The Science magazine event of the year was the collision of two neutron stars. This event, the first ever detected, produced both light and gravitational waves.
A neutron star forms following a supernova, the collapse of a massive star that squeezes protons and electrons together to form neutrons. To do this requires replacing one of a proton’s up quarks with a down quark to change it into a neutron.
This happens when the wave functions of an electron and a proton in an atom are squashed enough to overlap. Simply stated, the electron emits a W+ boson and becomes a neutrino. An up quark in the proton absorbs the W+ boson, which becomes a down quark.
Neutron stars rarely form as pairs. General relativity predicts that when they do the orbits will decay, giving off gravity waves. The theory predicts that after hundreds of million years, they will collide and give off a burst of energy.
When two LIGO (Laser Interferometer Gravitational-Wave Observatory) sites in Washington state and Louisiana recorded the first gravitational waves from the collision of two black holes, it verified yet another prediction of general relativity and won a Nobel Prize for its innovators, Rainer Weiss, Kip Thorne and Barry Barish.
After the initial LIGO discovery, they and the Virgo detector, located near Pisa, Italy, and newly operational, detected a second black hole collision. The third detector added a third dimension to the data. Using triangulation and the difference in arrival time of the signals, astrophysicists narrowed the location of the collision event to a size of only 60 square degrees in the sky, more than 10 times smaller than using the data available from only two LIGO interferometers.
The important difference between black hole collisions and neutron star collisions is that the less massive neutron star collisions produce a higher frequency of G-waves but also a flash of electromagnetic (EM) radiation. In theory, when neutron stars smash into one another, a spectrum of EM radiation blasts out: gamma rays, X-rays, radio waves. Everyone who has a window on the sky should be able to see the event.
The difficulty in the past has been in locating a colliding neutron star pair. Aiming the telescope randomly at the sky has a near-zero probability of finding them.
Observational verification came immediately after the high-frequency G-waves arrived from the colliding neutron stars. Within two seconds of the “chirp” of the gravity wave on the LIGO and Virgo instruments, a “blip” occurred on the Earth-orbiting Fermi Gamma-Ray Space Telescope. It recorded a brief flash of gamma rays, in a category of short gamma ray bursts that theoretical astrophysicists have suspected are the result of neutron stars colliding.
“Joy for all,” said David Shoemaker, spokesman for the LIGO Scientific Collaboration. “When we saw that, the adrenaline hit.”
Astrophysicists had long theorized that neutron star collisions should create heavy elements such as gold in a type of explosion never before witnessed, a kilonova.
After the collision, the resulting kilonova forged 10 Earth masses of gold along with silver, platinum and a smattering of other heavy elements such as uranium, thus verifying the last remaining mystery of how chemosynthesis forms the chemical elements.
The event of the year was much more than merely a chirp on LIGO and a gamma-ray blip. It was verification of several hypotheses of general relativity and the start of a new era in astronomy. Combining gravitational waves with light from a neutron star merger has been a long-held dream of astrophysicists.
Its detailed picture revealed the inner workings of neutron star collisions and the source of short gamma-ray bursts. Researchers also calculated how fast the universe is expanding and tested the properties of the odd material within neutron stars.
Richard Brill is a professor of science at Honolulu Community College. His column runs of the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
