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Supernova Shrapnel Discovered Inside Meteorite

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A meteorite than landed on Earth nearly 150 years ago has been found to contain microscopic shrapnel from a star that exploded about the time our solar system was born.

The chemical composition of the Orgueil meteorite, which struck France in 1864, indicates that a nearby star exploded in a supernova around 4.5 billion years ago, right when the sun's planets were forming. From the faint remnants of the stellar explosion, researchers are now in position to determine what kind of star exploded.

The research could solve the mystery of why levels of a metallic element, chromium, vary from one planet and meteorite to the next.

The study of the meteorite, which is embedded with rounded grains and is known as a chondrite, was led by University of Chicago researcher Nicolas Dauphas. The findings are detailed in the Sept. 10 issue of the Astrophysical Journal.

The tale of chromium 54

Previously, scientists believed that chromium 54, which is an isotope of the element, and other chemical elements were evenly dispersed throughout the cloud of gas and dust that collapsed to form our solar system.

"It was a very well-mixed soup. But it looks like some of the ingredients got in there and didn't get completely homogenized, and that's a pretty interesting result," said Bradley Meyer, a professor of astronomy and astrophysics at Clemson University, who did not work on the new research.

For four decades, scientists hypothesized that a supernova explosion occurred about 4.5 billion years ago, possibly triggering the birth of the sun.

"It seems likely that at least one massive star contributed material to the solar system or what was going to become the solar system shortly before its birth," Meyer said in a statement.

Inside the meteorite, traces of the chemical elements aluminum 26 and iron 60 two short-lived isotopes found in space rocks but not on Earth had led researchers to think they came from a core-collapse supernova, which is classified as a "Type II" event, describing a massive star that undergoes an internal collapse and violent burst.

A Type II supernova occurs when a star at least nine times heavier than the sun burns almost all of its fuel. The fusion engine in the center of the star begins to stutter, which triggers an internal collapse, followed by a violent explosion of the entire star.

By contrast, Type Ia supernova explosions occur in the death of a small but extremely dense white-dwarf star in a binary system (in which two stars orbit each other).

Type II supernova grains have been found in meteorites before, but until now, residual markers from a Type Ia supernova had never been detected.

Sifting through the grains

Scientists will now be able to analyze grains in the French meteorite for chemical markers that will help determine which type of supernova contributed to the rock's chromium 54 content.

"The test will be to measure calcium 48," Dauphas said. "You can make it in very large quantities in Type Ia, but it's very difficult to produce in Type II."

If the meteorite grains contain lots of calcium 48, then it probably came from a Type Ia supernova explosion.

Researchers in this field known as cosmochemists have sought the carrier of chromium 54 for 20 years, but recent advances in instrumentation suggest the answer may arrive soon.

The grains measured in the study were less than 100 nanometers in diameter, about a thousandth of the width of a human hair.

The findings suggest that after the supernova released these grains into space, dynamical processes in the early solar system sorted the fragments by size, which led the grains to become disproportionately incorporated into the meteorites and planets that were beginning to form around the sun.

"It's remarkable that you can look at an isotope like chromium 54 and potentially find out a whole lot about what happened in the very first period of the solar system's formation," Meyer said.