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Did a Massive Supernova Explosion Trigger the Formation of Our Solar System?

A new study published by University of Chicago researchers challenges the notion that the force of an exploding star prompted the formation of the solar system. In this study, published online last month in Earth and Planetary Science Letters, authors Haolan Tang and Nicolas Dauphas found the radioactive isotope iron 60—the telltale sign of an exploding star—low in abundance and well mixed in solar system material. As cosmochemists, they look for remnants of stellar explosions in meteorites to help determine the conditions under which the solar system formed. Some remnants are radioactive isotopes: unstable, energetic atoms that decay over time. Scientists in the past decade have found high amounts of the radioactive isotope iron 60 in early solar system materials.

"If you have iron 60 in high abundance in the solar system, that's a 'smoking gun'—evidence for the presence of a supernova," said Dauphas, professor in geophysical sciences. Iron 60 can only originate from a supernova, so scientists have tried to explain this apparent abundance by suggesting that a supernova occurred nearby, spreading the isotope through the explosion.

But Tang and Dauphas discovered that levels of iron 60 were uniform and low in early solar system material. They arrived at these conclusions by testing meteorite samples. To measure iron 60's abundance, they looked at the same materials that previous researchers had worked on, but used a different, more precise approach that yielded evidence of very low iron 60.

Previous methods kept the meteorite samples intact and did not remove impurities completely, which may have led to greater errors in measurement. Tang and Dauphas' approach, however, required that they "digest" their meteorite samples into solution before measurement, which allowed them to thoroughly remove the impurities. This process ultimately produced results with much smaller errors.

"Haolan has dedicated five years of very hard work to reach these conclusions, so we did not make those claims lightly. We've been extremely careful to reach a point where we're ready to go public on those measurements," Dauphas said.

To address whether iron 60 was widely distributed, Tang and Dauphas looked at another isotope of iron, iron 58. Supernovae produce both isotopes by the same processes, so they were able to trace the distribution of iron 60 by measuring the distribution of iron 58.

"The two isotopes act like inseparable twins: Once we knew where iron 58 was, we knew iron 60 couldn't be very far away," Dauphas explained. They found little variation of iron 58 in their measurements of various meteorite samples, which confirmed their conclusion that iron 60 was uniformly distributed.

To account for their unprecedented findings, Tang and Dauphas suggest that the low levels of iron 60 probably came from the long-term accumulation of iron 60 in the interstellar medium from the ashes of countless stars past, instead of a nearby cataclysmic event like a supernova. If this is true, Dauphas said, there is then "no need to invoke any nearby star to make iron 60."

However, it is more difficult to account for the high abundance of aluminum 26, which implies the presence of a nearby star. Instead of explaining this abundance by supernova, Tang and Dauphas propose that a massive star (perhaps more than 20 times the mass of the sun) sheds its gaseous outer layers through winds, spreading aluminum 26 and contaminating the material that would eventually form the solar system, while iron 60 remained locked inside the massive star's interior. If the solar system formed from this material, this alternate scenario would account for the abundances of both isotopes.

"In the future, this study must be considered when people build their story about solar system origin and formation," Tang said..

The image at the top of the page shows massive stars that exploded as supernovae, creating superbubbles in the surrounding gas. X-Ray (blue), Optical (yellow/green), Infrared (red) composite of N44 in the Large Magellanic Cloud

For more information: "Abundance distribution, and origin of 60Fe in the solar protoplanetary disk," by Haolan Tang and Nicolas Dauphas, Earth and Planetary Science Letters, December 2012. Journal reference: Earth and Planetary Science Letters



Farthest supernova yet marked death of very massive star

N SPACE - UNSPECIFIED DATE: In this handout provided by NASA, an X-ray image from the orbiting Chandra Observatory shows the nucleus of NGC 1260, the galaxy containing SN 2006gy, a massive star in what scientists are calling the brightest supernova ever recorded. Supernovas usually occur when massive stars exhaust their fuel and collapse under their own gravity, in this case the star could have possibly been 150 times larger than our own sun. Photo by NASA/CXC/UC Berkeley/N.Smith et al. via Getty Images

Astronomers are reaching ever further back in time, seeking events from the earliest days of the universe. Now, the discovery of the farthest (and thus oldest) supernova ever seen is raising hopes that astronomers will soon detect the explosive deaths of the first stars to form after the universe's birth.


These stars forged the first heavy elements, which helped create smaller and longer-lived stars like our own sun.

The earliest stars looked different from modern stars. The big bang produced only three light elements_hydrogen, helium, and a little lithium_but today, stars form in gas clouds that also contain heavier elements such as carbon and oxygen. These elements radiate away enough energy to eventually cool the clouds. When the clouds cool, they fragment into smaller clumps that collapse to spawn a plethora of mostly small stars.

But such fragmentation wasn't easy early in the universe's life, when stars formed from carbon- and oxygen-free gas clouds that remained warm. Because of their warmth, more gravity was needed to overwhelm the higher gas pressure_so when a cloud collapsed, it produced massive stars rather than small stars.

Astronomer Jeff Cooke of Swinburne University of Technology near Melbourne, Australia, and colleagues have been searching for the most distant, ancient supernovae by examining images from the Canada-France-Hawaii Telescope atop Mauna Kea in Hawaii. To discern even the faintest specks of light, the astronomers combine, or "stack," hundreds of images. In one image, taken in 2006 of a galaxy in Sextans (a faint constellation south of Leo), they spotted a very distant supernova indeed.

To find out just how far away it was, Cooke observed the galaxy's spectrum-- the combined light emitted from its stars, arranged by wavelength_at the Keck I telescope, also atop Mauna Kea. "It was quite exciting," he says. "As the spectrum was reading out, I could see the emission line for one of the features, and when I did a quick back-of-the-envelope calculation for the redshift, I saw how high it was."

The redshift is a measure of the supernova's distance. As the universe expands, it stretches the light waves traveling to us from a distant galaxy, shifting the galaxy's spectral lines to redder wavelengths; the farther the galaxy is and the more expanding space its light has traveled through, the greater its redshift. And as Cooke's team reports online Wednesday in Nature, the supernova's redshift is 3.90, which means it is 12.1 billion light-years from Earth_and it exploded just 1.6 billion years after the big bang. That makes it more than a billion light-years farther than the previous record holder.

Moreover, the supernova is anything but normal. It marked the death of a star that was more than 100 times as massive as the sun. During its brief life, such a star supports its great weight by generating so much light that the pressure of that outward radiation balances the inward pull of gravity. Unfortunately for the star, high-energy gamma rays supply much of this outward pressure, and when two gamma rays meet, they can convert their energy into a pair of particles, an electron and a positron.

This "pair production" robs the star of the support that the gamma rays' pressure had been providing. As a result, Cooke says, "The whole star collapses in on itself. It's one giant thermonuclear bomb, and it's incredibly bright." A pair-instability supernova emits about 10 times as much light as the brightest normal supernovae, which occur when white dwarf stars explode. Pair-instability supernovae are so rare that observers have previously seen only one good candidate_and that was in a fairly nearby galaxy.

Astronomer Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, calls the discovery of the distant pair-instability supernova a breakthrough. "It's the first demonstration that such events do take place at early cosmic times, and I think we will find many more of them in the future," he says. Astronomer Volker Bromm of the University of Texas, Austin, says: "This is a very, very promising sign for what we can expect in the coming years."

Cooke's team has also detected another pair-instability supernova 10.4 billion light-years from Earth. Neither supernova arose from a star that formed from pristine gas, so neither represents the very first generation of stars to form after the big bang. But the two explosions suggest that pair-instability supernovae_and thus very massive stars_were more common during the first few billion years of the universe's life than they are today.