The origin story: how the elements came into being

The most abundant elements in the universe, hydrogen and helium — the lightest elements which together constitute nearly 99% of the chemical abundance — were created during the Big Bang, the primordial hot and dense phase of the universe.

To begin with, it was all hydrogen (actually, a mixture of protons, neutrons, electrons, positrons and neutrinos). Then the neutrons combined with the protons to form deuterium and helium. A calculation of the thermonuclear reaction rates during the superhot dense phase of the early universe estimates that about one-fourth of the hydrogen would have become helium, a small fraction (approximately 0.01%) deuterium while also forming the lighter helium-3 isotope.

As the universe cooled rapidly, temperatures likely dropped too low for any heavier elements to form; only one in a billion of the nuclei would have been lithium. Deuterium and lithium are not made in stars, so the lithium in our lithium-ion batteries and the deuterium in heavy water, as well as the one used in hydrogen bombs, were formed during the Big Bang.

Thus, it is remarkable that except for the lightest elements — hydrogen, helium and lithium — all the other chemical elements that are crucial for biological metabolism were forged in the superhot nuclear furnaces in the cores of stars.

A question may be asked that if the thermonuclear conversion of hydrogen to helium is the main source of stellar energy, what about the helium made in the stars by conversion from hydrogen? It turns out that during the entire age of the galaxy, hardly 2% of hydrogen could be converted to helium inside the stars. However, the oldest objects show much higher helium abundance than this. This means that the bulk of the helium was generated in the first three minutes of the big bang.

A star like the Sun, converts about 560 million tonnes of hydrogen to helium every second. After much of the H fuses to form He, the core collapses and heats to 200 million degrees.

But it’s not easy to form carbon (C). Beryllium (Be) and Boron (B), are also light elements, and made in spallation processes due to cosmic rays and have a very low abundance. Beryllium-8 is very unstable decaying back to helium-4 nuclei. As carbon is crucial for life and must somehow be made (inside stars), Fred Hoyle conjectured the existence of a resonance level in the carbon nucleus which enabled Be-8 to capture an alpha particle forming C-12. This was later verified in the lab.

Something similar happens with neon nucleus, so that most oxygen remains (without becoming neon). Stars like the Sun, end up with cores of carbon and oxygen and ‘peacefully’ end as C-O white-dwarf. In more massive stars, the cores collapse further, heating up to 700 million degrees when Carbon fuses to form magnesium, silicon and oxygen to sulphur.

The star has meanwhile swelled to become a red giant or supergiant. The stellar core continues to collapse and evolve, the ash of one reaction becomes nuclear fuel for the next stage. The star is now pulsating as it adjusts to new sources of nuclear fuel, becoming layered like an onion, the inner layers being hotter and hotter with heavier elements being formed. The thermonuclear process finally ends when silicon burns to iron in the hottest innermost core region at temperatures of three billion degrees.

Heavier elements

Reactions stop at iron, as it has the maximum nuclear binding-energy. Heavier elements like molybdenum, iodine, selenium, etc arise in stars from the so-called slow neutron capture process. During the thermal pulsations of an intermediate star of an advanced stage, large number of C-13 nuclei fuse with Helium nuclei to produce an Oxygen nucleus and a free neutron.

In heavier stars with hotter interiors, Neon-22 fuses with Helium to form Magnesium-25 (Mg-25) and a free neutron. All these excess neutrons are ejected into the hot dense stellar nuclear cauldron where they attach themselves to heavy seed nuclei like iron.

More addition of neutrons can make the nucleus unstable in which case the neutron converts itself into a proton (ejecting an electron) increasing the atomic number and producing a heavier element. This large number of buzzing neutrons enables the nuclei already produced to successively capture neutrons followed by beta-decay, each neutron to proton conversion promoting the element up the periodic table.

With such reactions, evolving intermediate mass stars will eventually convert some of its iron into molybdenum, zinc, selenium and other trace elements. Lead, as also many of the rare-earth elements needed now for technology, are produced in such processes.

The heaviest elements like Platinum, Gold or Uranium are made by the so-called rapid neutron capture. This happens when a massive star explodes, with the core becoming a neutron star. This rapid neutron capture and successive beta decay by the seed nuclei, leads to formation of the heaviest elements on a very short time scale. 

Thus, we have been provided with a fantastic picture of how all chemical elements crucial for our existence were forged billions of years ago in the nuclear cauldrons inside massive stars and their explosive spectacular demise.

(The author is with Indian Institute of Astrophysics, Bengaluru)

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The origin story: how the elements came into being

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