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Originally appeared in Monday, February 20, 2006 The Daily Beacon
URL: http://dailybeacon.utk.edu/showarticle.php?articleid=49634

Scientists investigate stars’ role in creating planets, life

They are born, millions, maybe billions, every day. They pulse, like the heart. They age. Then, they die. Yet they aren’t animals or plants. They are stars. And when the largest stars die, they go out with a bang, having created in their deaths the chemical elements needed for life.

Without supergiant stars exploding as supernovae, “life as we know it would not exist,” said Anthony Mezzacappa, an astrophysicist at Oak Ridge National Laboratory. “We know that core collapse supernovae are the dominant source of elements between oxygen and iron in the universe.”

A core collapse supernova is the scientific label for the explosion of a star that is eight to ten times the mass of our sun.

Stars are gaseous balls made almost entirely of hydrogen and helium, the two lightest elements. At the center of the star, where pressure builds and temperatures reach 100 million degrees Kelvin, hydrogen particles slam into each other at speeds greater than 100 kilometers per second, or more than 2 million miles per hour.

At these speeds, the protons fuse to build helium, the next heavier chemical element. Every second, 564 million ton of hydrogen is converted to 560 million tons of helium, with 4 million tons of mass lost — converted into energy. Only seventh-tenths of one percent of mass is lost, a minuscule amount. But Einstein’s famous formula tells us, E=mc2, energy equals mass times the speed of light squared. So a little mass multiplied by the speed of light and then multiplied by the speed of light again converts into a vast amount of energy.

Michael Guidry, a physics professor at The University of Tennessee who does research on supernovae, said, “These fusion reactions are analogous to burning. At the center of the star, helium ash gets left behind at the core.”

Once the star depletes the hydrogen at its core, the center of the star contracts, heats up and starts to burn the ash of the previous burning stage, according to Guidry and Mezzaccappa. As heavier elements are created through fusion, the star’s core temperature continues to climb. At 600 million degrees Kelvin or higher, a star begins to fuse carbon nuclei.

“The same scenario repeats itself several times,” Guidry said. “The ash of each subsequent fusion remains with the most massive element at the core. So it would be something like neon, then carbon, then helium and then hydrogen on the outside. The sequence continues, and you get this layering onion structure, until iron is produced.”

As iron increases within the star’s core, nuclear fusion slows. Iron nuclei can merge, but the process absorbs rather than releases energy. A star that has 20 times more mass than the sun will burn hydrogen for 10 million years, carbon for a thousand, oxygen for one year, silicon for a week and iron for one day.

The iron core is under massive gravitational pressure, and, at a certain point, the star can no longer withstand the pressure or temperature of 10 billion degrees Kelvin. During fusion, an outward pressure balances the inward pull of gravity from the star, but as the iron core grows and fusions slows, there is insufficient pressure to counteract gravity.

“It is not a stable situation,” Guidry said. “At a certain point, known at the Chandrasekhar limit or 1.3 times the mass of the sun, the core collapses. So here you have this star that has lived for hundreds of millions of years, and it dies in seconds.”

Guidry equated the star’s size to the solar system. He said that the core would be about the size of the Earth, and the onion layers would extend from the sun all the way to Jupiter. The outer onion layers undergo one final flurry of fusion. Inside the star, core compression slows, stops when it reaches a density billions of times greater than the density of water, according to Guidry.

Mezzaccappa said that, like a rubber ball hitting a brick wall and bouncing back, the core rebounds, creating energetic shockwaves, causing a massive explosion that spews the layered elements into space.

“When astronomer Carl Sagan said, ‘We are star stuff,’ he meant it, literally,” said Guidry. “The energy from a supernova is so great that if we had one in the Milky Way galaxy, it would outshine the entire galaxy of stars, a sum of 200 billion stars. That energy blows apart the star and distributes the elements into the galaxy.”

The energy could be responsible for creating the elements heavier than iron like lead, uranium and plutonium, as well, according to Mezzacappa and Guidry. The remaining core of the star that did not explode becomes either a neutron star or a black hole.

“This proto-neutron star is radiating neutrinos, which are particles that are nearly massless and act like radiation,” Mezzacappa said. “They drive a wind off the proto-neutron star. Like the solar wind off the sun, which creates the aurora borealis, we believe that elements heavier than iron are formed in this wind. It is what we call rapid neutron capture.”

The scientists say that the explosion and wind blows the elements into the interstellar medium and these remnants were part of the primordial soup that aided in creating the solar system. The National Aeronautics and Space Administration will be one step closer to understanding the composition of space with the return of its Stardust spacecraft, which returned to Earth on Jan. 15. The craft captured space particles from space at the outer edge of the solar system in the winter and spring of 2000 and fall and winter of 2002.

Donald Brownlee, Stardust’s principal investigator, said in a Planetary Society interview, that the samples his team collects will provide insight into the origin of our solar system, the Sun, the planets and the structure of life itself.

Mezzacappa and Guidry agree and both are working on the TeraScale Supernova Initiative at Oak Ridge, which is the largest collaborating effort to focus on understanding “primordial soup.”