The collision of two neutron stars recorded on August 17 created an explosion called a kilonova. The event launched a colossal amount of incandescent matter into space that shone brightly for several days. Changes in the color and brightness of the kilonova provided the strongest evidence yet that the matter and energy released by colliding neutron stars is the source of some of the heaviest chemical elements in the universe. It is not known precisely which elements or how much of them was created in the explosion, but it almost certainly produced a large amount of uranium, gold, and other rare metals, such as platinum.
“Studying the radiation emitted by the kilonova will give us an idea of which elements were forged,” says Valdir Guimarães, a physicist at the University of São Paulo’s Institute of Physics (IF-USP) who did not participate in the observations, but closely followed the published results. “Some papers suggest that the amount of gold produced by the event could have a mass equal to that of Earth.” As well as gold and platinum, it is estimated that as many as 60 other elements from the periodic table were formed, together corresponding to less than 1% of the visible matter in the universe.
“This event strongly indicates that many of the heavy chemical elements found in nature were produced by kilonova explosions,” says Brazilian astrophysicist Vinicius Placco, a professor at the University of Notre Dame, USA. He studies the abundance of chemical elements in metal-poor stars in the Milky Way, comparing these observations with theoretical predictions regarding a higher energy phenomenon called a supernova—the explosive death of a star whose mass is dozens of times greater than our Sun—which also produces heavy elements. Alongside other Brazilians, Placco was part of the group that observed the August kilonova through the T80-South telescope in Chile (see article). He explains why it is not yet possible to know exactly what elements were produced by the event. “The brightness of the visible light range decreases by 360 times every 10 days, making it difficult to accurately measure the abundance of chemical elements formed,” he reports. “We need to observe more kilonova events to obtain such estimates.”
The August 17 kilonova was not the first to be discovered. In 2013, astrophysicist Nial Tanvir, from the University of Leicester, UK, observed another using the Swift and Hubble space telescopes. The afterglow was faint, however, and there was no information on the cause of the explosion—whether it was a collision between two neutron stars, or a neutron star and a black hole. The August event is one of the best documented by astronomers in recent years. Its light was recorded across the entire electromagnetic spectrum, and analysis of the gravitational waves emitted as the stars approached each other showed that their masses were 30% and 60% greater than the Sun’s.
Detection of the kilonova was announced on October 16, and over the following days there was a flood of scientific articles describing the phenomenon. Two weeks later, researchers from four North American universities consolidated the first summary of the kilonova observations and posted it on ArXiv, an online repository of scientific articles. Measurements taken by 38 telescopes for up to a month suggest that the colliding stars launched matter with 21,500 times the mass of Earth into space.
The explosion resulted in a black hole; a dark and extremely dense object, from which even light cannot escape. The energy involved in the explosion created a whole range of heavy chemical elements in less than a second when non-charged particles (neutrons) released by the stars collided with the nuclei of lighter chemical elements thrown into space by the blast. This mechanism, called rapid neutron capture or the r-process, produces elements as heavy as uranium, the nucleus of which has 92 protons (particles with a positive electric charge) and 146 neutrons. Heavier elements can form, but they are unstable and quickly break down, releasing other particles and energy in the form of electromagnetic radiation—especially gamma rays, a type of light that is invisible to the human eye.
The energy produced by the conversion of heavy and unstable elements into lighter, more stable ones changes the color of the kilonova. In the first few days, the telescopes captured blue light produced by a cloud with 5,300 times the mass of Earth, rich in elements lighter than lanthanum (57 protons and 139 neutrons), moving away from the collision at 81,000 kilometers per second, according to an article submitted for publication in Astrophysical Journal Letters. As the matter expanded and cooled, the central region of the kilonova turned purple and then reddish. “The change in color was caused by the radiative decay of heavier chemical elements whose masses are greater than lanthanum, which were concentrated in a slower-moving region of the cloud of matter,” explains Placco.
The merging of two neutron stars is believed to be a rare event that may occur just once every million years in our galaxy. Astrophysicists, however, expect the increased sensitivity of the LIGO and Virgo observatories to make detection of these events commonplace, an exciting prospect for astrophysicists and cosmologists. Joint observation of the gravitational waves and light produced by the colliding neutron stars could help resolve a long-standing dispute in cosmology: the value of the Hubble constant, a unit of measurement used to indicate the rate of expansion of the universe, and consequently, its age and composition.
Since measurements made by American astronomer Edwin Hubble in 1929 confirmed that the universe is expanding, various groups have attempted to accurately calculate the speed of its growth, which increases alongside distance. Hubble himself calculated that the speed at which celestial objects are moving increases by 500 kilometers per second every megaparsec (3.3 million light-years). It is now known that the value, measured using two strategies, is actually much lower.
One method involves estimating distances based on the luminosity of Cepheid stars, which pulsate regularly and have a known brightness. This technique results in a value of 73 kilometers per second per megaparsec (km/s/Mpc). But the method is not without its flaws. “For the Cepheid technique, the period-luminosity relation of these stars must be calibrated,” says astrophysicist Luis Raul Abramo, a professor at IF-USP. “This calibration is empirical, even though models of the interior of these stars are quite sophisticated.”
The other way to estimate the value of the Hubble constant is by using satellites in Earth’s orbit to measure cosmic microwave background radiation (CMBR), a form of invisible light that travelled through the universe 380,000 years after the Big Bang. Its distribution followed a certain pattern, at a time when the cosmos was denser. Physicists have calculated the constant based on how the density and geometry of this pattern varied, giving a value of 67 km/s/Mpc. This method is also inexact and can produce varying results, since it depends on the model used to explain the universe—the most accepted being that it is flat, composed of common matter, dark matter, and dark energy, and that its expansion is accelerating.
The difference between the two Hubble constant values is small (10%), but it still bothers cosmologists. “Either the measurements made using Cepheids need to be corrected, or there are problems with the most widely accepted cosmological model, which would have significant theoretical consequences in cosmology,” says astrophysicist Jailson Alcaniz, from the Brazilian National Observatory in Rio de Janeiro.
It is hoped that the dispute will be resolved with further distance measurements based on the gravitational waves emitted by neutron star collisions. The measurement taken in August resulted in an intermediate value: 70 km/s/Mpc, according to an article published in Nature. There is still a lack of precision involved. “Gravitational waves enable us to make a more direct measurement of these large distances, which in the case of neutron stars, can be compared with light analysis to verify the expansion speed,” says Abramo. “In my opinion, this dilemma will be solved through more observations of gravitational waves.”
ASHLEY VILLAR, V. et al. The complete ultraviolet, optical, and near-infrared light curves of the kilonova associated with the binary neutron star merger GW170817: Homogenized data set, analytic models, and physical implications. ArXiv. Online. Oct. 31, 2017 (submitted to The Astrophysical Journal Letters for publication).
THE LIGO SCIENTIFIC COLLABORATION AND THE VIRGO COLLABORATION. et al. A gravitational-wave standard siren measurement of the Hubble constant. Nature. V. 551, p. 85–8. Nov. 2, 2017. Online. Oct. 16, 2017.