We are stardust
We are golden
And we’ve got to get ourselves
Back to the garden
—Joni Mitchell’s song “Woodstock”, which was popularized by Crosby, Stills, Nash & Young
We are actually much older than you think, since every bit of you was made long before you were born. Almost two-thirds of your weight is water and essentially none of it was created during your lifetime. Up to half of the earth’s surface water is older than our sun. Additional water formed in the crust and atmosphere of the early earth, meaning much of the water in you is likely to be two to five billion years old.
That’s at a molecular level. At an atomic level, many of your atoms were created in long-dead stars that could have been up to a thousand times the mass of the sun. While it’s thought the three lightest elements—hydrogen, helium, and about one fifth of today’s lithium, all of which make up 98% of the ordinary matter in the universe—were made in the first three minutes after the Big Bang, elements with 3 through 26 protons—much of the additional lithium through iron—were made inside stars, with the exception of beryllium and boron (elements 4 and 5), which, on Earth, are made in our atmosphere.
Most of these atoms are roughly five to eight billion years old, although some might even date back to around 13.2 billion years—just 550 million years after the Big Bang. These ancient stars exploded as supernovas, scattering dust that drifted for millennia through interstellar space becoming parts of new stars that also ended up going supernova—perhaps repeating this process several times.
Some of it even left our galaxy for long periods in currents known as the circumgalactic medium, travelling out up to four times the width of our galaxy away from the Milky Way, before being pulled back in. Astronomer Jessica Werk, with the University of Washington, who had studied this, noted, “The same carbon in our bodies most likely spent a significant amount of time outside of the galaxy!”[1] This applies to other elements as well, suggesting that most of what we’re made of was once intergalactic. That’s stuff from the Milky Way that left and came back. Researchers estimate that half the stuff in us came from another galaxy, possibly from before our galaxy even existed.[2]
As we eat and breathe, new material enters our body, while some leaves. We’re constantly exchanging bits of us with the universe in a process of renewal. Obviously most of us wasn’t part of us when we were born and a large part of us only became part of us recently, but all of us are made from non-us elements.
When you look at a photograph of yourself when you were young, you know it’s you, but much of what you were made of then has been replaced. You have a sense of continuity, but you’re not the same as you were. You also think and act differently, and many of your interests and preferences have changed as well. If the you of then stood next to the you of now and you talked about your likes and dislikes, people would probably have trouble realizing the two of you are the same person. There are even some minor changes to your DNA.
Supernovas leave behind dead stars called neutron stars. Neutron stars are so compact and dense, that the protons and neutrons form what’s called nuclear pasta, which is a hundred trillion times denser than anything on earth and more than 10 billion times stronger than steel.
It’s now believed that many of the 90-plus heavier elements, which include nickel, copper, zinc, silver, tin, iodine, platinum, gold, mercury, lead, uranium, and plutonium, were formed when two neutron stars crashed together generating temperatures up to 1.4 trillion degrees Fahrenheit (800 billion C), shooting out these higher elements. Some might even come from smashups of a neutron star with a black hole. While you may think these smash-ups are rare, they’re actually thought to be quite common. Scientists estimate that there are actually about 100 million black holes and about a billion neutron stars in our galaxy alone.
 |
| (Top) This artist’s impression compares the size of a 12-mile-wide (19 km) neutron star with that of Manhattan. Neutron stars are the densest objects in the universe that we can directly observe. It takes matter equal to half-a-million Earths, compressed down to a sphere a few miles wide to form a neutron star. It’s so dense that on Earth one teaspoonful would weight a billion tons. A neutron star can spin faster than 700 times a second and, compared to the Earth, its magnetic field is a trillion times stronger. Astrophysicists think neutron stars consist of various oddities—such as a neutron superfluid—with a crust of ions and electrons.[3] (Bottom) Formed in a supernova a million years ago, this 17-mile-wide (28 km) neutron star named RX J18563.5-3754 is only about 200 light years away from us and travelling at 4,000 miles (6,500 km) per second. It will reach its closest point to Earth, 170 light years away, about 300,000 years from now. (Top credit: NASA's Goddard Space Flight Center. Bottom: NASA and F.M. Walter of State University of New York at Stony Brook.) |
An alternate idea is that some of the heavier elements came from collapsars. Collapsars are massive rapidly spinning stars that collapse before going supernova. Although rare, these might produce even more heavy elements than a neutron star collusion.[4]
There’s also evidence that up to 10% came from massive ejections of material from magnetars. Magnetars are neutron stars with extremely strong magnetic fields that are about a thousand times stronger than a normal neutron star. Their flairs could widely distribute these elements, without a collusion. Some magnetars are also pulsars.[5]
A minute amount, relatively speaking, of this dust from supernovas and neutron star collusions coalesced to form our solar system and, eventually, you. As author and astrophysicist Carl Sagan used to say, we are all made of star-stuff.
Moving from the origin of our atoms to the distribution of molecules, there’s an interesting exercise that professors present to their chemistry and physics students. On March 15th back in 44 b.c., Julius Caesar was stabbed to death at the Forum by sixty Roman senators. How likely is it that you have ever breathed in one of the molecules from Caesar’s final breath?
Assuming his dying exhalation was the average volume of a deep breath, which is a liter of air, then we can estimate that it consisted of something like 25 sextillion gas molecules. Making some assumptions about the lifetimes of molecules and how gases mix, we can calculate that on average, one of Caesar’s molecules is in every breath we have ever taken and every breath we will ever take.
There’s also a good chance that some of the water molecules in you right now once passed through a number of dinosaurs...along with many other interesting creatures—before getting into you. In fact, the same applies to all of the elements in your body. They too were once parts of dinosaurs, trilobites, trees, bot fly maggots, fungi, and rocks. The varied elements that came together to make us will eventually be dispersed back into the environment once again where they will become part of something else. In this way, everything on earth is related. Everything is made from the same stuff that’s continually recycled.
While the water in you is ancient, it only recently got into you. It comes from what you eat and drink. Most of it comes from tap water. Even if you don’t drink much water, it comes from the tap water used to make your soda or beer or reconstituted fruit juices. Within minutes of drinking it, water becomes part of your bloodstream, which is also mostly made of tap water (or well water or rain water or mountain-stream water, if that’s what you drink, but remember, filtered water is just tap water unless it comes directly from a well, stream, or rain and into a bottle). Your blood has some cells, salts and organic molecules in it, but it’s mostly water. If you think blood is thicker than water...well, not by much. From your blood, the water is distributed throughout your body, becoming a major part of all your organs.
Water passes through us like a stream. It’s unlikely that any of the molecules of water that are in you now will still be in you in two years’ time.
If all of this isn’t strange enough for you, your head is older than your feet are because of time dilation—a well-proven aspect of Einstein’s Special Theory of Relativity.
So on the day you turned 10 years old, next to none of you was actually 10. Most of you was much, much older, while some formations of those elements were much younger.
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[1] James Urton, University of Washington press release, “The carbon in our bodies probably left the galaxy and came back on cosmic ‘conveyer belt’ ”, UW News, January 3, 2025, https://www.washington.edu/news/2025/01/03/galaxy-carbon-conveyer-belt/, citing Samantha L. Garza, Jessica K. Werk, Trystyn A. M. Berg, Yakov Faerman, Benjamin D. Oppenheimer, Rongmon Bordoloi, and Sara L. Ellison, “The CIViL* Survey: The Discovery of a C iv Dichotomy in the Circumgalactic Medium of L* Galaxies”, The Astrophysical Journal Letters, 2024; 978 (1): L12, https://doi.org/10.3847/2041-8213/ad9c69.
[2] Ziya Tong, The Reality Bubble, Toronto: Prentice Hall Press, 2019.
[3] Francis Reddy, “NASA’s Swift Reveals New Phenomenon in a Neutron Star”, NASA’s Goddard Space Flight Center, May 29, 2013, https://www.nasa.gov/mission_pages/swift/bursts/new-phenom.html.
[4] Caleb A. Scharf, “Moon Blobs, Collapsars, and Long Planets”, Scientific American, May 10, 2019, https://www.scientificamerican.com/blog/life-unbounded/moon-blobs-collapsars-and-long-planets/.
[5]
Tatyana Woodall, Ohio State University press release, “Stellar collapse and
explosions distribute gold throughout the universe”, ScienceDaily, May 7, 2025, https://www.sciencedaily.com/releases/2025/05/250507130338.htm,
citing Anirudh Patel, Brian D. Metzger, Jakub Cehula, Eric Burns, Jared A.
Goldberg, and Todd A. Thompson, “Direct Evidence for r-process Nucleosynthesis
in Delayed MeV Emission from the SGR 1806–20 Magnetar Giant Flare”, The Astrophysical Journal Letters, 2025;
984 (1): L29, https://iopscience.iop.org/article/10.3847/2041-8213/adc9b0,
https://doi.org/10.3847/2041-8213/adc9b0.
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