Does fission exist in the universe? The Milky Way's ancient stars hold a secret

Author: Shen Wen, Editor of Principles

In theory, when two neutron stars collide, superheavy elements heavier than the heaviest elements on Earth will be produced. However, these superheavy elements are very unstable, and they will quickly break down through fission to produce lighter elements. But no one has been able to prove that such fission actually occurs in the universe. Until December 7, 2023, in a study published in the journal Science, researchers said they had found evidence for the fission of elements heavier than uranium for the first time. To understand how this is going, let's start with the origin of the elements.

The periodic table is very familiar to everyone, but the elements it contains have not always existed. In fact, the story of the origin of the elements goes back to the Big Bang 13.8 billion years ago. About 15 minutes after the Big Bang, the first chemical elements were produced in the universe: hydrogen, helium, and a small amount of lithium. These three elements are also the first three elements in the periodic table. Apart from these three elements, there are no other heavier elements in the early universe.

Over time, about 100 million years after the Big Bang, the first stars formed in the universe. After that, stars became factories for the elements. In the cores of stars, nuclear fusion turns hydrogen into helium. Fusion will continue as long as there is enough fuel. But like life, stars die. At the end of their evolution, as their fuel runs out, they produce heavy elements at an increasingly rapid rate. Over a period of time, the helium in a star will turn into carbon and oxygen. Later, the oxygen will fuse into silicon, phosphorus, and sulfur. In the last stages of a star's long life, it will produce metals like iron. Once a star starts producing iron, there is nothing to stop gravity from ruthlessly destroying it. In less than a second, the star collapses under its own gravity and explodes as a supernova, spewing the newly created elements into the universe.

The heaviest elements in the universe are created by the so-called r-process, or fast neutron capture, in which atomic nuclei rapidly absorb neutrons and then undergo radioactive decay, creating new elements, such as platinum, gold, and uranium.

Figure: When two neutron stars merge into each other, neutrons and atomic nuclei (such as iron nuclei) will fly out, and a bunch of neutrons will gather in the atomic nucleus in a very short time (usually less than a second), and then the neutrons in the atomic nucleus will decay to form heavy elements (such as gold).

The R process requires a lot of energy and a lot of neutrons to achieve, so it often occurs in a neutron-rich environment, such as the merger of two neutron stars. When stars with masses several times that of the sun die, their cores collapse into neutron stars. A few years ago, astronomers have confirmed in a major binary neutron star merger event that it will produce a large amount of heavy elements.

Although scientists have a general understanding of how the r-process works, the r-process that produces elements heavier than uranium is still poorly understood. And because the conditions under which the r-process occurs are so extreme, it is impossible to study in the laboratory.

So, in order to better understand the r-process, the researchers re-examined the data on various elements in 42 very old stars in the Milky Way. These stars all contain r-process elements.

Taking a broader look at the amounts of each heavy element found in these stars, they found that the abundance of the elements ruthenium, rhodium, palladium, and silver in these stars is correlated with a group of heavier elements, and that when an element in one group increases, the corresponding element in the other group also increases in a positive correlation.

The only plausible explanation for this phenomenon in different stars is that there is a unified process in the formation of heavy elements. After testing all possibilities, the researchers believe that fission is the only explanation that can reproduce this trend. Fission is basically the opposite of fusion. It is the process of releasing energy when heavy elements split to produce lighter elements.

The new results suggest that some r-process events can produce elements heavier than uranium, which then decay into the elements observed in stars. In other words, some elements in the middle of the periodic table, such as silver and rhodium, are likely the remnants of fission of some heavier elements. Even more surprising, the researchers found that the r-process can produce elements with an atomic mass number of at least 260 before fission, with far more neutrons than protons in their nuclei.

This research not only provides the first evidence of fission in the universe, but also greatly deepens our understanding of the formation of elements.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Shen Wen

Reviewer: Zhang Shuangnan, Researcher, Institute of High Energy Physics, Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.