guo said. “Our next goal is to optimize our entire AMS system to achieve even lower detection limits. Every bit of increased sensitivity opens up a world of possibilities.”
The team’s research was published on 24 May in the journal Nuclear Science and Techniques.
