Origins of the elements

The periodic table of the elements is pictured with shading indicating our current understanding of their cosmic origins. Only hydrogen, helium and a trace of lithium were created in the big bang. All other elements were forged in stellar interiors or stellar explosions. (Image — 5D55 Science Blog)

In my Jan. 2 column, I wrote of the detection of the kilonova by both gravitational and electromagnetic waves in August of 2017. The detections, themselves, were remarkable and ushered in the completely new era of Multi Messenger Astronomy. But the data from the event also gave evidence confirming the origins of very heavy elements such as gold and platinum.

In the initial fraction of a second of the Big Bang, the origin of the universe about 13.8 billion years ago, quarks materialized energy in the form of gamma rays as described by Einstein’s famous equation E = mc2. Since it was expanding, the universe quickly cooled too much to be able to produce after only one tenth of a billionth of a second. As the expansion and cooling continued, the strong nuclear force bound the quarks into trios to form protons and neutrons out of which atomic nuclei are made. Neutrons are a little more massive than protons, so fewer neutrons were made. In a the process of nuclear fusion, pairs of neutrons combined with pairs of protons to form helium nuclei whereas the remainder of the protons remained unbound as hydrogen nuclei. By the end of the first three minutes of its existence, the universe contained 75 percent hydrogen, 25 percent helium and a trace of lithium. All the other elements – those forming our flesh and bones, the air that we breathe, the rocks of the Adirondacks, the metals we shape into tools and use to power our nuclear reactors — were forged by stars.

The periodic table of the elements shown in the diagram gives our current understanding of the origins of the elements instead of their properties. Hydrogen, helium and lithium are the only ones that emerged from the Big Bang Fusion.

The cores of stars are now the only places in the universe hot and dense enough to fuse hydrogen into helium. The sun is doing this now, fusing over 6,000 aircraft carrier masses of hydrogen into helium each second! In each one of those fusions, 0.7 percent of the mass of the original hydrogen is converted to pure energy, again obeying E = mc2. This energy comes to us as the warming sunshine so welcome on these frigid winter days.

The sun, after some initial sputters, will manage the heat and density required to fuse helium into carbon and nitrogen before it runs out of fuel. At that time, its outer layers now enriched with helium, lithium, carbon and nitrogen will expand away in a planetary nebula (see The Wilderness Above, Dec. 5, 2017) and leave the core as a white dwarf star. As the planetary nebula expands, some processes are violent enough to create a smattering of heavier elements such as tin and lead. The figure shows which elements can be made in dying low-mass stars.

Larger stars, though, have enough density and heat in their cores to fuse all the elements up to iron. As described in The Wilderness Above for Dec. 17, 2017, formation of the iron core initiates the death of a giant star and causes it to explode in a Type II supernova. These exploding massive stars release all the fusion products — from oxygen to rubidium, as shown in the diagram — into the interstellar medium from which stars like the Sun then form with their attendant planets. The explosion crushes the cores of these stars into neutron stars or black holes.

The origins of heavier elements have puzzled astronomers. Unlike the light elements, iron and heavier elements absorb energy when they undergo fusion into heavier elements. This it takes extremely energetic events – more energetic that the explosions of giant stars — to create these elements.

Some are made in Type Ia supernovae, where a white dwarf in a binary pair acquires mass from its companion until it goes above 1.4 times the mass of the sun. At that point, it can no longer hold itself up against gravity and collapses to create a thermonuclear explosion. But the energy of these explosions is still only sufficient to create elements up to copper and zinc.

One of the problems with creating the elements along the bottom rows of the periodic table is that they have many more neutrons that protons. The most stable isotope of uranium, 238U, for example, has 92 protons and 146 neutrons (these sum to the isotope number, 238). To find an abundance of neutrons, some researchers started looking to neutron stars. In 1981, Joseph Taylor and Russsell Hulse of the University of Massachusetts-Amherst discovered a binary pulsar where the two ultra-dense stars were actually spiraling in toward each other as the system lost energy. This won them the 1993 Nobel Prize in Physics, made the collision of neutron stars a very real possibility and sparked work on theories explaining these as the events capable of creating the heaviest elements. It was expected that these collisions would also result in gamma ray bursts (GRBs).

On Aug. 17, 2017, the detection of gravitational waves and detection of a GRB 1.7 seconds later, caught the attention of many observers. 10.9 hours later, the Swope Telescope at Las Campanas Observatory in Chile observed the event in visible light as did five more telescopes within the following hour. These, along with observations in infared and ultraviolet over the next few days, revealed that the merger of these two neutron stars (with masses determined from the gravitational waves) had produced over 10,000 Earth masses of elements heavier than iron. Elements like silver, iodine and radium as shown on the diagram. About 100 Earth masses of it were gold.

Do you wear a gold or silver ring, a gold chain or silver earrings? Hold it in your hand and look at it. To produce it, the universe had to collide two neutron stars millions, if not billions of years before the sun and Earth were formed by gravity out of a cloud of gas and traces of heavier elements 5 billion years ago. As you look, breathe in and feel your lungs expand. It is the flexible chains of carbon from which your flesh is made allowing that expansion. That carbon most likely emerged from a planetary nebulae around stars slightly larger than the sun that lived their entire lives and died in the first 8 billion years of the universe to enrich that cloud. The nitrogen you breathe in as your lungs expand emerged with the carbon, but the oxygen, that precious gas on which our lives depend in every moment, was blown out of the core of a massive star that exploded in a supernova explosion. Some of the iron carrying oxygen to your brain as you read this also emerged from such explosions. Those very supernovae created the neutron stars that ultimately made the gold in your hand.

The stars flow through us. They flow through our organs, our breath, our blood, our bones and our society. Every element on Earth but hydrogen and some helium has come to us through ancient stellar processes that our observatories have only begun to reveal to us. What other wonders await our discovery in the subtle jiggles of electromagnetic fields and the fabric of spacetime itself?

The volunteer astronomers at the Adirondack Public Observatory are eager to show you cosmic wonders. The Roll Off Roof Observatory is open to the public on the first and third Fridays of each month approximately one half-hour after sunset. Whether you’re an avid amateur astronomer or have never visited an observatory, come and view through our telescopes and learn about the Wilderness Above. For updates and notices, check out our website at adirondackpublicobservatory.org and our Facebook page. On our public observing days you can also call the RORO at 518-359-6317 to talk with one of our astronomers.

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