The discovery of gravitational waves provides new tools for astronomical observations and enabled the real-time detection of a high-energy collision and the study of its repercussions. The study included work by many Israeli scientists
In February 2016, scientists from LIGO (Laser Interferometer Gravitational-Wave Observatory) reported the first measurements of gravitational waves. This happened some 100 years after Albert Einstein predicted their existence, and decades of failed attempts to detect such waves. Three of the observatory's establishers were awarded the Nobel Prize in Physics for 2017. Since then, two more gravitational wave events have been detected by LIGO and by the parallel observatory, VIRGO, in Italy. The three waves that were measured were created as a result of a collision, followed by a merger, between black holes. Now, an international team of scientists, including members from Israel, have reported that they have detected gravitational waves of a different origin – a collision between two neutron stars.
The recent observation opens a window to one of the highest-energy events to occur in the universe and to novel research tools that will enable the study of processes we still do not fully understand, like the formation of heavy elements in the universe. The discovery demonstrates that the ability to measure gravitational waves is an important tool for modern observational astronomy.
Watch the press conference during which LIGO scientists announced the discovery and explained their findings:
The densest of stars
Gravitational waves are ripples in the curvature of space-time generated by the motion of heavy objects. These waves have relatively small amplitudes, so in order for them to be detected in devices on Earth, they need to come from a powerful source. Collisions between black holes produce these waves. Following the proof-of-concept of detecting these collisions, scientists had hopes that other observations of similar events will follow – detected using gravitational waves and observed (also) using other tools.
One of the events that researchers hoped to measure was a neutron star collision. Towards the end of its life, our sun will turn into a red giant, and then shrink into a white dwarf. Stars 10 to 30 times larger than our sun, however, may end their lives as neutron stars, which are made from the densest material in the universe. In these stars, the core implodes under such a high pressure that the electrons in their atoms merge with the protons to become neutrons. What is left from the huge star is just a lump of material about 10 km in diameter, which has a mass double that of our sun.
When two neutron stars come into close proximity of one another, they begin orbiting around each other in progressively decreasing orbits, spiraling until they collide. The collision emits an enormous amount of radiation until the remainders of the colliding stars form a black hole. At the site of collision, however, the hot material lingers for days after the collision, a glowing reminder of the violent event that had occurred. This glow makes it possible to measure the unique events occurring around the collision site.
Up until now, scientists were not able to observe such collisions, since the sky is so vast, and it is difficult to know when and where to look. The event observed on August 17, 2017, in the two LIGO detectors (in Washington state and Louisiana) and in the VIRGO detector, enabled the scientists to detect the exact direction and distance of the source of these gravitational waves, and direct other telescopes with cameras and instruments that measure the type and magnitude of emitted radiation. Among others, they used the Hubble Space Telescope, along with telescopes that employ X-rays and other types of radiation. The process is analogous to identifying the exact location of a lightning to find where the thunder will be heard, just that in this case, the source of the event is 100 million light years away, in the Hydra constellation, and not just above the adjacent mountain range.
A process documented for the first time: The collision and merger of two neutron stars | Illustration: Science Photo Library
The origin of heavy elements
One of the most intriguing processes about which we can learn from neutron star collisions is the formation of heavy elements. Light elements, especially hydrogen and helium, are ubiquitous in the universe. Hydrogen is the simplest element: a nucleus consisting of a single proton. Nuclear fusion of two hydrogen atoms creates a helium atom, in the process that generates the heat in our sun and in other stars in the universe. Under certain conditions, helium atoms can fuse to become heavier elements, which, in turn, fuse to generate even heavier elements. The heavier the elements, however, the harder it is for them to form through these types of fusion processes, and therefore elements heavier than iron (atomic number 26) and nickel (28) are not generated at the cores of stars.
The common notion is that heavier elements form during explosions of stars called supernovas. However, even these explosions do not provide all the answers. Another hypothesis suggests that at least some heavy elements, like gold, platinum, and even uranium, form during high-energy cosmic events, such as neutron star collisions. The findings from documenting the neutron star merger and the radiation measurements that followed are now being published in a long line of scientific papers.
Some of the research teams analyzed the composition of the emitted light in an attempt to understand which elements are formed during these collisions. In a recently published paper in the journal Nature, researchers, including Prof. Tsvi Piran from The Hebrew University of Jerusalem, report that their model indicates that heavy elements form during the process. They report detecting a signature of lanthanide elements, of atomic numbers 57-71, which include transition metals such as ytterbium, cerium, and neodymium. In another Nature paper, another research team reports specific detection of a number of heavy elements, including cesium and tellurium. Three Weizmann Institute scientists were involved in this study – Prof. Avishay Gal-Yam, Dr. Ofer Yaron, and Ilan Manulis.
In a paper published in Science, a large team of researchers describe the pattern of radiation emitted during the neutron star collision, through which they validated that a merger has indeed occurred between these two stars. Prof. Ehud Nakar and graduate student Ore Gottlieb, Prof. Tsvi Piran and Dr. Assaf Horesh from the Hebrew University, and Dr. Eran Ofek from the Weizmann Institute of Science are among the Israeli researchers who took part in this work.
Another team of scientists monitored emission of residual radiation from the cosmic event and analyzed the radiation type and magnitude. Their research, published in Nature, shows that the radiation emitted from the hot material left after the merger comes from radioactive elements that were formed during the big explosion, which gradually decay, as is expected in these processes. This mechanism explains the formation of heavy elements during nuclear fusion processes that occur due to the big explosion. This study was led by Dr. Iair Arcavi, now a postdoctoral fellow at the University of California, Santa Barbara and Prof. Dan Maoz, Prof. Dovi Poznanski, and Michael Zaltzman, all from Tel Aviv University.
Looking into the future
Astronomy began as a visual field, in which scientists study what they can see within the visible light spectrum. Technological development enabled detecting other types of radiation invisible to humans, such as infra-red radiation, ultraviolet radiation, X-rays, and radio waves. The ability to detect gravitational waves expands the range of observations to a completely different kind of wave, which is not just another electromagnetic wavelength. These waves provide evidence for cosmic events “hidden” from us until now, or detected with exceptional luck. The new addition to the astronomical tool box enables scientists to acquire a more comprehensive picture of the universe and unravel another layer of its secrets.
Translated by Elee Shimshoni