LIGO makes its first observation of colliding neutron stars

Event provides evidence for how heavy elements form

Scientists announced Monday the first detection of gravitational waves from the collision of two neutron stars. Light from the collision was simultaneously observed by telescopes all around the world.

The LIGO-Virgo collaboration observed the signal on Aug. 17. LIGO, whose two interferometric observatories are located in Washington and Louisiana, has made four previous observations of gravitational waves from the collision of black holes.

This particular observation is special because it was a neutron star collision rather than a black hole collision, and it was observed not only by LIGO in the U.S. but also by Virgo in Europe.

Not only is this collision the first event detected by both gravitational-wave and optical astronomy, but it has provided a wealth of observations that will contribute to insights in many different fields of physics.

Detecting a neutron star collision

The gravitational-wave signal lasted 100 seconds, and its characteristics indicated that it likely originated from the collision of a pair of neutron stars with a combined mass of around 2.7 times that of the sun’s mass.

Neutron stars are small, dense stars that form when stars ten or twenty times the size of the sun reach the end of their lives and collapse in on themselves. They are the densest stars in the universe — if the stars they formed from had been any more massive, they would have collapsed further to become black holes.

Scientists simultaneously observed a short gamma ray burst located in roughly the same area of the sky as the gravitational-wave signal.

Over the days following the first gravitational wave and gamma ray observations, more than 70 telescopes observed the afterglow of the collision in X-ray, ultraviolet, optical, infrared, and radio wavelengths.

The explosion produced when two neutron stars collide is called a kilonova. Scientists have long thought that such events are the source of many of the elements heavier than iron in the universe.

Stars, during their normal lifetimes, fuse light elements to release energy. They do not usually fuse anything heavier than iron because these processes require more energy than they release. A large supply of neutrons makes fusion of heavier elements possible, which is why kilonova are thought to be a source of elements such as gold and platinum.

In the optical observations of the neutron star merger, scientists detected spectra characteristic of these elements, providing support for this theory.

The Virgo gravitational wave observatory

The addition of the Virgo detector allowed scientists to pinpoint the location of the gravitational wave signal much more accurately than in previous LIGO detections. The neutron star collision was pinpointed to an 28-degree area of the sky with a probability of 90%.

The Virgo detector, which is located near Pisa, Italy, started its first observing run this summer after being upgraded to Advanced Virgo.