How elements merged by gravity

Origin of the elements

Shortly after the Big Bang, there were only light elements, mainly hydrogen and helium. Heavier atoms only formed over billions of years through fusion processes in stars and huge explosions in space. Understanding this element synthesis is the subject of current research.

Nature created the chemical elements, of which all stars and planets, all organisms and also we humans are made, in two phases. The first phase ended just a few minutes after the Big Bang. Until then, only the lightest elements hydrogen and helium, as well as lithium and beryllium in small amounts, had been created. After that, the temperature and density in the expanding universe sank so much that heavier atomic nuclei could no longer be formed.

The second phase of nucleosynthesis did not begin until a few hundred million years later. At that time, the first stars formed from the primordial gas due to gravitational pressure. Nuclear reactions set in in their hot centers, in which the light elements hydrogen and helium gradually fused to form heavier elements down to iron. Atomic nuclei heavier than iron were formed in the last stages of development of massive stars, the so-called red giants, and in huge stellar explosions, the supernovae. The famous sentence: “We are made of stardust” is therefore not to be understood metaphorically, but in the literal sense: Every atom heavier than beryllium in our body or wherever in the universe owes its existence to the synthesis of elements inside the stars.

Supernova 1994D

Describing these processes using the laws of physics is a goal of nuclear astrophysics. The demands are high: it is an attempt to explain the frequency distribution of the elements quantitatively. Closely related to this is the question of how the astrophysical objects developed that produce the elements to this day. Without knowledge of the structure of atomic nuclei and the dynamics of nuclear reactions, this endeavor is hopeless. Nuclear astrophysics and nuclear physics are therefore closely linked.

The source of energy that makes stars shine is nuclear fusion. Here, light cores fuse to form heavier ones, creating new elements and releasing huge amounts of energy. Our sun, for example, burns more than 600 million tons of hydrogen into helium every second. Despite the enormous consumption, their large hydrogen reservoir is sufficient to achieve a lifespan of several billion years. Stellar fusion processes take place at such low energies that their reaction rates are extremely small. Direct measurements in laboratories on earth are almost impossible, even with the help of extremely intense particle beams, since cosmic radiation creates a much higher background of interference signals. Nevertheless, in the past few years, an international collaboration with an experiment in the Italian Gran Sasso laboratory has succeeded for the first time. This laboratory is located deep down in the Appenin mountain range, the rock of which absorbs a large part of the disturbing cosmic radiation. Only in this way could the very rare reaction signals be detected in the experiment.

From the supernova to the neutron star

When stars suddenly light up brightly in the sky, gigantic explosions are usually the cause, such as novae or supernovae. The largest explosions in space remain invisible to the human eye and can only be measured with special telescopes: huge bursts of gamma and X-rays, which may be due to the collision of two neutron stars. In the process, enormous amounts of energy are thrown into space, which are likely to be released through explosive nuclear reactions.

In these processes, short-lived nuclei that do not occur on earth also apparently play a central role. To describe the astrophysical events, it is important to know the properties of these nuclei such as mass, lifespan and reaction rates. To do this, the cores must be artificially manufactured in the laboratory. This has been achieved for some cores in recent years at research centers such as the GSI Helmholtz Center for Heavy Ion Research in Darmstadt or at the ISOLDE facility at CERN. Most of these exotic nuclei, however, will only be able to be produced and examined at the new FAIR particle accelerator facility in Darmstadt.

Structure of a star

A massive star creates increasingly heavy elements in its interior through the fusion of light atomic nuclei. In this way, a central area of ​​iron and nickel is created there. A further fusion of these elements is not possible because this would not release any more energy, but would require it. As soon as the fuel supply of light cores is used up, the star's internal energy source dries up. Until then, the fusion process had generated radiation pressure and thus compensated for the inwardly directed gravitational force. The moment the fusion ceases, the star's central region collapses under its own gravitational pressure.

This collapse continues until a huge “atomic nucleus” has formed inside with a radius of a few tens of kilometers, which combines about half the mass of the sun. Further matter that falls on this center is thrown back - comparable to a rubber ball that is thrown against a wall. A huge shock wave is created that runs outward from the central region of the star and blows away its outer shells. At the same time, violent nuclear reactions set in, producing enormous amounts of electrically neutral, almost massless elementary particles, so-called neutrinos. The neutrinos shoot out into space and drag matter with them. The hot expanding gas ball now lights up as a supernova. In this way, those elements also enter the interstellar space that the star has produced over the course of millions of years. Among other things, these include the nuclei of the elements oxygen and carbon, from which life arose on earth. After the supernova, what remains of the star is an extremely compressed structure, with a radius of 10 to 15 kilometers and a mass about one and a half times that of the sun - a neutron star.

In broad terms, supernovae are well understood. The physicists' ideas were, for example, confirmed by the various observations of the 1987a supernova in the Large Magellanic Cloud. However, many details cannot be described satisfactorily at the moment, such as the internal structure of neutron stars. It is still largely unknown in what form the highly compressed matter is present in the center of a neutron star. To do this, one would have to know how nuclear matter behaves at high densities and temperatures, in other words what the equation of state of nuclear matter looks like. The only way to investigate this question in the laboratory is to accelerate heavy atomic nuclei to high energies and cause them to collide. In the process, extremely hot and dense nuclear matter is generated for a short time. Such experiments are carried out at GSI in Darmstadt and at CERN in Geneva and are a central component of the research program at the future FAIR accelerator facility.

Synthesis of the heavy elements

Since no elements heavier than iron can form in the interior of stars through the fusion of lighter nuclei, heavy elements must have been formed through other processes. Nature has taken two different approaches, both of which are based on the fact that neutrons attach to existing nuclei. The captured neutrons are then converted into protons by beta decay, which increases the atomic number and creates new elements.

A process of element synthesis takes place mainly in the central region of stars, during the fusion of helium. Here the temperatures and neutron densities are relatively low and the capture of neutrons takes place relatively slowly, hence the name s-process (slow neutron capture). Here, a nucleus captures a neutron, so that the mass number of the atomic nucleus increases by one unit. The subsequent beta decay converts the neutron into a proton and increases the atomic number by one unit. This process, the principles of which are generally well researched in terms of nuclear physics, takes place many times in succession and finally ends with lead and bismuth. This creates about half of all stable atomic nuclei that are heavier than iron.

Illustration of a neutron star

The other half of the heavy nuclei and also all elements that are heavier than bismuth are produced in a second process, the fast r-process (rapid neutron capture). Existing nuclei absorb several neutrons at the same time and then quickly decay into stable neutron-rich nuclei or into unstable long-lived isotopes of uranium and plutonium. Since this r-process requires an extremely large neutron flux and takes a few seconds, it can only be imagined in an explosive scenario such as a supernova or the merging of two neutron stars.

The exact course of the nuclear physical reactions far beyond the core stability is still largely unclear. Therefore, the r-process currently represents one of the greatest challenges in experimental and theoretical nuclear astrophysics. New knowledge can only be gained here if it is possible to generate atomic nuclei with an extreme excess of neutrons. This is only possible with extremely intense beams of unstable nuclei, as will be available at the new FAIR accelerator facility. The combination of such rays with the appropriate experimental facilities allows the generation and investigation of atomic nuclei that otherwise only exist in a fleeting manner in supernovae.