A new study by a team of European and Japanese scientists has revealed that stellar processes producing silver differ from those producing gold.

The light elements hydrogen, helium and lithium came into being a few minutes after the Big Bang. All heavier elements were created later in the interior of stars or during star explosions, with each generation of stars contributing a little to enriching the Universe with chemical elements.

The elements a star can generate during its lifetime depend largely on its mass. At the end of their lives, stars about ten times the size of our Sun explode as so-called supernovae, producing elements sometimes heavier than iron.

Depending on how heavy the star originally was, silver and gold can also materialize in this way. When various stars of the same mass explode, the ratio of elements generated and hurled out into the Universe is identical. This constant relation is perpetuated in the subsequent generations of stars forming from the remnants of their predecessors.

The scientists, using observations from the UltraViolet Echelle Spectrograph at ESO’s Very Large Telescope and the twin 10-meter Keck telescopes, measured the amount of silver in a sample of 71 stars.

Their results, published in the journal Astronomy & Astrophysics (arXiv.org version), demonstrate that the amount of silver in the stars observed is completely independent of the amounts of other heavy elements like gold. This means that during a supernova silver is formed in a different fusion process from the one that forms gold. The scientists contend that silver cannot have originated together with gold. The elements must have materialized from stars of different masses.

“This is the first incontrovertible evidence for a special fusion process taking place during the explosion of a star,” said lead author Dr Camilla Juul Hansen of the Heidelberg University’s Centre for Astronomy.

“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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Bibliographic information: Hansen C. J. et al. 2012. Silver and palladium help unveil the nature of a second r-process. Astronomy & Astrophysics 545, A31: 28; doi: 10.1051/0004-6361/201118643