Your atoms, and the atoms inside the family dog or the tree growing outside, came from the stars. This is old news to scientists, but it’s also somewhat hazy: when exactly those elements arrived on Earth, and where exactly from, has been impossible to say.
Most of the chemical elements we’re made up of were forged from simpler atoms. The atoms that came directly out of the big bang were mostly hydrogen, a bit of helium, and trace amounts of lithium; heavier elements, including oxygen, carbon, calcium, and nitrogen, had to be cooked by thermonuclear reactions deep within stars’ cores. When a star dies, it spreads these elements out into space, either by puffing them gently away (as our sun will begin to do in five billion years or so) or by blasting them out in an explosion so powerful that, for a few days or weeks, the supernova can outshine the combined light of the galaxy’s other hundred billion stars, as Caleb Scharf recently explained. (The heaviest elements, including uranium, are too complex to be made inside a star; they’re only created during a supernova explosion itself.) The free-floating carbon, oxygen, and the rest eventually congeal to form new stars, along with the planets that orbit them. And on at least one planet we know of—Earth—the atoms and molecules have assembled themselves into living organisms.
The Technical University of Munich recently announced the discovery of “the first proven biological signature of a starburst on our earth.” A physicist named Shawn Bishop has extracted atoms of iron-60 from fossils of ancient bacteria buried under the floor of the Pacific Ocean. This is significant for a couple of reasons. First, iron-60, a radioactive cousin of ordinary iron, is created only in stars that explode as supernovas. Second, iron-60 decays relatively rapidly into a different atom, nickel-60 (not radioactive, but much less common than garden-variety nickel-58).
By looking at how much of the iron-60 has decayed, scientists can figure out how long ago it was created—in this case, 2.2 million years ago. They know that it came from a supernova blast, and that it had to be relatively nearby; otherwise, the iron-60 would have dispersed too widely to have fallen to Earth in any great quantity. No one knows exactly which star blew up, but one analysis suggests that it might have been part of the so-called Scorpius-Centaurus cluster of stars, about four-hundred and seventy light-years, or about two quadrillion miles, away.
That sounds far, but it’s practically next door in cosmic terms. There would have been no modern humans to witness the explosion, but our Homo erectus ancestors were walking the earth back then. They were presumably smarter than chimps, but less intelligent than Neanderthals, so it’s difficult to say whether they would have taken notice, much less what they might have thought about it. But the supernova would have been far brighter than anything in the sky except for the sun.
If the supernova had been closer—a few tens of light-years away—it could have done more than light up the skies and sprinkle a few iron-60 atoms. Some astrophysicists have suggested that gamma rays from a nearby supernova might have caused at least one of the mass extinctions that wiped out a large fraction of species on earth, allowing others to take over. If so, exploding stars might well have changed the course of evolution.
There was no mass extinction in this case—just a scattering of radioactive atoms, too few to cause any harm. But there were enough to leave their mark on microscopic creatures whose bodies had already been built out of stardust.
Michael Lemonick is a senior staff writer at Climate Central and lecturer at Princeton University; his most recent book is “Mirror Earth.”