2017’s biggest story
Why the world’s astronomers are so excited about colliding neutron stars.
“I can’t think of a similar situation in the field of science in my lifetime where a single event provides so many staggering insights about our universe.”
Astronomer Daniel Holz at the University of Chicago is referring to a collision that took place 130 million lightyears away between two neutron stars. It was detected on 17 August 2017, marking the first time scientists have ever witnessed such a cataclysm. They could do it because the two gravitational wave detectors of the Us-based Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo detector in Italy alerted them to the event. Astronomers could then point their telescopes to watch.
Together the observations confirm long-standing theories about how such an event would unfold. Papers about the findings have been published in Physical Review Letters, Science and Nature. A final compendium submitted to the Astrophysical Journal Letters has 3,500 authors – close to a third of the world’s astronomers.
The first gravitational waves ever detected by LIGO, in September 2015, were broadcast by a pair of colliding black holes. Detecting this cosmic cataclysm was a technological tour de force that won the Nobel Prize in 2017. It was a rather hard act to follow. A collision of two neutron stars – superdense collapsed corpses of exploded stars – seems a little dull in comparison. But physicists are every bit as excited.
With the colliding black holes there was nothing to see; their gravitational pull meant no light could escape. By contrast, the neutron-star collision (dubbed GW170817) allowed astronomers to watch.
LIGO’S two detectors and Virgo registered the high-pitched signal from the gravitational wave. By triangulation, the three well-spaced observatories were able to help astronomers narrow their search. Two seconds after a chirp from the gravitational wave, NASA’S Fermi space telescope detected a gamma-ray burst coming from a small region in the Hydra constellation, located in the southern hemisphere 130 million light-years away.
More than 70 observatories scattered across the globe gave astronomers their first-ever sight of a kilonova – the light radiating from the halo of material ejected from the coalesced stars.
Columbia University’s Brian Metzger had predicted just such an object in 2010, and coined the name kilonova, expecting it would be as bright as a thousand novae. Over the next few days the kilonova radiated light waves across the spectrum, from X-rays to blue to infrared.
The gamma-ray burst was an immediate result of the collision; the subsequent kilonova light was released by the shroud of material ejected during and after. Neutrons combined with surrounding elements to create heavy elements. From their spectral signatures we now know the heaviest elements are created in such cataclysms, solving a 60-year-old mystery since E. Margaret Burbidge, Fred Hoyle and colleagues showed elements heavier than iron were too unstable to form within stars.
The grandaddy finding of them all is the near-simultaneous arrival of the gravitational chirp and the gammaray burst (just two seconds apart). That confirms Einstein’s 100-year-old prediction that gravitational waves travel at the speed of light.
Astronomers are gobsmacked that so many long-standing predictions have been confirmed in the blink of an eye. “We didn’t expect to see this so soon,” says Eric Howell of the University of Western Australia.