For astronomers and scientists around the world, this was “the event we’ve all been waiting for.”
It was 8:41 a.m. Eastern time on Aug. 17 when several devices stationed in the U.S. and Europe detected a series of gravitational waves, or ripples in space and time, from two high-density neutron stars that collided about 130 million years ago, or 130 million light years from Earth. As the stars spiraled faster and closer together, they stretched and distorted the surrounding space-time, giving off energy in the form of powerful gravitational waves before smashing into each other.
At nearly the same time, a telescope operated by NASA detected a flash of light in the form of gamma rays that was visible for just two seconds. But the brief episode of light allowed scientists to observe the spectacular collision of two neutron stars for the first time ever, an event that confirms theories about gravitational wave sources that had been developed over decades.
“The new thing this time is that not only did they detect gravitational waves from neutron stars but the really new thing here is that we also have detected light coming from the same location in the sky,” said Raffaella Margutti, assistant professor in the Department of Physics and Astronomy at Northwestern University. “It’s the very first time we have been able to detect gravitational waves and light from the same source.”
Scientists had long predicted that neutron star collisions produced powerful explosions. During the historic observation on Aug. 17, scientists were able to determine that the violent and bright collision was indeed so powerful that it forged materials such as gold and platinum, solving a decadeslong mystery about where many heavy elements central to life on Earth are produced.
“The reason that this particular object is interesting is that it brings together so much physics,” said James Annis, a senior scientist and astrophysicist at Fermilab. “If you look at the periodic table there are these rows that don’t fit. That first of the rows that don’t fit, that whole row was made in this weird neutron star merger. In that row was gold and platinum – so just knowing where gold and platinum come from in the universe is pretty cool. ... The origin of the gold in my wedding ring almost certainly came from an event just like this 10 billion years ago.”
“This was the event we’ve all been waiting for,” said Northwestern University astrophysicist Shane L. Larson in a press release, noting that all four previous gravitational wave detections were of black holes and therefore invisible to scientists. “Black holes don’t emit light, but neutron stars are made of matter that does emit light. These skeletons of dead stars are telling us something about how stars live and evolve in the universe and how they fill the stellar graveyard with whatever it is that they’ve done during their life.”
A Northwestern press release states: “Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovae. A neutron star is about 12 miles in diameter and is so dense that a teaspoon of neutron star material has a mass of about a billion tons.”
The discovery of the colliding neutron stars was made by thousands of scientists and engineers at the U.S.-based Laser Interferometer Gravitational-Wave Observatory, or LIGO; the Italy-based Virgo gravitational wave detector; and roughly 70 ground- and space-based observatories, including NASA’s Hubble Space Telescope. The international research effort features four Northwestern astronomers, including Vicky Kalogera, the leading astrophysicist in the LIGO Scientific Collaboration.
On Monday, Kalogera and a handful of other experts will discuss the findings as part of a panel sponsored by the National Science Foundation and hosted at the National Press Club in Washington, D.C.
The groundbreaking observation comes about two years after gravitational waves were directly detected for the first time on Sept. 14, 2015, by LIGO, stationed in Hanford, Washington. The discovery confirmed Albert Einstein’s theory of general relativity, which predicts that gravitational waves should travel at the speed of light, and led to LIGO’s architects receiving the 2017 Nobel Prize in Physics.
Once word of the gravitational-wave signal spread to astronomers the morning of Aug. 17, they had to hurry to point their telescopes to a certain area of the sky before sunset so that they would be ready to detect electromagnetic radiation signals produced by the collision.
Each of the two types of signals provides critical information about the same cosmic event: Gravitational waves tell scientists what the objects are, the masses of the objects and their approximate location in the sky. Electromagnetic waves, meanwhile, are used to pinpoint the event’s exact location in the sky.
Detection of gravitational waves by LIGO and Virgo detectors, coupled with gamma-ray detection by NASA’s Fermi space telescope, enabled follow-up observations by telescopes around the world, including in Chile and New Mexico, both locations where Northwestern astronomers work.
“I remember waking up that day to a flurry of emails saying there is a gamma-ray burst in conjunction with a gravitational wave event,” said Wen-Fai Fong, a Hubble postdoctoral fellow at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics, in a press release. “I thought, ‘This is what we’ve been waiting for our entire careers.’”
The discovery of two colliding neutron stars raises a number of questions. For example, astronomers know that the stars merged into one object, but what exactly is it?
“This is just the beginning for us,” Kalogera said in a press release. “The more sources like this we can detect, the more we can learn. The universe doesn’t stop with one such collision, and not all collisions will be the same. We’re bound to find new mysteries.”
Note: This story was originally published on Oct. 16.
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