Most of the atoms in our bodies were made in the stars

We are made of the stuff of the stars

When we look around us, for example, on a walk in the countryside, we can see rocks, the water flowing in a river, or breathe the air. Rocks may have silicon, calcium and carbon. Water is made up of molecules with two hydrogen atoms and one oxygen atom, and may have dissolved minerals such as magnesium or sodium. When we breathe, we inhale oxygen molecules, which will flow into our blood, where iron also runs. There is calcium in our bones, magnesium in our muscles and phosphorus in our brain.

Everything around us and ourselves are made up of atoms. The concept of atom, tiny particle, appeared in antiquity, in Greece and India. Throughout the history of science, several scientists have been identifying chemical elements, that is, those substances that are made up of the same type of atom.

In 1869, Russian chemist Dmitri Mendeleev tried to systematize the knowledge already gathered about chemical elements, trying to organize them according to their physical properties and chemical behavior. So produced the first periodic table, with the 70 elements known at the time.

This was 150 years ago. 2019 was declared by Unesco as the international year of the periodic table of the chemical elements. One of the goals is to raise awareness of the importance of this instrument, which is like a window on everything that exists in the Universe, including ourselves.

But where do all these chemical elements come from? which is like asking ourselves how the material we are made of was produced.


Where did atoms come from?

In 1869 Mendeleev could not explain why the properties that allowed him to organize chemical elements into groups. It was not until 1911 that Ernest Rutherford proposed a model of the atom which, although later perfected by quantum mechanics, gives us a realistic idea of ​​the structure of the atom.

At its center is the nucleus, with a positive electric charge and where most of its mass is concentrated. Around this 'orbit' electrons – or in the case of hydrogen, just one electron – particles with a negative electrical charge.

The positive electrical charge of the nucleus is due to other particles, the protons. What distinguishes the different chemical elements is the number of protons present in the nucleus.

But since the protons are of the same electrical charge, they should repel each other by the effect of the electromagnetic force. Something will have supplied enough energy to counteract this electromagnetic force to the point where the protons are extremely close together. Then another force of nature comes into play, the strong nuclear force, which made them cling to each other.

George Gamow, born in Ukraine and naturalized American, was one of the biggest supporters of the idea that the Universe had a very dense and hot origin. Under these initial conditions, he argued in an article published in 1946, there would be enough energy to produce in sequence all the chemical elements, those with the greatest mass from the lighter ones.

There were, however, two gaps, two positions in this sequence that interrupted it and that prevented the heavier elements from forming. The answer would then have to be elsewhere.

At the same time, Fred Hoyle, a native of York, England, and a professor at Cambridge University, had begun to look to the stars as potential factories for chemical elements. He was the first to introduce the concept of stellar nucleosynthesis.

It is worth remembering that Fred Hoyle was one of the biggest critics of the theory of a hot and dense early Universe, defended by Gamow. It was Hoyle who sarcastically coined the expression “Big Bang”. It is curious that astronomers designate the Big Bang theory, now with strong scientific evidence, with an expression created by someone who did not believe in it.



the energy of the stars

Continuing with the concept of stellar nucleosynthesis, let's go back to 1920. This year, Arthur Eddington was the first to suggest that the energy of the Sun and other stars, the energy that gives us light and heat, would be of nuclear origin, that is, produced by converting matter – in this case, nuclear particles – into energy.

It was not until nearly twenty years later, in 1939, that Hans Bethe, a German-born American physicist, described two processes by which the Sun could withdraw its energy in this way and in sufficient quantity to remain active for billions of years. In both processes, hydrogen nuclei – that is, simple protons – are converted to helium nuclei, the first two elements of Mendeleev's periodic table.

In the same decade of the 30s, William Fowler, an engineer who, however, had dedicated himself to nuclear physics, was working at the California Institute of Technology in a laboratory named Kellogg. Funded by the “king” of breakfast cereals, this laboratory aimed to study treatments for cancer using X-rays, but studies in nuclear physics began to be developed there.

Fowler realized that the nuclear reactions he was analyzing in the laboratory were the same ones that Hans Bethe claimed took place inside stars. But both Bethe and later Fowler were interested in learning about the ability of stars to produce thermonuclear energy.

Fred Hoyle's perspective, however, was different. Hoyle wondered if, once the hydrogen was consumed by the processes described by Bethe, the resulting helium could become the basis for producing more massive nuclei.


We are made of the stuff of the stars

In the late 40s and early 50s, Hoyle developed models of the interior of stars and how they evolved, or aged, into red giants. He then began to collaborate with William Fowler, with voyages for both sides of the Atlantic.

Fowler had in recent years gathered experimental data on nuclear reaction rates. The objective was to reconcile a model of the interior of stars with the productivity of nuclear reactions observed in the laboratory. And both should still agree with the abundance of chemical elements, both on Earth and in the Universe, through the study of meteorites and the analysis of starlight.

As a result of this work on three fronts, they published in 1957, together with Margaret Burbidge and Geoffrey Burbidge, one of the most important articles in the history of science, “Synthesis of Elements in Stars".

In this article they explained how, in stars more evolved than the Sun, in the red giant phase, the chemical elements from helium to iron (this one with 26 protons in the nucleus) are produced. Then, only in the final phase of the life of some stars with greater mass, the remaining elements found in nature are produced. At this stage, these stars explode, in a phenomenon known as a supernova, and in the explosion they release all that material into space.

However, the amount of helium in the Universe cannot be explained by stellar fabrication alone. The current abundances of deuterium (an isotope of hydrogen, that is, a nucleus consisting of a proton and a neutron) and helium are due to the initial conditions of the Universe, as initially proposed by George Gamow in 1946, and demonstrated together with his doctoral student Ralph Alpher two years later.

It was only in the furnaces of the first generations of stars in the Universe that all the other heavier elements were created from carbon (with 6 protons in the nucleus), essential elements for the later formation of planets and life forms like us. For this reason Carl Sagan vulgarized the famous phrase “We are made of the material of the stars”.

In 1983, William Fowler received the Nobel Prize for Physics, shared with Subrahmanyan Chandrasekhar, but was surprised not to share it with Fred Hoyle, his English friend with whom he was hiking in Scotland.

This was one of the injustices of the Swedish Academy of Sciences, which also did not include in the 1974 Nobel Prize. Jocelyn bell, another “star” already released in this series. Interestingly, Fred Hoyle was then one of the most aggressive critics, though perhaps less correctly, of this exclusion of Bell from the highest award in the world of physics.



Author Sérgio Pereira – Science Communication Group of the Institute of Astrophysics and Space Sciences.

"Stars that shine in time” is a rubric with which the Institute of Astrophysics and Space Sciences associates itself with the celebration of the 100 years of the International Astronomical Union (IAU), recalling important figures in the history of astronomy over the past 100 years.
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