July 2007

Star material

Simulating the origin of everything – from the iron in our blood to the planet we live on.

How, from the primordial Big Bang eons ago, were the building blocks of life created?

Over the past several centuries scientists have pried loose some clues, but a definitive answer has yet to be found.

That may soon change, thanks to collaboration between Department of Energy and university scientists.

Hubble Space Telescope image of remnants of supernova 1987A

Hubble Space Telescope image of remnants of supernova 1987A

The final revelation of the source of life’s core elements could come from their investigation of what astrophysicists call core-collapse supernovae – the deaths of massive stars.

“Understanding how massive stars die is a key link in the chain of origin from the Big Bang to us, which is tantamount to understanding how we came to be in the universe,” explains project head Anthony Mezzacappa, a corporate fellow at DOE’s Oak Ridge National Laboratory.  That makes it “one of the most important unsolved problems in astrophysics.”

Core-collapse supernovae are the most energetic events in the cosmos and are responsible for the chemistry and thermal evolution of the universe.  They are, in essence, responsible for the atoms that make up our bodies as well as the rest of the material universe.

Core-collapse supernovae start with stars at least eight times the size of our sun.  Radiation from these stars drive powerful “winds.” The solar wind from our sun, by comparison, produces dramatic aurora borealis, or northern lights, when it strikes Earth’s magnetic fields.  Solar winds from massive stars are many times more energetic.

Since exploding stars cannot be reproduced on Earth and the type of matter that remains after a star dies doesn’t exist on our planet, DOE researchers use supercomputers to model and study core-collapse supernova explosions.

The group uses Oak Ridge’s Jaguar, a Cray XT3/4, supported by DOE’s Office of Advanced Scientific Computing Research.

Jaguar runs at 119 teraflops – 119 trillion calculations per second.  This allows three-dimensional modeling of some of what happens during the death of a massive star, but a thorough simulation will require even bigger computers.

A sugar cube-sized chunk of a star’s collapsed core would weigh as much as Earth’s entire human population.

“The computational power required to simulate three-dimensional core-collapse supernovae events with all the physics will be at the multi-petaflops scale,” says Mezzacappa, who is an astrophysicist.

A petaflops is 1,000 teraflops, or a quadrillion calculations per second – that’s a 1 followed by 15 zeroes.  Mezzacappa expects the DOE to have that capability within five years.

Such a potent computer, coupled with astronomical observation, would give researchers a virtual cosmic laboratory.  They could study fundamental nuclear particle physics in a way that’s impossible with terrestrial experiments.

Teraflops computers, meanwhile, already are enabling physicists to peel away secrets hidden in the onion-like layers of core-collapse supernovae.

development of instability of the core collapse supernova shock wave

This shows development of the computationally discovered instability of the core collapse supernova shock wave in three dimensions, from a simulation developed by John Blondin of North Carolina State University and Anthony Mezzacappa of Oak Ridge National Laboratory. The instability leads the shock wave, represented by the surface in this picture, to deform away from round. The deformations lead to circulating flow below the wave. This shows the angular momentum in the stellar core fluid below the shock. Visualization by Kwan-Liu Ma of the University of California at Davis.

Only massive stars become core-collapse supernovae.  Smaller stars can die as thermonuclear supernovae, like thermonuclear bombs on a stellar scale, leaving little behind.

Core-collapse supernovae, the type the Oak Ridge group studies, are different.  Such stars die when their cores collapse.

What’s often left is a relatively small, extremely dense, rapidly rotating object that shoots out radio waves in a swiftly pulsating pattern.  These rotating neutron stars are called pulsars.

A core-collapse supernova’s death stems from the star’s relatively short life cycle of some millions of years.  Smaller stars, like the sun, live for billions of years.

“More massive stars evolve more quickly because of their increased gravitational pull,” Mezzacappa says.  “Gravity is always pulling inward and their nuclear fuel burns more quickly as a result.”

These massive stars have onion-like layers as they approach the ends of their lives.  A silicon layer surrounds an iron core.  An oxygen layer surrounds that, then carbon and helium layers.  A hydrogen envelope is on the outside.

The iron core, at 1.25 billion degrees Fahrenheit, eventually can’t support itself against the pull of gravity and collapses, causing the supernova.

The collapsing core splits into inner and outer layers and eventually cannot compress any further.  Like a rubber ball, it springs back, creating a gigantic shock wave that obliterates the other layers in a cataclysmic explosion.

On a cosmic scale, all of this takes place in the blink of an eye, Mezzacappa says.  The core collapses in about a tenth of a second, it rebounds in about a second, and the outer layers that took millions of years to form are destroyed within a few hours.

What remains of the core is no longer iron but a material so densely packed with neutrons that a sugar cube-sized chunk would weigh as much as Earth’s entire human population.  Such a material cannot exist here on Earth, either naturally or in a laboratory.

In fact, the density of this material is such that these neutron stars weigh more than our sun within a radius of only about six to 12 miles.  Our sun’s radius is 434,000 miles.

The dynamics of the explosion that creates them causes some neutron stars to rotate rapidly – a fast as one hundredth of a second for a complete revolution.

By chance, Mezzacappa’s team came upon “the mechanism whereby these neutron stars may be spun up, and, therefore, the mechanism whereby pulsars may be born.  It was a secondary benefit to our research effort,” he says.  “We were trying to understand one phenomenon and in the process we came to understand a different one.”

supernova movie

Opening frame of a movie showing density in the stellar core fluid below the shock in a three-dimensional simulation of instability of the core collapse supernova shock wave.

Mezzacappa and his collaborators are working toward simulating a massive collapsing star with all the necessary physics in two dimensions.

“In doing so,” Mezzacappa says, “we will also advance the three-dimensional simulations that were used to discover the pulsar spin because those simulations had only a subset of the physics in them.”

Petaflops computing capability also could help researchers understand another key phenomenon: Why the shock wave propagating out from the collapsing core seems to stall.

“We know that it stalls because it loses energy as it plows through the star.  Eventually it loses enough energy to stall,” Mezzacappa says.  “The fundamental question in supernova theory, which is not yet answered, is how is the shock wave revived?”

Answering this and other complex questions relating to core-collapse supernovae events will lead to a more thorough understanding of how the elements of the periodic table are formed.  It’s an important concern of DOE’s nuclear physics program.

Core-collapse supernovae are the main source of elements in the periodic table between oxygen and iron.  It’s believed, but not known with certainty, that they also are responsible for half the elements heavier than iron.

“As such, these events would be the dominant source of elements in the periodic table, without which life as you and I know it – human life – would not exist,” Mezzacappa says. “Many elements – the iron in your blood, the oxygen that you breathe, other metals that your body needs for its processes, even the gold in your wedding band – come from these core-collapse supernova events,” he adds.