"Long before it's in the papers"
September 07, 2012

RETURN TO THE WORLD SCIENCE HOME PAGE

Silver, gold come from bursting stars—but different types, study finds

Sept. 7, 2012
Courtesy of the University of Heidelberg
and World Science staff

Sil­ver and gold orig­i­nate in ex­plod­ing stars, but dif­fer­ent types of stars, new re­search sug­gests.

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.”

* * *

Silver and gold originate in exploding stars, but different types of stars, new research suggests. 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 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 ten 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 in 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 materialised from stars of different weights. Silver and gold were found together in the stars examined 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 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 are 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.”