An artist’s impression shows two tiny but very dense neutron stars at the point at which they merge and explode as a kilonova © University of Warwick/Mark Garlick
Astronomers have entered a new era, combining observations of gravitational waves and light for the first time. They have recorded a cataclysmic collision between two neutron stars in a distant galaxy, which not only set the universe aquiver but also propelled newly created atoms of gold, uranium and other heavy metals into space.
Excited scientists announced the discovery at briefings around the world on Monday. One of them, Professor Stephen Smartt of Queen’s University Belfast, said: “This opens up a new dawn in physics and astronomy. We had only just begun to detect gravitational waves and now we have used telescopes to detect light from the object that caused them: a merger of two neutron stars, which had been predicted but never seen before.”
Neutron stars are the densest form of matter known — the collapsed cores of massive stars. Each has a mass greater than the Sun in a region of space just 10km across. Putting it another way, a teaspoon of neutron star would weigh a billion tonnes.
When gravitational waves from the neutron star collision 130m light years away reached Earth on August 17, these tiny distortions in space-time were picked up by scientists at the new Ligo detector in the US and Virgo, its European counterpart in Italy. The first four gravitational wave detections since 2015 had all resulted from giant black holes colliding and gave off no visible radiation.
This fifth one looked quite different and fitted the theoretical predictions for colliding neutron stars. It lasted for 100 seconds — much longer than the previous four detections — reflecting a final death spiral by the two neutron stars, followed by a mega-explosion called a kilonova that astronomers had not previously observed.
The gravitational wave teams immediately alerted astronomers across the world. More than 70 observatories on Earth and in space then detected light — electromagnetic radiation at various wavelengths — arriving from the collision, which took place in the previously obscure NGC 4993 galaxy in the Hydra constellation. This ranged from a short burst of ultra-energetic gamma rays through visible and ultraviolet light to radio waves.
This image from the VIMOS instrument on ESO’s Very Large Telescope at the Paranal Observatory in Chile shows the galaxy NGC 4993, about 130m light years from Earth
All astronomical observations used to depend entirely on electromagnetic radiation. Gravitational waves, whose discovery was recognised with the 2017 Nobel physics prize this month, provide another window on the most violent events in the universe: possibly including reverberations from the Big Bang that began it all.
Observations of the neutron star collision and kilonova explosion may answer an important question in astrophysics: where did the heavy chemical elements come from?
They did not form in the Big Bang and the nuclear processes in ordinary stars are not energetic enough to produce anything heavier than iron.
Astronomers had assumed that heavy elements originated in massive stellar explosions called supernovae but kilonovae now emerge as another source.
“We have discovered that this neutron star merger scattered heavy chemical elements such as gold and platinum out into space at high speeds,” said Kate Maguire of Queen’s University Belfast, who was in the international team analysing light bursting from the neutron star collision.
“These new results have significantly contributed to solving the long-debated mystery of the origin of elements heavier than iron in the periodic table,” said Ms Maguire.
Or, as Professor Sheila Rowan of Glasgow University, put it: “We now know that the violent collision of neutron stars is a gold factory.”
The first papers about the event appeared online in several scientific journals on Monday, with much more analysis and interpretation expected over the next few months. At the same time the gravitational wave detectors will be listening out for cataclysmic cosmic collisions.