If we exclude dark matter, by far the majority of the universe is made of hydrogen, and the majority of what remains is made of helium. Famously, all elements with more than two protons collectively make up about two percent of the matter in the universe. Still, most of that hydrogen and helium is unavailable to humans, trapped in one of several trillion stars or in enormous clouds of gas. It’s only when those stars explode that they begin to donate heavier elements to the cosmos, fusing basic atoms together into larger ones in the punishing furnace of a dying star. When the star goes nova, large quantities of these heavier elements are thrown out to fly through space and slowly aggregate into, among other things, the planet Earth. Yet supernovae were not just features of the primordial universe; like people, stars die every day, and they don’t stop affecting our planet just because we’ve decided that its development ought to be “done.” Supernovae scare us, sending blasts of gamma radiation to lick at the protective layers of our atmosphere, but now we’re coming to appreciate their intrusions as powerful windows into the final leg of the stellar life cycle.

In 2004, research into ocean sedimentary layers found non-terrestrial iron isotopes with half-lives of less than three million years, unstable and energetic forms that can only be formed in a supernova. Shawn Bishop of the Technical University of Munich wanted to know if this iron isotope, 60Fe, could be found in the fossil record itself, proving that it has been incorporated into living organisms. The most logical target was a class of bacteria known as magnetotactic bacteria, which have fascinating little organs that allow them to align with the Earth’s magnetic field. The organ actually forces the bacteria into alignment, even after death! The tiny crystals called magnetite (not Magnemite) have the formula Fe 3 O 4 , and Bishop found precisely the proportion of stellar isotope you’d expect if the Earth had endured a nearby supernova roughly 2.2 million years ago. The unstable 60Fe isotope has already undergone almost an entire half-life since being deposited on Earth.

Since these living creatures died on the biological timescale (unlike the sediment, which was deposited on the geological timescale) this work helps us to date the nova much more accurately than prior evidence. When testing samples dated between 1.7 and 3.3 million years ago, it was only in the range of about 2.2 million years ago that the bacteria used 60Fe in making magnetite. Previous estimates had gone as high as 2.8 million years. The researchers estimate the star must have been a few tens of parsecs out when it exploded (around under a hundred light years) to be found in its current abundance. To put that in perspective, our closest star is about 1 parsec away, and the closest galaxy about 500,000.

Beyond being a powerful confirmation of the date of this supernova, the findings have implications for abiogenesis, or the initial formation of life from non-living chemicals. Many claim that the transition from chemistry to biology was impossible on primordial Earth, claiming that the process demands too wide an array of complex molecules. They have looked to the stars as a possible source of these molecules. Of course, supernovae give off elements rather than complex organic molecules, but the elements are deposited in similar ways. Just a few weeks ago, researchers analyzed dust from a passing meteor and found grains of silica previously thought to be “mythical” in nature. Finding that any alien molecules can fall to Earth and be quickly usable by life gives hope for our complex organic molecules as well.

Now read: NASA discovers three Earth-sized planets right in the habitable zone

