A very spirited Carl Sagan once said in one of the episodes of his 1980 TV miniseries Cosmos: “The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star stuff.” Filled with conviction and all the gravitas that his voice famously commandeers, he said it in such a matter-of-fact-ly way that you just sit there and nod in agreement—or at least that’s what I’m inclined to believe how people felt when they heard him say it on TV all those years ago.
Admittedly, it’s enough of a challenge to ponder the answers to questions like these. Sure, we know that planets and stars can form from accretion disks of vast clouds of gas and debris that just huddle together from their own collective gravity to form stuff like our own Sun or planets like Earth and Neptune. But the question beyond that—where the elements, the stuff that we (and everything else) are made of, come from—is one that eludes everyday life. It’s the sort of question that maybe necessitates our very own Carl Sagan figure to bring out the curiosity inside us.
Or, perhaps, just a pair of astronomers who probably should have instead listened to some smart folks talk it out at a conference. That seems to have been the case a few years ago, when a certain diagram made its way across scientific discussions online.
Scribbling Down Star Stuff
At first glance, this just looks like a periodic table of elements after your little sibling got hold of your colored markers. It’s either that or the sort of distracted scribbling students all over the world would have been all too familiar with. Its colorful, innocent, almost childlike guise clevely belies a century of intense research by astronomers and related researchers the world over—that and the exasperation of two astronomers with some time on their hands.
During a 2008 conference called Kavli Institute of Theoretical Physics (KITP) Conference: Back to the Galaxy II, astronomers Jennifer Johnson and Inese Ivans were in attendance, listening to the premier researchers of their field discuss the latest breakthroughs in unraveling the history of the Milky Way Galaxy. Experts on topics ranging from dark matter to distant galaxies came together to share their findings with their colleagues, and perhaps take a peek into what makes our own galactic neighborhood tick throughout the eons since it formed.
At that point, new research was pouring in on the chemical compositions of stars. Naturally, it often led to questions such as “where does this element come from?”—a question that the two astronomers had had enough listening to on repeat. Grabbing some colored markers from a nearby drugstore and a printout of the periodic table of elements, the two proceeded to color in each element according to the then-current understanding of how they are formed. Powered by Carl Sagan’s spirited speech—”we are made of star stuff”—and using different colors to represent different processes, each element was colored in, with stripes and shades to show rough estimates of how much a certain process creates how much of a certain element.
What came out was a table that at first glance appears to have been crafted by studious young researchers equipped with notebooks and magnifying glasses instead of computer simulations and large ground-based telescopes, but what lies underneath is a colorful, albeit messy, summary of a century of research in that field that the two astronomers waived in the faces of anyone interested in looking at their piece of work during that fated conference.
A Stellar Work in Progress
By 1920, SIr Arthur Eddington proposed that the Sun was powered by the nuclear fusion of hydrogen into helium. Nobel laureate Hans Bethe later proved the fact, but the link between what powers stars and what stars, planets, and indeed us, were made of, was not yet established—not because we didn’t know how stars work, but because we had yet to understand atomic nuclei at the time. It took the efforts of Fred Hoyle, Willy Fowler, and Margaret and George Burbidge in the 1950s to finally show that elements beyond helium were forged from stellar cores and were dispersed across the universe upon their deaths. According to Johnson herself, all these discoveries, and a summation of years’ worth of observations of stellar compositions, computer models of stellar fusion, and the flashes of light produced by decaying radioactive elements in the cosmos, were now represented by streaks and smears “that Inese and I converted into splotches of teal, orange, and yellow in 2008.”
Johnson jokes that astronomers really only recognize three elements in the periodic table: hydrogen, helium, and the “metals.” The “metals” conveniently group all the other elements produced outside the Big Bang together, which works well enough for their purposes; of course, the universe is never really that simple. As she said it herself: “A star’s atmosphere is a preserved sample of its natal gas, a fossil record etched onto the surfaces of the stars; and it is the sum of the nucleosynthesis of its stellar ancestors. If we can decode the chemical fingerprint, we can know about the stars that came before. The number of massive stars that have ever existed is not easy to measure by counting black holes, but observing the accumulated ‘vomit’ left as black holes form is much easier.”
This version above, presented at the winter meeting of the American Astronomical Society in 2017, would show things a bit clearer. Johnson was part of the team that presented results from the Sloan Digital Sky Survey (SDSS) collaboration, which measured the composition of more than 100,000 stars across the night sky. Naturally, the chart is also a work in progress; as more information is discovered about the origins of these elements, revisions will be added to adjust for them. As of now, the “middle elements” like tin, molybdenum, or arsenic—Johnson’s favorite element—are generally still up in the air; it will be difficult to find the “smoking gun” for these elements that aren’t key to powering stars and aren’t produced in stellar explosions either. And as of now, the most recent revision came in 2017 after the detection of gravitational waves from merging neutron stars by the Laser Interferometer Gravitational-Wave Observatory (LIGO); further studies into these gravitational waves and their light sources confirmed the theory that these oddball stellar explosions produce most of the gold and other r-process (rapid neutron capture process) elements in the universe.
We Are Made of Star Stuff
At the end of the day, Johnson and Ivans never really made a breakthrough—they simply summarized what the giants of their field had gathered from decades before. It’s not some peer-reviewed, government-funded research project; it’s just a plain white printout funded by spirited astronomers and supplied by the nearest drugstore. What was groundbreaking, however, was the human aspect of it all. What seemed to be just scribbles on a periodic table instead gave personality to the efforts of scientists the world over spanning a century. Now we see that the carbon in our bodies, the oxygen we breathe, and the gold we wear all come from the stars in the sky, scattering their contents across the universe upon their death.
Perhaps it’s this sense of personality, imparted through the scribbles and footnotes on the empty spaces between hydrogen and helium on the table, that gave this seemingly simple picture its own life, creating waves of news online about how a couple of astronomers helped change the way we see the periodic table, and in the process bringing science closer to the people—just like how Carl Sagan did it all those years ago.
Bibliography
- Crew, B. (2017, January 23). This Awesome Periodic Table Shows The Origins of Every Atom in Your Body. ScienceAlert. Retrieved August 16, 2021, from https://www.sciencealert.com/this-awesome-periodic-table-shows-the-origins-of-every-atom-in-your-body
- Johnson, J. (2017, January 9). Origin of the Elements in the Solar System. Science Blog from the SDSS. Retrieved August 16, 2021, from http://blog.sdss.org/2017/01/09/origin-of-the-elements-in-the-solar-system/
- Johnson, J. (2018, September 1). A Chemical History of the Universe. American Scientist. Retrieved August 16, 2021, from https://www.americanscientist.org/article/a-chemical-history-of-the-universe
