The universe was shaped by furious events of ages past, says S.Ananthanarayanan.
While many elements on the earth were born in nuclear fusion, which is the process of the hydrogen bomb, it took a process many times more energetic to create the heavier elements.
Benoit Côté, Marius Eichler, Andrés Yagüe López, Nicole Vassh, Matthew R. Mumpower, Blanka Világos, Benjámin Soós, Almudena Arcones, Trevor M. Sprouse, Rebecca Surman, Marco Pignatari, Mária K. Peto, Benjamin Wehmeyer, Thomas Rauscher and Maria Lugaro from Konkoly Observatory and Eötvös Loránd University, Budapest, Technische Universität and the particle accelerator, at Darmstadt, Germany, Michigan State University, University of Notre Dame, Los Alamos National Laboratory, in USA, and University of Hull, Hull and University of Hertfordshire, UK, look at meteorites of the early Solar System, to create a snapshot of what the ancient processes looked like.
In a paper in the journal, Science, the team examines the radioactive nuclei of two elements, remnants of the high energy events where they were created. A helpful feature of these nuclei, that they decay at the same rate, enables the team to narrow uncertainties in the nature of radiation at the time when these nuclei were formed.
The first of the elements created, from the matter that arose in the Big Bang, was the simplest, the hydrogen nucleus, which consists of a single positively charged particle, the proton. And it was of hydrogen that the early universe consisted. Over billions of years, hydrogen clouds drew close, a result of gravity, till they were compressed many million times over. The stupendous pressure brought the mutually repelling protons so close together that short range, attractive nuclear forces come into play and the nuclei merged, as nuclei of the element helium. While it takes great energy to force protons together in this way, once they merge, they fall into a stable, low energy state, and give off huge energy. It is like a golf ball falling into a deep hole that is at the top of a slope in the golf course. It takes a powerful stroke to drive the ball up the slope, but when it gets there and into the hole, it speeds up to be faster than it was at the start!
The huge energy released in the fusion of hydrogen nuclei, and this is the energy source of the stars, and our own sun, makes the cloud expand. But when it has expanded as much as it can, it collapses again, to set off more fusion reactions and more expansions. When the hydrogen nuclei get used up, the helium nuclei, which have two protons, begin to fuse, to form elements with more protons in the nucleus, along with equally heavy, but neutral particles called neutrons, that help the nucleus stabilize. These heavier nuclei then participate in fusion reactions, to create even heavier nuclei, till the nuclei of iron, 56Fe, which consist of 56 particles (26 protons and 30 neutrons), are formed.
The process stops at iron because iron is at the bottom of the pit, in the matter of releasing energy when a proton is added. Adding protons to the iron nucleus hence cannot sustain itself, but it takes more energy to bring about the fusion than the fusion releases. The elements that arise in the process of the creation of the stars thus have nuclei only till the element iron – and iron is also the most abundant constituent of meteorites and the oldest stars.
Where then, do the heavier elements, like cobalt, nickel, copper, zinc, or lead, silver, gold, platinum, and so on, come from? Well they come from more energetic events, where there is a high density of free neutrons. The process is ‘neutron capture,’ where a nucleus captures a neutron, which then undergoes radioactive decay, to turn into a proton. This can take place in the late stages of evolution of large stars, to account for about half the heavier elements, but for the rest, we need higher energy events like supernova explosions or collision of the heaviest stars.
One of the end points of the life of a star could be that matter is compressed till the protons and electrons coalesce into neutrons, which enables them to be packed closer still. The result, the neutron star, is an extremely dense object, some 10 kilometers across, but with mass well over that of the sun. The shrinking diameter also leads to a tremendous rate of spin. As the object has a strong magnetic field, there could be radiation in pulses, which leads to the object being called a ‘pulsar’. One can imagine that collision of objects like neutron stars (or black holes) would be energetic indeed. And it is such events that lead to the neutron densities for the heaviest of elements to be formed. The paper in Science observes that the optical radiation that accompanied the neutron star merger, which led to gravitational waves that were detected in 2017, shows that at least some of the heaviest elements were produced during the event. And it is believed that the heaviest elements, produced in this way, got to the Solar System through meteorites that came from far out in space.
A question of what, in fact, were the kinds of events that led to formation of the heaviest elements, however, has not been answered. As we can imagine, the only witnesses to these events are the meteorites that crash into the earth. The team writing in Science, however, has identified contents of these meteorites which can illuminate the conditions, particularly of the density of neutrons, at the time the meteorites were formed.
The paper refers to other studies which document the presence of radioactive nuclei that are produced in high energy neutron capture, in meteorites. Just as in the case of carbon dating of archeological artefacts or fossils, measurement of what proportion of the radioactive nuclei in the meteorites has decayed could reveal things about conditions when the nuclei were created. This, however, is not possible in the present case because there may have been more than one instance of “enrichment” of the parent nucleus, as well as other uncertainties, the paper says.
However, as a stroke of fortune, the paper says, two of the nuclei, found in meteorites, have almost the same rate of decay. The nuclei are iodine-129 and curium-247, and both of them have almost the same half-life, of about 15.6 million years. The ratio in which the two nuclei are found at present, having come down, since their creation, at the same rate of decay, would thus represent the ratio in which they were created.
129I, as it has 53 protons (and 76 neutrons), and needs fewer neutrons to be added. High neutron density, however, would favour production of 247Cm, which has 96 protons (and 151 neutrons) and needs more neutrons to be added. The number of 129I nuclei in meteorites was found to be higher, at over 400 times the number of 247Cm. This indicates greater production of 129I when the nuclei were created, which suggests that the density of neutrons was modest, favouring the more easily created nucleus.
.The calculations and values are tentative, as there are many factors to consider, but the approach is one that opens a window to imagine conditions that obtained 4.6 billion years ago, the time when the Solar System was formed.
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