Physicist Camilla Hansen of the University of Heidelberg in Germany and colleagues reached the conclusions after measurements of various heavy stars. These in turn allowed for a reconstruction of how elements were formed within them. The research is published in the September issue of the journal Astronomy & Astrophysics.
All elements, except for a handful of the lightest ones, are produced inside stars, either during their normal lives or around the time of the explosions that they undergo as they run out of fuel. This production of elements—including metals—occurs because the process that provides energy for stars, called fusion, involves combining lighter elements to make heavier ones.
Each generation of stars therefore contributes a little to enriching the universe with chemical elements. The elements a star can generate in its lifetime depend largely on its mass, or weight. At the end of their lives, stars about 10 times the size of our sun explode as so-called supernovas, producing elements sometimes heavier than iron that are released by the blast. Depending on how heavy the star originally was, silver and gold can also materialize this way.
When equal-mass stars explode, the relative amount of the elements generated and hurled out into space is identical, Hansen explained. This pattern continues in subsequent generations of stars that form from the remnants of their predecessors.
But Hansen and colleagues’ investigations have also found that the amount of silver in the stars measured is independent of the amounts of other heavy elements like gold. This means that during a supernova, or stellar explosion, silver arises through a different fusion process from the one that forms gold, she explained. Thus, the scientists contend that silver cannot have originated together with gold; they must have materialized from stars of different weights.
Silver and gold were found together in the measured stars only because these are not the actual stars of their origin, but later generations of stars that formed from their remnants, Hansen explained. The type of star measured is “the descendant of the supernovae that actually created silver (in one type of supernova) and gold at a different site (e.g. a more massive supernova or a merger event),” she wrote in an email.
“The elements are created in (or just after) the explosion and sent into space. Here they then clump, cool and following form stars,” she added. The study methodology was not unlike “genetically testing a baby to learn about the DNA of the parents.”
The scientists argue that the types of explosion that produce silver also produce the precious metal palladium, which is used for purposes including cleaning up car exhaust, electronics applications and jewelry.
“This is the first incontrovertible evidence for a special fusion process taking place during the explosion of a star,” said Hansen. “Up to now this had been mere speculation. After this discovery, we must now use simulations of these processes in supernova explosions to investigate more precisely when the conditions for the formation of silver are present. That way we can find out how heavy the stars were that could produce silver during their dramatic demise.”
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