“We are stardust, we are golden / We are billion-year-old carbon.” So wrote Joni Mitchell, also noting that she dreamed of bomber jet planes turning into butterflies. The 1970 song has lost none of its relevance today. It’s also a lyrical description of dry and timeless realities of human existence: we are made from elements and those elements have been around for an unfathomably long time (“a billion years” is poetic license and a severe understatement). As Mitchell says, most of the elements that make up us – from carbon to iron – originated under the conditions of intense pressure and heat that exist in the core of stars, all of which is spewed out into the universe when they die.
As the poets at NASA put it, “from the carbon in our DNA to the calcium in our bones, nearly all of the elements in our bodies were forged in the fiery hearts and death throes of stars.” And they’ve been around far longer than we have. Light elements started forming an estimated 14 billion years ago, actually, in the first few minutes after the Big Bang, though others didn’t come around till a few hundred thousand years later when the universe cooled down enough for electrons to stay in orbit around atomic nuclei.
The only elements that preceded these were much simpler: hydrogen, helium and a bit of lithium — yes, the stuff they also prescribe for bipolar disorder, though probably not enough to cheer you up in the dark, lonely vastness of the early universe.
Things have gotten better since then. A survey of over 150,000 stars, announced in 2017, cataloged the distribution of the life-essential elements of carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.
“From the carbon in our DNA to the calcium in our bones, nearly all of the elements in our bodies were forged in the fiery hearts and death throes of stars.”
“We are now able to map the abundance of all of the major elements found in the human body across hundreds of thousands of stars in our Milky Way,” said Jennifer Johnson, a leader of the study team and an observational astronomer based at The Ohio State University, in a press release at the time.
More recently, a paper published just this July in the journal Science Advances details the finding of stardust liberally sprinkled on a nearby asteroid, this time with the bonus of close-range, lab-based study of this exotic yet oh-so-familiar substance.
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Dr. Ann Nguyen of NASA, a space scientist and the director of the NanoSIMS Research Facility at the NASA Johnson Space Center, is the lead author among the dozens of contributors from multiple countries who helped to analyze samples of the asteroid Ryugu.
“Presolar stardust grains are dust that condensed from the gaseous outflows of stars that died billions of years before our solar system formed,” she told Salon in an email. “They have isotopic compositions that are vastly different than the isotopic compositions of any material that formed within our solar system.”
The unusual isotopic ratios — proportions of particular versions of the elements found — allow the researchers to determine that they have come from far, far away and also to understand how they were formed.
Nguyen and her colleagues mapped the Ryugu samples for carbon, nitrogen, oxygen and silicon, all essential parts of who we are today — only, of course, while these are elements familiar to Earthlings like us, the isotopic ratio tells us that these particular ones came from somewhere else.
"These grains seeded our solar system and constituted some of the original building blocks."
“This is because their isotopic ratios result from nucleosynthetic reactions that occurred deep within the parent stars. Some of these grains were incorporated into our solar system and survived processing on the asteroid or comet host,” Nguyen explained. Because the presolar grains are, on average, a quarter of a micrometer in diameter, this isn’t something you can see in a telescope or other far-range analysis. Instead, the researchers used specialized equipment capable of measurements on the scale of a nanometer, down to the atomic level.
But you don’t just kill a star and get an entire cupboard of elements suitable for whipping up whatever material good — whether Uranus or, with apologies, your anus — you’re after. Specific types of stars and the way in which they die release different elements into the universe.
“Stars of different masses have different fates,” Nguyen told Salon. “Stars with low to intermediate masses (lower than about 8 solar masses) evolve to the AGB phase, whereas more massive stars evolve to become supernovae. The more massive stars reach higher temperatures and pressures in their interiors, which allows for different nucleosynthetic reactions to occur.”
(The AGB phase Nguyen mentioned refers to the so-called "asymptotic giant branch," a life stage of relatively skimpy stars in which they become cool and luminous balls of occasionally explosively-igniting helium fusion around a carbon and oxygen core, then swell to become a red giant, lose mass to stellar winds, and may end up as very pretty protoplanetary nebulae — a necessarily incomplete description of a complex and sometimes varied process stretching out over around a hundred thousand years.)
The team targeted carbon, nitrogen and oxygen specifically because their isotopic composition provides clear indications of what sort of star they came from: supernova, nova or AGB star, for example. And the chemical makeup of a presolar grain tells secrets about the chemistry of the star it came from. “For instance,” Nguyen offered, “presolar oxides and silicates come from stars that are oxygen-rich, and presolar SiC [a compound of silicon and carbon] and graphite [a form of carbon] come from carbon-rich stars.”
Ok, but how did this stuff find its way to us so very long ago? “We know we are made of stardust because bona fide examples of grains have been identified in asteroid and comet samples," Nguyen explained. "These grains seeded our solar system and constituted some of the original building blocks. The organic matter that I study formed through abiotic processes and is different than prebiotic organic matter — but it is possible that bodies like Ryugu or comets delivered the ingredients necessary for life on Earth. If so, the presolar grains and organic matter would also have been delivered.”
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Scientists have just figured out how to open a stuck canister of dust from 101955 Bennu, a carbon-filled near-Earth asteroid — damaged fasteners had rendered it astronomer-proof, but not for long! And now Nguyen is deep in analysis of the Bennu samples from the NASA sample-return mission, which returned to Earth in September but only fully revealed its dusty secrets last month. Samples of the samples are now being studied by scientists all over the world and we’re getting tantalizing tidbits already (lots of water and phosphate). Perhaps we will soon know more about the origin of us and everything around us.
In the meantime, those of us with a sense of awe can be pretty happy knowing our earth dust, and our bodies, which will one day return to dust, do indeed derive from the glittering reaches of space and the unfathomable intensity of stars in their death throes.
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