“Truly a eureka moment”, “Everything I ever hoped for”, “A dream come true” — Normally restrained scientists reached for the stars yesterday to describe the feelings that accompany a “once-in-a-lifetime” event.
The trigger for this meteor shower of superlatives was the smash-up of two unimaginably dense neutron stars 130mn years ago.
Evidence of this cosmic clash hurtled through space and reached Earth on August 17 at exactly 12.41GMT, setting in motion a secret, sleepless, weeks-long blitzkrieg of star-gazing and number-crunching involving hundreds of telescopes and thousands of astronomers and astrophysicists around the world.
It was as if a dormant network of super-spies simultaneously sprung into action.
The stellar smash-up made itself known in two ways: it created ripples called gravitational waves in Einstein’s time-space continuum, and lit up the entire electromagnetic spectrum of light, from gamma rays to radio waves.
Scientists had detected gravitational waves four times before, a feat acknowledged with a Nobel Physics Prize earlier this month.
But each of those events, generated by the collision of black holes, lasted just a few seconds, and remained invisible to Earth- and space-based telescopes.
The neutron star collision was different. It generated gravitational waves — picked up by two US-based observatories known as LIGO, and another one in Italy called Virgo — that lasted an astounding 100 seconds.
Less than two seconds later, a Nasa satellite recorded a burst of gamma rays. This set off a mad dash to locate what was almost certainly the single source for both.
“It is the first time that we’ve observed a cataclysmic astrophysical event in both gravitational and electromagnetic waves,” said LIGO executive director David Reitze, a professor at the California Institute of Technology (Caltech) in Pasadena Initial calculations had narrowed the zone to a patch of sky in the southern hemisphere spanning five or six galaxies, but frustrated astronomers had to wait for nightfall to continue the search.
Finally, at around 2200GMT, a telescope array in the northern desert of Chile nailed it: the stellar merger had taken place in a galaxy known as NGC 4993.
Stephen Smartt, who led observations for the European Space Observatory’s New Technology Telescope, was gobsmacked when the spectrum lit up his screens. “I had never seen anything like it,” he recalled.
Scientists everywhere were stunned.
“This event was truly a eureka moment,” said Bangalore Sathyaprakash, head of the Gravitational Physics Group at Cardiff University. “The 12 hours that followed are inarguably the most exciting of my scientific life.”
“There are rare occasions when a scientist has the chance to witness a new era at its beginning — this is one such time,” said Elena Pian, an astronomer at the National Institute for Astrophysics in Rome.
LIGO-affiliated astronomers at Caltech had spent decades preparing for the off chance — calculated at 80,000-to-one odds — of witnessing a neutron star merger.
“On that morning, all of our dreams came true,” said Alan Weinstein, head of astrophysical data analysis for LIGO at Caltech.
“This discovery was everything I always hoped for, packed into a single event,” added Francesco Pannarale, an astrophysicist at Cardiff University in Wales.
For these and thousands of other scientists, GW170817 — the neutron star burst’s tag — will become a “do you remember where you were?” kind of moment.
“I was sitting in my dentist’s chair when I got the text message,” said Benoit Mours, an astrophysicist at France’s National Centre for Research and the French co-ordinator for Virgo. “I jumped up and rushed to my lab.”
Patrick Sutton, head of the gravitational physics group at Cardiff and a member of the LIGO team, was stuck on a long-haul bus, struggling to download hundreds of e-mails crowding his inbox.
Rumours swirled within and beyond the astronomy community as scientists hastened to prepare initial findings for publication yesterday in a dozen articles spread across several of the world’s leading journals.

What are neutron stars?
Thrilled physicists and astronomers announced yesterday the first-ever observation of the merger of two neutron stars, one of the most spectacularly violent phenomena in the Universe.
But what are they? 
We asked Patrick Sutton, head of Cardiff University’s gravitational physics department, who contributed to the discovery.
Q: What are neutron stars? 
A: You can think of them as the collapsed, burnt-out cores of dead stars. When large stars reach the end of their lives, their core will collapse, the outer layers of the star blown off.  You’re left with an extremely exotic object, this neutron star.
A neutron star typically would have a mass that’s perhaps half-a-million times the mass of the Earth, but they’re only about 20kms across (about the size of London). A handful of material from this star would weigh as much as Mount Everest.
They are very hot, perhaps a million degrees, they are highly radioactive, they have incredibly intense magnetic fields... They are arguably the most hostile environments in the Universe today.
Q: Why do neutron stars merge?
A: It’s very common for stars...in the Universe to actually be formed in pairs by a given gas cloud. If the stars are large enough, then at the end of their life they explode and they leave behind neutron star cores, and the neutron stars will continue orbiting each other.
As they orbit, they give off gravitational waves and the waves carry away energy and so the stars slowly fall closer and closer together. As they get closer together they orbit faster and faster and the gravitational wave emission speeds up.
You get a runaway process where the two stars in the last few moments of their life, they’ll be orbiting each other several hundred times per second, so moving at very close to the speed of light, and eventually they will merge.
Q: What happens then? 
A: Because we don’t understand exactly the mechanics of how these neutron stars work on the interior, it’s not certain what the final fate is. If the stars are heavy enough, we’re sure they will collapse to form a black hole and some of the remaining matter... will form what is called an accretion disk orbiting just around the black hole.
It may be that if the stars are light enough, that they will actually form a single, very heavy neutron star instead of a black hole. That may be stable and stay as a neutron star forever, or it may be unstable and eventually collapse into a black hole.