WHAT made the gold and platinum in your favourite jewellery? The answer may
lie in some of the most violent events in the Universe: collisions between
superdense neutron stars. 鈥淧robably many of the heavy elements we鈥檙e familiar
with on Earth were made in this way,鈥� says Stephan Rosswog of the University of
Leicester.
Shortly after the big bang, the Universe contained only the light elements
hydrogen and helium. When these materials later formed stars, heavier elements
such as carbon and oxygen were forged in the stars鈥� nuclear furnaces. And even
heavier elements were created when very massive stars exploded as supernovae.
Such explosions also blast debris into space where another generation of stars
and planets form.
But when it comes to the heaviest elements, such as gold and platinum,
astronomers are not sure that supernovae can create enough of them. Most of
these metals must be made in a nuclear reaction called the 鈥渞-process鈥�, in which
a nucleus consumes many neutrons in quick succession. 鈥淵ou would need much more
extreme conditions than you get in supernova simulations,鈥� says Rosswog.
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He suspected the r-process might flourish during collisions between neutron
stars鈥攖he collapsed remains of stellar cores left behind after some
supernova explosions. Sometimes two neutron stars orbit each other, and they can
spiral ever closer together before eventually merging in a violent explosion.
These mergers are rare, probably happening about once every 100,000 years in our
Galaxy.
To test the idea, Rosswog鈥檚 team simulated the collision of two neutron stars
on a supercomputer at the University of Leicester, taking account of everything
from the laws of quantum physics to Einstein鈥檚 theory of general relativity. The
merged stars collapsed to form a black hole, but in the process they spewed out
quantities of very hot, dense, neutron-rich ash in which the r-process could
thrive.
Rosswog told the National Astronomy Meeting in Cambridge this week that
neutron star mergers could easily produce the amount of heavy elements we see.
鈥淩ight now, they鈥檙e the best candidate for these elements,鈥� he says. He plans to
look at the distribution of the heaviest elements in old populations of stars.
Because neutron star collisions are so rare, there should be an uneven
distribution of these heavy elements in the early Universe. 鈥淭here should be
some clumpiness,鈥� says Rosswog.
Stan Woosley of the University of California at Santa Cruz doesn鈥檛 think it鈥檚
a closed case yet. 鈥淧eople still don鈥檛 know just how and where the r-process
happens,鈥� he says. 鈥淭he new calculations make merging neutron stars more
attractive but will not put the issue to rest.鈥� Woosley favours an alternative
scenario in which, under the right conditions, supernova explosions could leave
behind neutron stars that spout jets of neutrons into space. These might fuel
enough r-process reactions to build up the amounts of heavy elements we see.