Dead stars collide, unlock clues to universe

Such explosions are thought to have created the universe's heavy elements like gold, silver, uranium.

Astronomers announced on Monday that they had seen and heard a pair of dead stars collide, giving them their first glimpse of the violent process by which most of the gold and silver in the universe was created.

The collision, known as a kilonova, rattled the galaxy in which it happened 130 million light years from here in the southern constellation of Hydra, and sent fireworks across the universe. On August 17, the event set off sensors in space and on Earth, as well as producing a loud chirp in antennas designed to study ripples in the cosmic fabric. It sent astronomers stampeding to their telescopes, in hopes of answering one of the long-sought mysteries of the universe.

Such explosions, astronomers have long suspected, produced many of the heavier elements in the universe, including precious metals like gold, silver and uranium. All the atoms in your wedding band, in the pharaoh’s treasures and the bombs that destroyed Hiroshima and still threaten us all, so the story goes, have been formed in cosmic gong shows that reverberated across the heavens.

This gong show happened when a pair of neutron stars, the shrunken dense cores of stars that have exploded and died, collided at nearly the speed of light. These stars are masses as great as the sun packed into a region the size of Manhattan brimming with magnetic and gravitational fields.

Studying the fireball from this explosion, astronomers have concluded that it had created a cloud of gold dust many times more massive than the Earth, confirming kilonovas as agents of ancient cosmic alchemy.

“For the first time ever, we have proof,” said Vicky Kalogera, an astronomer at Northwestern University. She was one of thousands of astronomers that reported their results on Monday in a globe-girdling set of news conferences and academic conferences.

A blizzard of papers is being published, including one in The Physical Review Letters that has some 4,000 authors. “That paper almost killed the paperwriting team,” said Kalogera, one of 10 people who did the actual writing. More papers are appearing in Nature, the Astrophysical Journal Letters and in Science, on topics including nuclear physics and cosmology.

“It’s the greatest fireworks show in the universe,” said David Reitze of the California Institute of Technology and the executive director of the Laser Interferometer Gravitational-Wave Observatory, or LIGO.

Daniel Holz, an astrophysicist at the University of Chicago and a member of the LIGO Scientific Collaboration, a larger group that studies gravitational waves, said, “I can’t think of a similar situation in the field of science in my lifetime, where a single event provides so many staggering insights about our universe.”

The key to the discovery was the detection of gravitational waves, emanating like ripples in a pond vibrating the cosmic fabric, from the distant galaxy. It was a century ago that Albert Einstein predicted that space and time could shake like a bowl of jelly when massive things like black holes moved around. But such waves were finally confirmed only in 2016, when LIGO recorded the sound of two giant black holes colliding, causing a sensation that eventually led this month to a Nobel Prize.

For the researchers, this is in some ways an even bigger bonanza than the original discovery. This is the first time they have discovered anything that regular astronomers could see and study. All of LIGO’s previous discoveries have
involved colliding black holes, which are composed of empty tortured space-time — there is nothing for the eye or the telescope to see.

But neutron stars are full of stuff, matter packed at the density of Mount Everest in a teaspoon. When neutron stars slam together, all kinds of things burst out: gamma rays, X-rays, radio waves. Something for everyone who has a window on the sky. “Joy for all,” said David Shoemaker, a physicist at the Massachusetts Institute of Technology who is the spokesman for the LIGO Scientific Collaboration.

This is the story of a gold rush in the sky.

It began on the morning of August 17, Eastern time. Shoemaker was on a Skype call when alarms went off. One of the LIGO antennas, in Hanford, Washington, had recorded an auspicious signal and sent out an automatic alert.

Twin antennas, in Washington and Livingston, Louisiana, monitor the distance between a pair of mirrors to detect submicroscopic stretching and squeezing of space caused by a passing gravitational wave. Transformed into sound, the Hanford signal was a long 100-second chirp, that ended in a sudden whoop to 1,000 cycles per second, two octaves above middle C. Such a high frequency indicated that whatever was zooming around was lighter than a black hole.

Checking the data from Livingston to find out why it had not also phoned in an alert, Shoemaker and his colleagues found a big glitch partly obscuring the same chirp.

Meanwhile, the Fermi Gamma-Ray Space Telescope, which orbits Earth looking at the highest-energy radiation in the universe, recorded a brief flash of gamma rays just two seconds after the LIGO chirp. Fermi sent out its own alert. The gamma-ray burst lasted about two seconds, which put it in a category of short gamma ray bursts, which astronomers suspect are neutron stars colliding. “When we saw that,” Shoemaker said, “the adrenaline hit.”

Luckily the European Virgo antenna had joined the gravitational wave network only two weeks before, and it also showed a faint chirp at the same time. The fact that it was so weak allowed the group to localise the signal to a small region of the sky in the Hydra constellation that was in Virgo’s blind spot.

Quest for the fireball

The hunt was on. By then Hydra was setting in the southern sky. It would be 11 hours before astronomers in Chile could take up the chase.

One of them was Ryan Foley, who was working with a team on the Swope telescope run by the Carnegie Institution on Cerro Las Campanas in Chile. His team made a list of the biggest galaxies in that region and set off to photograph them all systematically.

The fireball showed up in the ninth galaxy photographed, as a new bluish pinprick of light in the outer regions of NGC 4993, a swirl of stars about 130 million light years from here. “These are the first optical photons from a kilonova humankind has ever collected,” Foley said.

Nine days later, the orbiting Chandra X-ray Observatory detected X-rays coming from the location of the burst, and a week after that, the Very Large Array in New Mexico recorded radio emissions. By then the fireball faded from blue to red. From all this, scientists have begun patching together a tentative story of what happened in the NGC 4993 galaxy.

“It’s actually surprising how well we were able to anticipate what we’re seeing,” said Brian David Metzger, a theorist at Columbia University who coined the term kilonova back in 2010.

One burning question is what happened to the remnant of this collision. According to the LIGO measurements, it was about as massive as 2.6 suns. Scientists say that for now they are unable to tell whether it collapsed straight into a black hole, formed a fat neutron star that hung around in this universe for a few seconds before vanishing, or remained as a neutron star. They may never know, Kalogera said.

Neutron stars are the densest form of stable matter known. Adding any more mass over a certain limit will cause one to collapse into a black hole, but nobody knows what that limit is. Future observations of more kilonovas could help understand where the line of no return actually is.

Holz said, “I still can’t believe how lucky we all are,” reciting a list of fortuitous circumstances. They had three detectors running for only a few weeks, and it was the closest gamma-ray burst ever recorded and the loudest gravitational wave yet recorded. “It’s all just too good to be true. But as far as we can tell it’s really true. We’re living the dream.”