How terrifying would a supernova explosion be if it occurred near Earth?

About 2.5 million years ago, an Australopithecus afarensis was resting under a tree. As the sun dipped below the horizon, he observed a strange phenomenon. A star in the night sky began to shine brighter than all the others, even outshining the moon. It continued to grow brighter, even during daylight hours, until it was more luminous than the sun itself. This spectacle continued for several days, the star growing ever brighter, before it gradually faded over the course of months and finally disappeared among the stars.

What the ancient hominin witnessed was a supernova explosion, a cataclysmic event in the universe. When a massive star exhausts its nuclear fuel, it collapses under its own gravity. The outer layers are blasted away, creating an expanding sphere of scorching gas. This process accelerates charged particles to nearly the speed of light, emitting deadly high-energy radiation and illuminating the entire galaxy. The energy unleashed could wipe out all life within a 50-light-year radius.

The impact of a supernova depends on its distance from us. We've observed supernovae safely from thousands of light-years away. But if one were to explode just 100 light-years from Earth, the radiation over decades would erode the ozone layer and ionize the atmosphere, leading to perpetual rain. This could reflect so much sunlight back into space that Earth would plunge into an ice age.

Researchers have also found radioactive atoms in seabed minerals. This evidence suggests that shortly after our Australopithecus afarensis ancestor lay down, a nearby supernova explosion may have triggered the onset of the Quaternary Ice Age on Earth.