Neutron Stars’ Death Merger Rattles The Fabric Of Spacetime

Simulation of the the two neutron stars that collided in the galaxy NGC 4993 in Hydra to create both a visible explosion and ripples in spacetime

It happened 130 million years, but we had to wait till August 17 to find out. That’s when scientists directly detected gravitational waves — ripples in in the fabric of spacetime — as well as light from the spectacular collision of two neutron stars. On four previous occasions, the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO) has recorded shockwaves in spacetime from black hole mergers, but this event was unique because we saw it in visible light, too.  The merger created a brief flash of light trillions of times as bright as the sun and released as much energy as the sun will in its 10-billion-year lifetime.

The discovery was made using LIGO, the Europe-based Virgo detector and some 70 ground- and space-based observatories.

This illustration imagines the appearance of two merging neutron stars. The rippling space-time grid represents gravitational waves that travel out from the collision, while the narrow beams show the bursts of gamma rays that are shot out just seconds after the gravitational waves. Swirling clouds of material ejected from the merging stars are also depicted. The clouds glow with visible and other wavelengths of light. Credit: NSF/LIGO/Sonoma State University/A. Simonnet

Neutron stars are the smallest, densest stars known — a teaspoon’s worth weighs a billion tons —  and form when massive stars collapse and explode as supernovae. Gravity’s grip on the star’s core during the collapse crushes all the electrons and protons of the various elements there into pure neutrons squeezed into a white-hot sphere just 12.5 miles (20 km) across. Since two neutron stars were involved, the pair was likely a binary or double star, each of which evolved into a neutron star in close orbit about the other. How they could survive dual supernova explosions and neither be ejected is testament to gravity’s might.

Listen to the chirp of the merging neutron stars as recorded by the LIGO instrument. It’s toward the end.

As far back as 1915 in his General Theory of Relativity, Einstein predicted that massive objects accelerating around each other would lose energy in the form of gravitational waves. This would cause them to orbit ever more closely over time until neither could resist the gravity of the other, ultimately merging into one. On Aug. 17, two neutron stars in the galaxy NGC 4993 in Hydra spiraled together and emitted gravitational waves that LIGO detected as a loud “chirp” for about 100 seconds. At the same time, they produced a powerful barrage of gamma rays seen by Earth-orbiting telescopes about two seconds after the gravitational waves.

In the days and weeks following the smashup, other forms of light, including X-ray, ultraviolet, optical, infrared, and radio waves were seen in the afterglow.

Astronomers not only saw gravitational waves and gamma rays from the event, but also visible and near-infrared light as shown in these images made with the Swope and Magellan telescopes. Credit: 1M2H/UC Santa Cruz and Carnegie Observatories/Ryan Foley

Space-time is the union of the three familiar dimensions with time to create a four-dimensional world. Everything exists in space-time. Massive objects like the Earth and sun bend or warp space-time in four dimensions much like a bowling ball placed on a trampoline creates a dimple or depression in three.  Gravitational waves ripple the fabric of space-time like a wave lifts and lowers water. In other words, it briefly shifts an object’s position. Not by much, mind you. LIGO’s detectors are designed to detect a shift of 1/10,000th the wide of a single proton!

Astronomers coordinated worldwide to observe and share information about the event. Credit: LIGO

During the merger and seen in the video, a plume of dust rich with newly-formed heavy elements including gold, silver and platinum was shot into space and seen by the U.S. Gemini Observatory, the European Very Large Telescope, and the Hubble Space Telescope. Astronomers couldn’t have been happier, since the question of exactly how these heavy elements form has been a vexing one for decades.

“From informing detailed models of the inner workings of neutron stars and the emissions they produce, to more fundamental physics such as general relativity, this event is just so rich. It is a gift that will keep on giving,” said David Shoemaker, spokesperson for the LIGO Scientific Collaboration

The gravitational signal, named GW170817, was detected by two identical LIGO detectors in Hanford, Washington and Livingston Louisiana on Aug. 17 at 8:41 a.m. Eastern time. At nearly the same time, the Gamma-ray Burst Monitor on NASA’s orbiting Fermi space telescope had detected a burst of gamma rays. This was not a chance coincidence. Observatories around the world were quickly alerted to the event and gathered photos and spectra (spreading the light into a rainbow of colors to look for signatures of elements present) of the afterglow as it faded from bright blue to red in the coming days.

How LIGO works. A single laser beam is split into two beams, reflected off mirrors and redirected back to a detector where they cancel out. Gravitational waves cause the instrument to stretch and squeeze, throwing the beams very slightly out alignment. Scale this up to 2.5 miles on a side and you’ve got a LIGO detector. Credit: NSF with additions by the author

LIGO detects these submicroscopic blips in space-time by monitoring the distance between two mirrors using lasers. Normally, the lasers arrive at the same point in the exact same time after being reflected from the mirrors, but if something big rocks space-time, their arrival times will vary ever so slightly. There was a delay of 1.7 seconds between the ripple detection and when the gamma rays were picked up by satellite because the jet takes that long to produce the radiation. Both gravitational waves and all forms of light (gamma rays, X-rays, visible, etc.) travel at the speed of light.