This article on NS-NS collisions is a guest article by Jwalant Yagnik, an M.Sc Astrophysics student in the Netherlands.
With this article, I wish to talk about one of the most brilliant discoveries of the previous decade! A discovery named as the breakthrough of the year in 2017 by the famous journal: Nature. A discovery that has far-reaching implications, not just in the field of science but also in our lives.
The Periodic Table For Astronomers
Before we talk about this breakthrough, we need to revisit some of the fundamentals of science that we studied in schools. So, let us take a trip down memory lane and revisit our Chemistry class from high school. Do you remember something called the periodic table? I used to hate memorizing all the elements according to their atomic numbers, properties, and classifications as metals, non-metals, and so on.
But here is an image of the Periodic table for your reference. The table also shows the processes by which the elements were synthesized. Lucky enough for me, I chose to become an astrophysicist, and for observational astronomers, the periodic table looks something like this:
As funny as it may seem, it is a fact! For our purpose, all the elements which were not synthesized during the big bang are heavy elements, and we call them metals. Only Hydrogen and Helium were directly produced in the initial moments of the big bang. All other elements are made in some processes in the atmosphere of planets or the cores of stars.
For example, elements like Lithium are produced during interactions of cosmic rays with our atmosphere. Please note that when I say element, I mean the nucleus of that element. Because we are talking about highly energetic processes here, and at such high energies, the elements get ionized, meaning they lose all their electrons.
So, in the periodic table, there are nuclei whose origin we know. Like, Li, Be, B, C, N, O, up to Fe, Co, and Ni. All of them are a product of some nuclear process in a planet or a star, and we have observational evidence for these.
The heaviest ones, Iron, Nickel, etc., are created during the violent deaths of stars, namely supernovae. Above that, we do not have observational proof about how nuclei heavier than Iron are created. Until now, we thought that they might be a product of supernovae, too, because the formation process is similar.
- Nuclear reactions in stars
- LIGO and the detection of the gravitational waves
- Detecting gravitational waves from a pair of white dwarfs
Lessons From Nuclear Physics
To understand how these processes occur, we need to understand a little bit of Nuclear Physics. We know that the nucleus is made up of two particles: Protons and neutrons. Protons, being positively charged, neutrons being neutral. We know that two positively charged particles would repel each other. So, to make stable or semi-stable nuclei, beyond one point, we need something more inside the nucleus to hold it together against the repulsive force. So, as we go from lighter nuclei towards heavier ones, we start noticing the number of neutrons become much larger than protons.
We know from theory and experiments on radioactivity that if a nucleus is bombarded with neutrons, it will absorb a neutron and eventually decay in such a way that it becomes stable. To gain stability, depending on which nucleus we are talking about, sometimes, a neutron in the nucleus splits into a proton and an electron (called beta radiation), hence maintaining the charge neutrality, and also emit a neutrino (but we do not need to worry about the neutrino here).
In this process, we created an element whose proton number increased by 1 due to a neutron splitting into a proton and an electron. This way, we made a higher atomic numbered nucleus. This is a very crude way of explaining this process, as I do not wish to make it more complicated to understand. But, please note that this is a very, very diluted version of what actually happens. The actual process is significantly more complicated. To synthesize much heavier nuclei, we need to imagine this process on a dramatically larger scale. We need to have access to an enormous amount of neutrons in an extreme environment.
Do we know of any cosmic entity which has these conditions? Have you heard of Neutron Stars?
The Neutron Stars
Neutron Stars: Extremely dense, super-heavy, small objects, which are just remains of dead stars. These objects can be about 20 km in diameter yet have masses up to 1.5 times the mass of our Sun. The gravitational force in this dead star is so overwhelming that it collapsed into a state where the atoms’ electrons were captured by the nuclei and fused with the protons to make neutrons, and hence the name: A neutron star.
So we now know an object which is mostly made of neutrons. But, it is a dead star. How can it make heavy nuclei? It is already dead! Nuclear fusion is no longer going on.
It turns out that when we set our telescopes onto the night sky and see it, we observe that a vast majority of stars exist in pairs. Is it possible that there might be a pair of neutron stars? In 2015, we detected the gravitational waves coming from a merger of a binary black hole pair. It does not seem impossible to find a pair of neutron stars, and if we can see two of that merge, maybe we will see the birth of such heavy elements by the process described in the previous part.
This observation is easier said than done. Firstly, because neutron stars are only about 20 km in diameter, so they are tiny. Second, they hardly emit any visible radiation. Dead star, remember? How do you see something which does not emit light but is immensely heavy?
Well, we already know the answer to this question! Gravitational waves! We try to detect gravitational waves coming from a binary neutron star merger. Once we detect this, we point all the telescopes available to us towards the GW signal’s direction. Theory suggests that there will be a lot of heavy nuclear production, and in that process, there will also be a lot of nuclear radiation! Hence electromagnetic radiation should be abundant to actually see the event!
August 17, 2017
One such gravitational wave signal is detected in LIGO detectors from merging neutron stars, and Astrophysicists worldwide are alerted about it. Immediately, all the telescopes on earth as well as in space were pointed towards the direction of the signal (calculated with the help of LIGO and VIRGO detectors), and voila! We have visual confirmation of light coming from the NS merger!
Multiple spectra were taken in all available bands, and it was confirmed that the heavy elements were indeed produced there! Just to put this into perspective, a rough estimate suggests that Gold was one of the products in this nucleosynthesis, and the amount of Gold produced was equal to 5 to 6 times the entire mass of the earth!
The Importance of This Discovery
What does this discovery mean for science? This is the first time we saw an electromagnetic signal along with a GW signal. We now have access to a completely new window to look at the universe! This was also the experimental confirmation that heavy elements are produced in NS-NS mergers.
What does this discovery mean to us? So far, the only process which produces elements like Gold, Platinum, Uranium, and so on is binary neutron star merger. We have all of these elements on Earth. This implies that we may be the product of one such process, and we owe our existence to it!
For me, the field of GW, especially the optical counterpart, is fascinating, and that is also the subject for my master’s thesis project. We have developed special telescopes to observe these optical signals and study them in much more depth. I am a part of the team that built these three telescopes (BlackGEM array of telescopes) in Chile, in South America, to detect these optical light signals coming from colliding Neutron Stars.