A single merger of neutron stars can produce up to 8 x 10 22 tons of gold. The total effect of such astronomical events can explain the chemical composition of our entire Galaxy. Astrophysicists came to this conclusion after analyzing the heavy elements that are obtained as a result of such mergers. The main source of new data is the first reliably recorded burst of gravitational waves GW170817. The study is published in The Astrophysical Journal.
The original composition of the universe included only hydrogen and helium with small impurities. Elements up to the iron group are formed in the bowels of ordinary stars in the process of thermonuclear fusion. For heavier nuclei, this process becomes energetically unfavorable; therefore, they are formed as a result of neutron capture and subsequent β-decay.
There are two types of neutron capture: slow (s-process) and fast (r-process). With the help of the first one it is possible to obtain stable or long-lived nuclei, the half-life of which is much longer than the characteristic time of absorption of the next neutron. The result can be elements such as lead, bismuth and polonium. As a result of rapid capture, other elements can also be formed, since the nuclei do not have time to decay, but absorb the next neutron or even several at once. Accordingly, the r-process occurs only under the condition of a very high concentration of free neutrons. This is how elements such as gold and europium appear.
For a long time, astrophysicists argue when the r-process occurs. Some are inclined towards supernova explosions, others towards mergers of neutron stars. In a new work, scientists have found out whether it is possible to explain the amount of heavy elements observed in the Galaxy through the merger of neutron stars. Due to the fact that recently, for the first time, undeniably discovered such an event, scientists were able to roughly estimate the frequency of such phenomena. It turned out to be equal to 320-4740 pieces per cubic gigaparsec per year.
One such fusion should lead to the appearance of an amount of europium equal to 1 - 5 Earth masses (one Earth mass is about 5.97 x 1021 tons) and 3 - 13 Earth masses in the form of gold. If the event GW170817 is a typical merger of neutron stars, then the authors conclude that it is precisely these phenomena that can explain the amount of europium in the Milky Way. These mergers are the main places where the r-process takes place.