Multiple Ancient Supernovae

If a supernova were to go off somewhere in our galaxy, the minimum safe distance for Earthbound life would be about 50 light years.  Any closer than that, and we would experience an intense blast of high energy radiation and an eventual shower of radioactive particles.  It would be like nuclear bombs were set off all around the Earth, causing little destruction but a lot of radioactive fallout.  Supernovae are incredibly powerful to be able to cause such damage at 50 light years, but even at larger distances, we can see evidence of their effects here on Earth.

False color image of Cassiopeia A using Hubble and Spitzer telescopes and Chandra X-ray Observatory. Credit: NASA/JPL-Caltech

A team of scientists from the Australian National University have found evidence that several supernova events showered Earth with radioactive elements between 3.2 and 1.6 million years ago. The evidence comes in the form of radioactive Iron-60 found in sediment and crust samples from the Pacific, Atlantic, and Indian oceans.  If you’re thinking the Iron-60 was already here, think again.  Iron-60 decays with a half-life of 2.6 million years, and is extremely rare.  Any amount from the Earth’s formation would have long since decayed, and so it had to have come from elsewhere.

These events are very recent in astronomical terms, and the fact that the Iron-60 is spread over such a long period of time suggests multiple nearby supernovae deposited radioactive elements over the long term. There is also the interesting coincidence that this period is aligned with the cooling of the Earth and the transition from the Pliocene period to the Pleistocene period.  There are theories that suggest that an increase in cosmic rays (a byproduct of supernovae) can increase cloud cover, cooling the planet.  But for now, this is just conjecture, and more evidence is needed.

Based on the concentrations of Iron-60, the researchers believe the supernova or supernovae would have had to be within 300 light years from Earth, much farther than the dangerous 50 light year minimum.  At 300 light years, life would have been unaffected biologically, but if climate changes were triggered, this would definitely have affected life on Earth.

So where did the supernovae come from? Several million years is a lot of time for a supernova remnant to disperse, but an independent study has proposed an aging star cluster that has an absence of massive stars and is moving away from Earth.  This suggests that the cluster’s massive stars have already exploded as supernovae and dispersed, possibly while the cluster was closer to us.

It gives you a sense of just how incredibly powerful a supernova is.  With all the technology we have on Earth, we can’t come close to moving anything across such incredible distances.  The massive explosions accelerate particles across the galaxy, seeding new stars with the raw materials needed for life. If you’re worried about a nearby supernova, there are no candidates close enough to cause us any harm. I hope we see a supernova soon, if only to see a beautiful display in the sky.

 

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