Astrophysicists have directly detected a spectacular collision of two neutron stars 130 million light years away, via both gravitational waves—ripples in space and time—and light. This marks the first time that a single cosmic event has been viewed in both gravitational waves and light.
This simulation from Caltech shows the final stages of the merging of two neutron stars. The merger shown in the simulation is happening much faster in reality, within less than a hundredth of a second, and produces strong gravitational waves. This illustrates one of the possible scenarios for the merger event GW170817, detected by the LIGO-Virgo gravitational-wave network. The result of the merger could have been a neutron star or a black hole, the latter of which is shown here.
Credit: W. Kastaun/T. Kawamura/B. Giacomazzo/R. Ciolfi/A. Endrizzi
The discovery was first made two months ago, on August 17, 2017, Laser Interferometer Gravitational-Wave Observatory (LIGO), the Europe-based Virgo detector, and some 70 ground- and space-based observatories.
“The idea that two neutron stars could merge was predicted more than 30 years ago, but has never before been definitively observed,” said Michael Shara, one of the researchers involved in the discovery, and a curator in the Museum’s Department of Astrophysics. “Detection of the merging neutron stars’ gravitational wave alone is cause for major celebration. But that we also saw an optical counterpart to this event some 130 million light years away makes this an incredible find for astrophysics.”
Curator Michael Shara explains Einstein’s general theory of relativity and describes the latest discovery in today’s podcast.
Neutron stars are the smallest, densest stars known to exist and are the remnants of massive stars’ explosions in supernovas. As these two neutron stars in a binary system spiraled in towards each other, they emitted gravitational waves that were detectable for about 100 seconds.
This movie shows a possible trajectory of the neutron stars that merged in an event called GW170817. The pair of stars—a neutron star and a normal star—orbit quietly (green), until the normal star goes supernova, spawning a second neutron star and “kicking” the system into an elliptical orbit (purple). The two neutron stars merge and generate gravitational waves, a gamma-ray burst, and a "kilonova" explosion. Other potential lives of the star pair are shown in the thinner lines and circles.
Credit: LIGO-Virgo/A. Geller/Northwestern University
When they collided and merged, a flash of light in the form of gamma rays was emitted and seen on Earth about two seconds after the gravitational waves. In the days and weeks following the smashup, other forms of light, or electromagnetic radiation—including X-rays, ultraviolet, optical, infrared, and radio waves—were detected.
The event demonstrates that, in addition to gravitational waves, merging neutron stars are also the sources of some short gamma ray bursts and extremely luminous novae, or new stars that appear suddenly and then fade into oblivion.
On hearing word of the event, Shara worked with the director and staff of the Southern African Large Telescope (SALT) to get an optical spectrum of the newly formed star.
“That SALT spectrum is similar to what has been predicted to be emitted by the merger of two neutron stars,” Shara said. “The light we detected in the first two days after the merger was very blue, then quickly became very red. That may be because we were seeing different sets of rare, heavy chemical elements ejected by different layers within the neutron stars as time progressed. There is no trace of the lightest elements, namely hydrogen, helium, carbon, nitrogen, or oxygen, which make up 99.9 percent of the matter in the universe.”
The SALT spectrum is part of the capstone paper in The Astrophysical Journal Letters that describes this major astrophysical detection.
These observations have given astronomers an unprecedented opportunity to probe a collision of two neutron stars. For example, observations made by SALT, the U.S. Gemini Observatory, the European Very Large Telescope, and NASA’s Hubble Space Telescope suggest the presence of recently synthesized material, including the rare earth elements mined on Earth for essential components in cell phones and other electronics.
“The new light-based observations hint that the lanthanide and actinide families of chemical elements are created in these collisions and subsequently distributed throughout the universe,” Shara said. “This would solve a decades-long mystery of the origin of these rare but vital chemical elements.“