The collision of two stars in a galaxy far, far away, has been detected by a remarkable collaborative effort involving thousands of researchers around the world that potentially opens up a whole new field of science.
Among much else, the titanic collision expelled the equivalent of an amount of gold the size of Earth, by one estimate. As well as gold, platinum, uranium and other heavy metals are produced by such extreme events, physicists understand
When two stars collide
Scientists from The Australian National University (ANU) and around the world have detected for the first time ripples in space and time, known as gravitational waves, from the collision of two very dense stars, called neutron stars.
The detection on August 17 of the two neutron stars merging also prompted a frantic burst of scientific research. In just eight weeks, it produced enough new science for a slew of papers to appear in journals such as Nature, Science and Physical Review Letters in a co-ordinated release on Tuesday.
Australian researchers played significant roles, from the construction of the detecting devices to the rapid response of astronomical teams that were among the first to identify the optical and radio signals from the collision in a galaxy NGC 4993, about 130 million light years away.
A neutron star is the dense, collapsed core of a massive star that exploded as a supernova. Two of them spiralling into a merger triggered the emissions that have scientists ‘ecstatic’. Photo: NASA/Dana Berry.
“We’re in a state of frenzy at the moment,” Susan Scott, a professor at the Australian National University and a leading researcher of neutron stars, said. “It’s an enormous breakthrough which unlocks the possibility to see these events [again] and therefore to study them.”
Until now, the merger of such stars – so dense that something the size of a sugar cube would weigh 1 billion tonnes on Earth – had been theorised. One has now been observed for roughly the last 100 seconds before the collision and apparent collapse into a black hole.
The discovery builds on the confirmation two years ago that the merger of black holes created gravitational waves, as predicted a century earlier by Albert Einstein. Such waves are ripples that spread at the speed of light, independent but similar to electromagnetic radiation, such as radio waves.
Those findings helped win the 2017 Nobel Prize for Physics, announced just a fortnight ago, for American scientists Rainer Weiss, Barry Barish and Kip Thorne.
Three other black hole mergers had been detected since the original discovery in 2015 – each of which caused a stir among scientists. But it was the discovery using gravitational waves of a neutron star merger about to happen that has scientists predicting that a revolution in astronomy is about to unfold.
The Nobel Prize committee’s award was “so last year”, jokes Paul Lasky, a postdoctoral research fellow in gravitational-wave astrophysics at Monash. “The fact that the Nobel Prize keeps us interested for only a week now is just indicative that it is the birth of a new science.”
That new field, gravitational wave astronomy, will likely produce more observations of neutron star collisions and other phenomena as scientists know what to look for and where, and have the instruments to do so.
“Over the next five years, we’re going to see lots of these [events],” Dr Lasky said. “The amount of physics and the amount of science we’re going to be able to do with these is just remarkable.”
Teams involved in the LIGO Scientific Collaboration themselves totalled about 1100 scientists, while the researchers involved in turning what Professor Scott calls “an avalanche” of space-based and terrestrial telescopes to a portion of the night sky numbered another 2400 people.
Those operating the two detectors at LIGO – or Laser Interferometer Gravitational-Wave Observatory – spotted the last moments of the death spiral before the two neutron stars merged. They then sent out alerts to a private email list of collaborators.
Tara Murphy, an associate professor of physics at the University of Sydney and an author of the Science paper out on Tuesday, happened to be at a conference on Variable Radio Sources – “the exact topic of this work”, she says – when the message went out on the morning of August 17.
Professor Murphy then jumped on the instant messaging system on her phone to alert her colleague David Kaplan, also at the event.
Then followed much ducking in and out of the conference, and private meetings among the 40-odd delegates, as the scientists started spreading the word to their teams back home.
“We were frantically emailing people to get the telescopes pointing to the right patch of the sky,” Professor Murphy said.
‘Did anyone else see anything?’
Detecting the imminent collision was one thing. Within 1.7 seconds of the merger, gamma rays emitted from the event were also being picked up by NASA’s Fermi space telescope.
“The gamma ray team also put alerts, effectively saying ‘we’ve detected this amazing burst, did anyone else see anything?'” Dr Lasky said.
“In this 1.7 seconds of the gravitational wave signal, the 50-year hypothesis [that neutron star mergers may be the source of gamma rays] was immediately confirmed,” Professor Scott said.
Dr Lasky, though, did not immediately see the gamma-ray alerts. He was working on his laptop in his Melbourne home but remembers getting “quite excited” by the gravitational wave data coming through.
“The signal we were seeing in the those first 10, 20, 30 minutes was just absolutely beautiful – a very clean, crisp signal that people had been imagining for a number of years how a binary neutron star would look like,” he said.
Dr Lasky said scientists “had no idea” when the gravitational waves detected from black holes had been formed. This time, though, the combination with traditional astronomy had allowed researchers to pin down their speed “with remarkable precision”.
Einstein, it turns out “passed another test”, Dr Lasky said. Gravitational waves do travel at the speed of light, as the great theoretical physicist predicted.
Professor Scott, who also happened to be working late that night at her home in Canberra, could barely believe what she was reading.
Having been closely involved in the gravitational wave research identifying the four separate black hole collisions, she had to re-read the first missive several times to realise this was something else.
Her work though a “large portion of the night” then involved ensuring the SkyMapper and 2.3-metre telescopes at the ANU’s Siding Spring Observatory in northern NSW were programmed to scan the right galaxy for the optical signal that they hoped to detect.
While the Swope Telescope in Chile would be the first to make a tentative detection of the light signal about 10 hours after the merger, the SkyMapper would be among first to confirm it, Professor Scott said.
“Australia was the first to get the colour of the resultant fireball from the collision, and to measure its temperature at 6000 degrees, or about the surface temperature of the sun,” Professor Scott said.
Below is an image captured by the SkyMapper of the fireball ejected by the collision. (Supplied by ANU’s Christian Wolf et al.)
‘Not confident at all’
Australian work was also crucial for the detection of radio waves from the event, which would require about 16 days of careful monitoring.
Professor Murphy and her PhD candidate Dougal Dobie sprang into action to convince CSIRO to give their team what would eventually total 40 hours of precious time on the Australia Telescope Compact Array at Narrabri, also in northern NSW.
“We weren’t confident at all,” Professor Murphy said. “We truly did not know whether we were going to detect anything or not.”
The team knew to direct the array to an area about 150 times the size of the moon in the night sky. After a tentative detection by US counterparts, the Sydney University team were the first to confirm the radio signal on September 5.
While each part of the spectrum played a role, the radio wave detection should be best placed to distinguish which of the astrophysical models worked best, Professor Murphy said.
Also of interest will be learning more about how gold and other elements are formed.
“It’s a big mystery where half of all the elements of the universe come from,” said Professor Murphy. “We know that elements heavier than iron must be produced in supernova explosions [which form neutron stars] because there’s no other thing with enough energy to produce them.”
In this instance, a lot of gold would have been ejected in the event, Dr Lasky said.
“Our best bet is that around about one Earth’s mass of gold was produced, which is a pretty phenomenal amount,” he said.
Other Australian researchers were involved through the ARC Centre of Excellence for Gravitational Wave Discovery, or Ozgrav.
“For the first time in history we can now combine light signals with gravitational waves to provide a totally new way to probe the universe,” said Ashley Ruiter from UNSW Canberra’s School of Physical, Environmental, and Mathematical Sciences, whose work appears in a Nature paper out on Tuesday.
Dr Ruiter and colleague Ivo Seitenzahl were part of the ePESSTO (extended Public ESO Spectroscopic Survey of Transient Objects) collaboration, which took the first spectrum of the event.
Dr Lasky, who contributed sections of several papers and has more to come, said it had been “an amazingly busy eight weeks”, with so much more to understand, including what happened immediately after the collision.
The stars may have formed a black hole immediately or their instruments are not yet sensitive enough to pick up the gravitational waves.
“Either way it’s very interesting,” Dr Lasky said.
Professor Scott said it looks like it collapsed into a black hole, but we’re still examining our data for any interim signals” to if see the merged object “had a stable moment”.
“We’re trying to get a handle on [the complex nuclear physics involved],” Dr Lasky said. “This event has allowed us to make a huge leap towards that understanding”.