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We can learn a lot about our galaxy by looking to the stars, but we can also reveal a lot about our cosmic history from... Dead trees?

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Most of the time, studying astronomy means looking up — you know, where all the stars and planets are. But occasionally, astronomers can get their data much closer to home, in old, dead trees.

In fact, cosmic history preserved in thousand-year-old wood is helping shed light on some of the most dramatic events in our planet’s past. And it’s begun to reveal just how events across the galaxy may have affected our little planet. Now, we’ve known for decades that Earth isn’t an isolated bubble, and events from space can sometimes mess with our atmosphere.

Like, if you’ve ever seen the northern, those colorful flashes are an indirect result of electrons traveling from the Sun into the atmosphere. But these electrons aren’t the only particles messing with our planet. Other high-energy particles from distant events like supernovas and black hole jets also reach the Earth from time to time, and these can have much more subtle effects.

It all starts when these high-energy particles, known as cosmic rays, hit the atmosphere. Here, they collide with atoms in the air, breaking them apart and freeing up neutrons. Those free neutrons then collide with nitrogen atoms, knocking out a proton, which transforms the nitrogen into carbon.

But these new carbon atoms have two extra neutrons in their nucleus — one from the collision, and one leftover from their original days as nitrogen atoms. That gives them an atomic mass of 14 instead of 12. So, appropriately, we call them carbon-14 atoms.

But just like regular carbon, carbon-14 bonds with oxygen to form carbon dioxide. So, when plants absorb CO2 for photosynthesis and use it to build their tissues, they capture these products of cosmic radiation, as well. And with trees, this is especially obvious.

Since trees grow by adding a ring of tissue to their trunks each year they capture annual snapshots of carbon-14 levels in each ring. So, by analyzing the carbon-14 within these rings, astronomers can actually reconstruct a history of cosmic events. The tricky thing about this technique is that it’s hard to find old trees that haven’t already decayed.

But now and then, trees get buried alive in catastrophic events like floods, and these can be preserved for thousands of years — long before written records began. In recent years, various teams of astronomers have compiled a database of carbon-14 levels in ancient trees. And when they looked at the record in fine detail, they discovered something remarkable:.

On top of gradual, decades-long fluctuations tied to the Sun’s cycles of activity, they spotted sharp spikes in carbon-14 concentrations. One of these spikes was first identified in Japanese cedar trees dating back over 1200 years. And it revealed that around 774 to 775 C.

E., its carbon-14 levels jumped by 20 times the normal amount of variation. Later studies found the same spike, at the same time in trees from around the world, including in Russia and the United States. So this was clearly a global phenomenon.

And it wasn’t the only one. Similar spikes have also appeared during various other years. And that’s left scientists with a challenge: figuring out what caused these massive spikes.

What happened in space to shower our planet with cosmic rays? So far, no one knows for sure, but there are a few possibilities. For instance, maybe the spikes were the result of supernova explosions in our galaxy.

It’s possible that these exploding stars may have delivered a bunch of high-energy particles to Earth. And models do suggest that events like this could cause a dramatic spike in carbon-14. So researchers have actually gone looking for supernova remnants in our galaxy, to see if any could possibly be connected with the carbon-14 spikes in trees.

And in 2020, a researcher published a study identifying eight possible supernova remnants that seemed to line up with various spikes in the record. Some dated back nearly 13,000 years. They even found one that they hypothesize could have caused the spike from 774.

Now, the tricky thing here is that the technique used to date the supernova remnants is imprecise. Basically, you’re just looking at how far the remnants of a supernova have expanded, and working backward. And those estimates can be off by as much as 1500 years.

So eventually, scientists hope to learn more by looking at ancient historical records or other environmental records like sediments that might preserve other clues to the nature of these events. If they succeed, that would give us a better picture of how we fit into our cosmic neighborhood, and it could even help us estimate the risk of future events like these. After all, any high-energy event strong enough to leave its mark in tree rings could fry our electronics and communications networks, and could even directly affect living things.

I know this script was a ton of fun until we found that Supernova might, mess with civilization. So by studying the surprising astronomical record in trees, we can begin to understand not just what happens in space, but how major cosmic events play out on Earth. Thanks for watching this episode of SciShow Space!

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