In February 2016, scientists from the LIGO observatory reported the first measurements of gravitational waves. This was about 100 years after Albert Einstein predicted their existence, followed by dozens of years 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 in the LIGO observatory and in 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 some 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 us to study 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 spacetime in space-time that are generated as a result of 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 that detecting these collisions is possible, scientists were hoping that other observations of similar events, which can be detected using gravitational waves, will be made, and then they can also be observed using other tools.
One of the events that researchers were hoping 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. Larger stars that are 10 to 30 times bigger 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 to 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. Nevertheless, at the site of collision, the hot material lingers for several days after the fact, as a glowing reminder of the violent event that had occurred. This glow makes it possible to measure the unique events occurring around the site of collision.
Up until now, scientists have not been able to observe these collisions, since the sky is so vast, and it is difficult to know when and where to look. The event observed on August 17th 2017, in the two LIGO observatories (in Washington state and Louisianna) and in the VIRGO observatory, allowed the scientists to detect the exact direction and distance of the source of these gravitational waves, and then direct other telescopes with cameras and devices 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 can be compared to identifying the exact location of lightning in order 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.
The origin of heavy elements
One of the most intriguing processes that we can learn about from neutron star collisions is the formation of heavy elements. Light elements, especially hydrogen and helium, are very ubiquitous in the universe. Hydrogen is the simplest element: a nucleus consisting only one proton. Nuclear fusion of two hydrogen atoms creates a helium atom, which is the process that generates the heat of our sun and of other stars in the universe. Under certain conditions, helium atoms can fuse to become heavier elements, and in turn they fuse to generate even heavier elements. The heavier the elements, however, the harder it is for them to form via 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 states 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 attempt to understand what elements are formed during these collisions. In a recently published paper in the journal Nature, researchers, including Prof. Tsvi Piran from the Hebrew University in 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. Another research team reports in a different Nature paper specific detection of a number of heavy elements, including cesium and tellurium. Three scientists from the Weizmann Institute were involved in this study – Prof. Avishay Gal-Yam, Dr. Ofer Yaron and Ilan Manulis.
In a paper published in the journal 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 in deed occurred between these two stars. The following Israeli researchers took part in this work – Prof. Ehud Nakar and graduate student Ore Gottlieb; Prof. Tsvi Piran and Dr. Assaf Horesh from the Hebrew University in Jerusalem; and Dr. Eran Ofek from the Weizmann Institute of Science.
Another team of researchers monitored emission of residual radiation from the cosmic event and analyzed the type and magnitude of radiation. They show, in their study that was published in Nature, that the radiation emitted from the hot material remaining following the merger comes from radioactive elements that were formed during the big explosion, which gradually decay, as 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, which is now a postdoctoral fellow at University of California, Santa Barbara, joined by Prof. Dan Maoz, Prof. Dovi Poznanski and Michael Zaltzman, all three 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. As technology developed, we were able to detect other types of radiation that are not visible 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 that were hidden from our eyes until now, or that we could only detect if we were very lucky. This ability enables scientists to acquire a more comprehensive picture of the universe, and unravel another layer of its secrets.
Translated by Elee Shimshoni