A galactic collision of a different type was detected at Hanford’s special space observatory this summer.
Astronomers at LIGO — the Laser Interferometer Gravitational-wave Observatory — discovered the gravitational waves passing through the Earth the morning of Aug. 17, according to a Monday announcement.
But these gravitational waves — or ripples through space and time — were detected for 100 seconds, unlike the waves that lasted as little as a fraction of a second from earlier detections from the collision of massive black holes.
It was an immediate clue that the ripples came from the massive, fiery collision of two neutron stars. The crash spewed material that radioactively decayed, creating heavy metals like gold and platinum.
Neutron stars are the collapsed cores of large stars and are the smallest, densest stars known to exist.
The neutron stars observed in August orbited each other and created the gravitational waves detected at LIGO in the 100 seconds before they merged into an ultradense object, emitting a fireball of gamma rays.
The collision was 130 million light years from Earth, occurring at the end of the Jurassic period on Earth, said Amber Strunk, Hanford LIGO outreach coordinator.
The gravitational waves passed through LIGO, and 1.7 seconds later the Fermi space telescope orbiting the Earth detected gamma rays.
In another 11 hours, telescopes saw light, said officials at a morning press briefing. Over the next two weeks other forms of light, or electromagnetic radiation, were observed, including X-ray, ultraviolet, infrared and radio waves.
It was the first time that a cosmic event has been viewed in both gravitational waves and light, giving scientists a new way of learning about the universe.
“This discovery marks the inception of multi-messenger astronomy,” said Michael Landry, head of the Hanford LIGO, in the desert northwest of Richland.
About a quarter of the world’s professional astronomers have been involved in the follow-up of the initial discovery, he said.
“It may be the most studied astrophysical object,” he said.
In addition to the Hanford LIGO, its twin LIGO observatory in Louisiana and the Virgo observatory in Italy recorded the gravitational waves, providing essential information to identify and pinpoint the location of the collision in the sky, allowing additional astronomers to find the collision site and make observations.
Theorists have predicted that what follows the initial fireball is a “kilonova,” a phenomenon by which the material that is left over from the neutron star collision, which glows with light, is blown out of the immediate region and far out into space.
The light-based observations made after the gravitational waves were detected Aug. 17 showed that elements heavier than iron are created in these collisions and then distributed through the universe.
Such neutron star collisions could account for half of the gold and plutonium in existence, scientists said.
Before the detection, scientists could not say for certain what causes short gamma ray bursts. Now they know that at least one source is colliding neutron stars.
Because the detection of gravitational waves and the initial gamma rays detection occurred within just seconds, scientists also were able to confirm that Albert Einstein’s theory of relativity was correct in its prediction that gravitational waves should travel at the speed of light.
While the observation of the neutron star merger answered some questions, it also raised others.
The short gamma ray burst observed was one of the closest seen to Earth to date, but it was surprisingly weak for its distance, scientists said.
The neutron stars were estimated to have about 1.1 to 1.6 times the mass of the sun, but each was only about the size of a city. Neutron stars are so dense that a teaspoon of neutron star material has a mass of about a billion tons.
LIGO at Hanford made its first detection of gravitational waves in September 2015, nearly 100 years after Albert Einstein predicted their existence.
The gravity waves were created as two black holes spiraled toward each other and collided in a distant galaxy more than 1 billion light years from the Earth.
The observation, after four decades of development and then operation of the LIGO observatories, led to the award of the 2017 Nobel Prize in Physics to key players in the project.
Since the first direct detection of gravitational waves, the Hanford and Louisiana LIGOs have detected gravitational waves three more times, most recently in conjunction with a third observatory in Italy.
Each time the gravitational waves were created from energy released as pairs of black holes merged. Black hole mergers do not emit visible light, preventing astronomers from observing the mergers with telescopes to gather more information about them..
Gravitational waves create ripples through the fabric of space and time, stretching space in one direction and contracting it in another.
The LIGO observatories look for that distortion, which effectively turns a circle into an ellipse.
At the Hanford observatory two vacuum tubes extend for 2.5 miles across the Hanford shrub steppe landscape north of Richland at right angles. At the end of each, a mirror is suspended on fine wires.
A high-power laser beam is split to go down each tube, bouncing off the mirrors at each end. If the beam is undisturbed, it will bounce back and recombine perfectly.
But if a powerful enough gravitational wave is pulsing through the Earth, the beam will be disturbed as the waves slightly stretch one vacuum tube and compress the other. The movement is so small that it would take 10 trillion such movements to equal the width of a human hair.
The gravitational waves detected most recently were the longest, closest and loudest gravitational waves LIGO has detected.
About 5:30 a.m. Aug. 17 Corey Gray, the lead operator at Hanford LIGO, received a gamma ray alert, but did not think much of it. He gets several alerts a week, he said. Shortly after that he received an alert of data that indicated a possible gravitational wave detection.
At that time the Hanford LIGO was nearing the end of a seven-month run and operators were looking toward the end of the session’s 24-hour-a-day control-room operations.
He signed into a conference call, expecting it to be routine, but something was different.
“People were talking very fast, people were talking over each other,” he said.
Within a few sentences he heard the words “neutron stars” and knew this was going to be monumental, he said.
“My first thought was we were connected to the world of astronomy and other astronomers,” he said.
The Hanford and Louisiana LIGOs are currently shut down for about a year’s work to increase their sensitivity.
When they come back online, they could make observations every few weeks or weekly, said David Shoemaker, of the Massachusetts Institute of Technology, the spokesman for the LIGO Scientific Collaboration.