Almost all of the elements that make up the human body, except hydrogen, are formed in the interiors of stars and in supernova explosions. Physicists still describe the exact course of these nuclear reactions only approximately. Two new studies have filled in part of this gap. The amount of radioactive titanium produced during an explosion turned out to be one-third greater than calculations had predicted.
A Century of Observations Without the Details
Astronomers have been recording supernova explosions for several centuries. Despite this, the process itself is understood only in broad terms.
The issue is not the brightness or shape of the remnant, but the nuclear reactions taking place inside. These determine which chemical elements are expelled and in what quantities.
The Marker Used for Counting
A useful reference point for these calculations is the radioactive isotope titanium-44. It is produced during the explosion itself and decays slowly, so it remains long after the visible glow fades. Its half-life is about 60 years.
Until now, the amount of this material produced had been estimated theoretically. A laboratory measurement showed that there is 35% more of it than expected. The study was published in the July issue of the peer-reviewed journal Physical Review Letters.
Gamma radiation from the isotope is still being detected in the roughly 350-year-old supernova remnant Cassiopeia A. An error of about one-third means that these data will have to be interpreted again.
Where Oxygen Comes From
According to estimates from the Chandra X-ray Observatory, explosions of massive stars produced nearly all of the oxygen in the human body, about half of the calcium in our bones, and roughly 40% of the iron in our blood. The rest of the iron was formed in the deaths of white dwarfs, while carbon and nitrogen came from stars of moderate mass.
In the same Cassiopeia A remnant, astronomers found phosphorus, without which neither DNA nor bone tissue could exist. In other words, the object now being used to refine estimates of titanium production has already served as evidence that the elements essential for life really are formed in explosions of massive stars.
Explosions on the Surfaces of Neutron Stars
The second study, reviewed by Space.com, appeared in the same issue and concerns a different type of phenomenon. These occur in binary systems where the gravity of a neutron star pulls material away from a neighboring star.
The compact remnant contains between one and two solar masses compressed into a sphere only about 20 kilometers in diameter. Matter reaches its surface and triggers a thermonuclear flash in which heavy elements are also formed.
Trapping Nuclear Material
Physicists at the Facility for Rare Isotope Beams (FRIB) in Michigan examined the reaction that drives the X-ray emission from these events in greater detail than anyone had before. The main question concerned the so-called nickel-copper cycle, in which nuclear material is temporarily trapped.
Until now, it was not known whether this actually occurred. The experiment confirmed the effect, although the fraction of trapped material turned out to be small.
Christopher Cousins of the nuclear physics group at the University of Surrey notes that a measurement like this would have been considered unattainable only a few decades ago. His colleague Gavin Lotay adds that the nuclear reactions taking place in the most spectacular stellar explosions are still only partly understood.
Both studies make it possible to compare computer models more closely with what astronomers observe in real supernova remnants. The nearest test case is Cassiopeia A, where gamma radiation from titanium-44 has been measured for more than two decades.
