When Stars Collide: A New Era in Astronomy

16 October 2017

The observation of colliding neutron stars heralds a powerful new approach to studying the universe

Imagine you’re asked to piece together a jigsaw puzzle blindfolded, or else with your hands tied behind your back. The loss of either sight or movement would likely render the task impossible, for you need both together to create the bigger picture. The same is true of modern science. It is only by exploiting multiple approaches that we can solve the great puzzles of the cosmos.

That’s why today marks such an important moment in scientific history. Astronomers have observed a never before seen event: the collision between two neutron stars. This discovery, and the observations of its aftermath, resulted from many collaborations around the world, and from spacecraft, telescopes and gravitational wave detectors working in unison.

None of these approaches could have pieced the puzzle together alone; it is only by combining them that we have been able to see the full event unfolding. And that is tremendously exciting, both for all the new information that it reveals, and for what it represents: a new era in astronomy. With multiple ways of ‘seeing’, we are now better equipped than ever before to unravel the mysteries of the universe.

Colliding Stars

So, what exactly did we see and why does it matter?

When a massive star dies, it collapses in on itself, leaving behind the iron-rich core, while the outer part of the star is blasted away in a supernova explosion. It is this core that becomes a neutron star, about as massive as the sun but roughly the size of a city, and incredibly dense; even a teaspoonful weighs a billion tonnes.

The collision of two neutron stars has long been anticipated as a powerful source of gravitational waves, and is widely thought to be the engine responsible for short gamma ray bursts. These are among the most powerful explosions in the cosmos and produce an intense flash of high-energy gamma rays that momentarily outshine the combined light from all other sources in the entire universe.

It has also been proposed that short gamma ray bursts could be followed by a ‘kilonova’ – a follow-on explosion to the gamma ray burst that expels bits of dead neutron star out into the universe. Kilonovae could provide a crucial way of making heavy elements, like platinum and gold, and releasing them into the cosmos.

However, no-one knew for sure if the pieces of this puzzle fitted together, because we’d never identified a neutron star collision and linked it directly to a short gamma ray burst and a kilonova. We just didn’t have the capability; that is, until now.

The Big Event

Watch original version on ESO website.
Credit: N. Risinger (skysurvey.org), LIGO-Virgo, Digitized Sky Survey 2, ESO. Music: Johan Monell.

On the 17th August at 8:41am, a gravitational-wave signal reached the US-based LIGO Laboratory and its European counterpart Virgo. This long, drawn out ‘chirp’ indicated ripples in the fabric of spacetime caused by a cataclysmic event in space, matching beautifully the waveform expected from the collision of two neutron stars.

At almost the exact same moment, NASA and ESA spacecraft detected a gamma ray burst coming from the same region of sky. Together, the gamma rays and the gravitational waves strongly confirmed the long-suspected link between colliding neutron stars and short gamma ray bursts.

Very quickly after, ESO’s state-of-the-art suite of telescopes in Chile all pointed up at the same section of sky. And before long, we had the concrete evidence we were looking for to complete the picture; the telescopes identified traces of visible light, convincingly matching the pattern of brightness expected for a kilonova.

But it didn’t end there: in the weeks following the event, telescopes and observatories found further evidence of the stars’ cataclysmic end and equally dramatic aftermath – including the signatures of heavy elements like gold and platinum.

Ultimately, by using the varied data gathered from gravitational waves, gamma rays, visible light and other electromagnetic emissions, scientists were able to confirm the first sighting of a neutron star collision and its impact, all occurring 130 million light years away.

Piecing Together the Puzzle

Together, these techniques have provided us with an unprecedented insight into neutron stars. Using LIGO and Virgo, we could identify gravitational waves from the collision and its impact on spacetime. With NASA and ESA’s space observatories, we were able to link the collision to the generation of gamma rays. And with ESO’s, and many other, telescopes on the ground, we were able to identify the light produced by the kilonova, and confirm how this cataclysmic sequence of events was a factory for producing heavy elements.

And so, as a result of multiple approaches – from gravitational wave detectors, to spacecraft, to telescopes sensitive to light in all its forms – we now have the answers we’ve been chasing, along with a new and deeper appreciation of the significant role played by neutron stars in shaping our cosmos.

But the real story is even bigger than that; for this moment marks a step change in our approach to exploring the cosmos. Thanks to recent advances in technology – many of which are directly supported by STFC – we now have a suite of advanced techniques at our disposal. And whilst approaches like gravitational wave detection are already hugely significant when used alone, it is when they are combined with existing approaches, which study light, that they become truly revolutionary.

Working together, these techniques are poised to reveal a fuller picture of rare cosmic events and broaden our understanding of the universe. Neutron stars are the first great cosmological mystery to be unravelled in this way, but they certainly won’t be the last.

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