Popcorn Supernovas

A NASA mission launched in Mid-January 2003 is studying the remains of some uncomfortably close supernova explosions.

Australopithecus squinted at the blue African sky. He had never seen a star in broad daylight before, but he could see one today. White. Piercing. Not as bright as the Sun, yet much more than a full moon. Was it dangerous? He stared for a long time, puzzled, but nothing happened, and after a while he strode across the savanna unconcerned.

Millions of years later, we know better.

"That star was a supernova, one of many that exploded in our part of the galaxy during the past 10 million years," says astronomer Mark Hurwitz of the University of California-Berkeley.

Supernovas near Earth are rare today, but during the Pliocene era of Australopithecus supernovas happened more often. Their source was an interstellar cloud called "Sco-Cen" that was slowly gliding by the solar system. Within it, dense knots coalesced to form short-lived massive stars, which exploded like popcorn.

Researchers estimate (with considerable uncertainty) that a supernova less than 25 light years away would extinguish much of the life on Earth. The blast needn't incinerate our planet. All it would take is enough cosmic rays to damage the ozone layer and let through lethal doses of ultraviolet (UV) radiation. Our ancestors survived the Pliocene blasts only because the supernovas weren't quite so close.

We know because we can still see the cloud today. It's 450 light years from Earth and receding in the direction of the constellations Scorpius and Centaurus (hence the cloud's name, "Sco-Cen"). Astronomer Jesús Maíz-Apellániz of Johns Hopkins University have backtracked Sco-Cen's motion and measured its closest approach: 130 light years away about 5 million years ago.

Sco-Cen was still nearby only two million years ago when many plankton, mollusks, and other UV-sensitive marine creatures on Earth mysteriously died. Paleontologists mark it as the transition between the Pliocene and Pleistocene epochs. Around the same time, according to German scientists who have examined deep-sea sediments from the Pliocene era, Earth was peppered with Fe⁶⁰, an isotope produced by supernova explosions.

Coincidence?

No one knows. It's a puzzle researchers are still piecing together.