Hubble and fleet of telescopes watch as neutron stars collide

The Hubble space telescope and around 70 other observatories have witnessed one of the biggest and rarest explosions in the universe, as two incredibly dense neutron stars collided.

An artist’s concept of a neutron star merger. Image credit: Illustration by Robin Dienel courtesy of the Carnegie Institution for Science

The blast was so great that it distorted the fabric of space and time, and showered gold, metal and other rare metals out into the cosmos.

Ripples were detected on Earth as a rare gravitational wave. Giant telescopes of all types around the world were swiftly turned in the direction from which it had come in a bid to find the source.

First to track it down was a team led by Dr Ben Shappee, using the 1-metre (40-inch) Swope Telescope at Cerro Las Campanas in Chile.

They quickly identified a previously unknown point of light in a faint lenticular (lens-shaped) galaxy known as NGC 4993, about 130 million light-years away, in the constellation of Hydra.

Astronomers say it is the first time they have seen a kilonova – a phenomenon that was predicted more than 30 years ago, where two neutron stars circling each other get too close and crash.

The drama, which was revealed today, began on August 17 when two sensitive instruments, the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the USA, and the Virgo Interferometer, in Italy, both detected a gravitational wave. The event was labelled event named GW170817.

About two seconds later, a massive explosion known as a gamma-ray burst was observed in the same direction by two space telescopes – the European Space Agency’s Integral satellite, and NASA’s Fermi orbiter.

Hubble observed the kilonova gradually fading over the course of six days, in the galaxy known as NGC 4993. Image credit: NASA and ESA. Acknowledgment: A.J. Levan (U. Warwick), N.R. Tanvir (U. Leicester), and A. Fruchter and O. Fox (STScI)

The following night, the world’s largest telescopes began their bid to find the source in optical light. Hubble, operated jointly by NASA and ESA, managed to capture the explosion’s glow with its hi-resolution imaging instrument.

This proved to the astronomers that they had caught a kilonova, an explosion much rarer and more powerful than a supernova, as two incredibly dense objects collided.

It is believed they were neutron stars, the collapsed cores of earlier supernova blasts. They are so dense that a teaspoonful of material would weigh a billion tonnes.

Dr Shappee, of the University of Hawaii’s Institute for Astronomy, was thrilled that his observers were first to spot the light from the cataclysmic event. He said: “It is amazing that with the multitude of teams searching for this source, our small team of mostly young astronomers was the first to image, discover and report it.”

The Hubble team was led by Professor Andrew Levan of the University of Warwick, aided by Professor Nial Tanvir of the University of Leicester, both in the UK.

Professor Levan said: “Once I saw that there had been a trigger from LIGO and Virgo at the same time as a gamma-ray burst I was blown away.

“When I realised that it looked like neutron stars were involved, I was even more amazed. We’ve been waiting a long time for an opportunity like this!”

He added: “The spectrum of the kilonova looked exactly like how theoretical physicists had predicted the outcome of the merger of two neutron stars would appear.

“It ties this object to the gravitational wave source beyond all reasonable doubt.”

Professor Tanvir said: “It was surprising just how closely the behaviour of the kilonova matched the predictions.

“It looked nothing like known supernovae, which this object could have been, and so confidence was soon very high that this was the real deal.”

About 70 telescopes, observing in all parts of the spectrum, and all over the world and in space, took part in the study of this phenomenon, making it one of the greatest collaborative events ever. Their results are published today in Nature and other journals.


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