Origin of Universe’s heavy elements, ranging from gold to uranium, has finally been confirmed, after a gravitational wave source was seen and heard for the first time ever by an international collaboration of astronomers and astrophysicists.
Merging neutron stars. Image credit: Mark Garlick, University of Warwick.
The neutron star merger GW170817 was detected on August 17, 2017, and immediately telegraphed to astronomers around the world, who turned their telescopes on the region of the sky from which it came.
The ripples in space and time that LIGO and Virgo detectors measured suggested a neutron star merger, since each star of the binary weighed between 1 and 2 times the mass of our Sun.
Only 1.7 seconds after the gravitational waves were recorded, Fermi Gamma-ray Space Telescope detected a short burst of gamma rays from the same region, evidence that concentrated jets of energy are produced during the merger of neutron stars.
Less than 11 hours later, astronomers caught their first glimpse of visible light from the source. It was localized to the lenticular galaxy NGC 4993, situated about 130 million light years from Earth in the direction of the constellation Hydra.
“In NGC 4993, two neutron stars once spiraled around each other at blinding speed,” the scientists said.
“As they drew closer together, they whirled even faster, spinning as fast as a blender near the end.”
“Powerful tidal forces ripped off huge chunks while the remainder collided and merged, forming a larger neutron star or perhaps a black hole. Leftovers spewed out into space. Freed from the crushing pressure, neutrons turned back into protons and electrons, forming a variety of chemical elements heavier than iron.”
The merging of two neutron stars produces a violent explosion known as a kilonova. Such an event is expected to expel heavy chemical elements into space. This picture shows some of these elements, along with their atomic numbers. Image credit: ESO / L. Calcada / M. Kornmesser.
Huge amounts of gold, platinum, uranium and other heavy elements were created in this collision, and were pumped out into the Universe — unlocking the mystery of how gold on wedding rings and jewelry is originally formed.
“The collision produced as much gold as the mass of the Earth,” the researchers noted.
“How the heaviest elements came to be has been one of the longest standing questions of our cosmic origins,” said Dr. Daniel Kasen, a scientist in the Nuclear Science Division at the Department of Energy’s Lawrence Berkeley National Laboratory.
“Now we have for the first time directly witnessed a cloud of freshly made precious metals right at their production site.”
“Once we saw the data, we realized we had caught a new kind of astrophysical object. This ushers in the era of multi-messenger astronomy, it is like being able to see and hear for the first time,” said Professor Andrew Levan, of the University of Warwick.
“Thanks to this multi-messenger event, we know for a fact that neutron star mergers can produce heavy elements such as gold, silver and iron, which are so important to us on this planet,” added Dr. Raffaella Margutti, of Northwestern University.
“This discovery has answered three questions that astronomers have been puzzling for decades: what happens when neutron stars merge? What causes the short duration gamma-ray bursts? Where are the heavy elements, like gold, made? In the space of about a week all three of these mysteries were solved,” said Dr. Samantha Oates, of the University of Warwick.
“The exquisite observations obtained in a few days showed we were observing a kilonova, an object whose light is powered by extreme nuclear reactions. This tells us that the heavy elements, like the gold or platinum in jewelry are the cinders, forged in the billion degree remnants of a merging neutron star,” said Dr. Joe Lyman, also from the University of Warwick.
Scientific papers describing and interpreting these observations are published in the journal Nature and the Astrophysical Journal Letters.
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E. Pian et al. Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger. Nature, published online October 16, 2017; doi: 10.1038/nature24298
N.R. Tanvir et al. 2017. The Emergence of a Lanthanide-rich Kilonova Following the Merger of Two Neutron Stars. ApJL 848, L27; doi: 10.3847/2041-8213/aa90b6
