The first detection of gravitational waves from the cataclysmic merger of two neutron stars, and the observation of visible light in the aftermath of that merger, finally answer a long-standing question in astrophysics: Where do the heaviest elements, ranging from silver and other precious metals to uranium, come from?
Based on the brightness and color of the light emitted following the merger, which closely match theoretical predictions by University of California, Berkeley and Lawrence Berkeley National Laboratory physicists, astronomers can now say that the gold or platinum in your wedding ring was in all likelihood forged during the brief but violent merger of two orbiting neutron stars somewhere in the universe.
This is the first detection of a neutron star merger by the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in the United States, whose leaders were awarded the Nobel Prize in Physics two weeks ago, and the Virgo detector in Italy. LIGO had previously detected gravitational waves from four black hole mergers, and Virgo one, but such events should be completely dark. This is the first time that light associated with a source of gravitational waves has been detected.
“We have been working for years to predict what the light from a neutron merger would look like,” said Daniel Kasen, an associate professor of physics and of astronomy at UC Berkeley and a scientist at Berkeley Lab. “Now that theoretical speculation has suddenly come to life.”
Watch a short video on the discovery, hosted by Dan Kasen.
The neutron star merger, dubbed GW170817, was detected on August 17 and immediately telegraphed to observers around the world, who turned their small and large telescopes on the region of the sky from which it came. The ripples in spacetime that LIGO/Virgo measured suggested a neutron star merger, since each star of the binary weighed between 1 and 2 times the mass of our sun. Apart from black holes, neutron stars are the densest objects known in the universe. They are created when a massive star exhausts its fuel and collapses onto itself, compressing a mass comparable to that of the sun into a sphere only 10 miles across.
Only 1.7 seconds after the gravitational waves were recorded, the Fermi 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, observers caught their first glimpse of visible light from the source. It was localized to a known galaxy, NGC 4993, situated about 130 million light years from Earth in the direction of the constellation Hydra.
Artist’s conception of a binary neutron star merger, which creates a whirling cloud of radioactive debris and a short gamma-ray burst (jets). (Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet)
The detection of a neutron star merger was surprising, because neutron stars are much smaller than black holes and their mergers produce much weaker gravitational waves than do black hole mergers. According to Berkeley professor of astronomy and physics Eliot Quataert, “We were anticipating LIGO finding a neutron star merger in the coming years but to see it so nearby – for astronomers – and so bright in normal light has exceeded all of our wildest expectations. And, even more amazingly, it turns out that most of our predictions of what neutron star mergers would look like as seen by normal telescopes were right!”
The LIGO/Virgo observations of gravitational waves and the detection of their optical counterpart will be discussed at a 10 a.m. EDT press conference on Monday, Oct. 16, at the National Press Club in Washington, D.C. Simultaneously, several dozen papers discussing the observations will be published online by Nature, Science and the Astrophysical Journal Letters.
Genesis of the elements
While hydrogen and helium were formed in the Big Bang 13.8 billion years ago, heavier elements like carbon and oxygen were formed later in the cores of stars through nuclear fusion of hydrogen and helium. But this process can only build elements up to iron. Making the heaviest elements requires a special environment in which atoms are repeatedly bombarded by free neutrons. As neutrons stick to the atomic nuclei, elements higher up the periodic table are built.