National Lab astrophysicists explore supernovae with an eye on national security
Scientists will receive a $2 million, three-year grant from DOE's Scientific Discovery through Advanced Computing (SciDAC) program to research supernovae, the cataclysmic deaths of stars
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November 5—The SciDAC program within DOE's Office of Advanced Scientific Computing Research supports High Energy and Nuclear Physics research in the use of terascale computers to dramatically extend exploration of the fundamental processes of nature, as well as advance the ability to predict the behavior of a broad range of complex natural and engineered systems.
"With this grant, we are trying to understand some of the most challenging issues in theoretical and computational physics," said Rob Hoffman of LLNL, one of the principal scientists for the project. "These processes include hydrodynamics, neutrino and radiation transport, the nuclear equation of state, convection, thermonuclear fusion, and flame propagation. These are precisely the issues at the forefront of research at the national laboratories, and progress in these areas advances our national security interests as well as our understanding of basic science."
A supernova is one of nature's most awesome spectacles, literally the explosion of a star. Observed in nearby galaxies at a rate of more than one per week, these titanic events release immense amounts of energy that can temporarily rival that of their host galaxy. "A supernova releases as much kinetic energy as the sun will radiate over its entire lifetime," said Hoffman. "They are the best bang since the big one."
The most recent supernova to be seen from Earth without scientific equipment occurred in 1987. Known as Supernova 1987A, the event occurred in the Large Magellanic Cloud, a satellite galaxy of the Milky Way.
Hoffman and Frank Dietrich of N-Division at LLNL, Chris Fryer and Mike Warren of LANL, Stan Woosley (principle investigator) and Gary Glatzmaier of UC Santa Cruz, and Adam Burrows and Phil Pinto of the University of Arizona have teamed up to model supernovae, to discover how these explosions occur, and to study in detail the complex physical processes that take place in supernovae.
The three-year SciDAC grant allows each of the researchers and their respective institutions to apply their specialties to supernova research.
Supernovae are broadly classified by two types based on the presence (Type 2) or absence (Type 1) of hydrogen in their light spectrum. Although both types release similar amounts of energy in optical light, they are totally different astrophysical systems.
A Type 2 supernova is the end result of the evolution of a massive star. All stars spend most of their lives "burning" hydrogen to make helium, and releasing energy as a byproduct. The energy liberated during nuclear burning provides the star with pressure support against the force of gravity, which would otherwise cause it to collapse.
Massive stars (at least eight times more massive than the sun) can burn successively heavier fuels (the ashes of one burning cycle serve as the fuel for the next) and thereby make successively heavier elements, such as carbon, oxygen, silicon, calcium, etc. The sun won't make anything much heavier than oxygen.
Eventually, a massive star encounters natural limits that prevent it from burning a fuel heavier than silicon. Once depleted of fuel, the star will collapse, turning into a huge "gravity bomb." During the final nuclear burning cycle, the central core of a massive star is transformed into iron, which then quickly turns into one of nature's most bizarre objects, a "neutron star", an object with the mass of the sun compacted into a sphere the size of a small city.
Once born, the nascent neutron star will liberate its mass (gravitational binding energy) in an enormous burst of energetic neutrinos, expelling the outer mantle of the star into space and seeding the galaxy with freshly minted heavy elements out of which future stars and planetary systems can form.
"Since the beginning of time, the nuclear burning processes in massive stars have created nearly all of the chemical elements heavier than boron," Hoffman said. "Nucleosynthesis is but one of the many fascinating things about supernovae. We are all composed of material that at one time burned inside a huge star."
A Type 1 supernova is composed of two stars, one the ancient core of an old star like our own sun (a white dwarf, made of carbon and oxygen), the other is either a young (main sequence star like the sun) or a middle-aged (red-giant) star. The stars must orbit each other closely enough that gravity can pull material from the envelope of the younger star onto the surface of the white dwarf. Once enough matter builds up, the temperature and density of the white dwarf reach a point where a thermonuclear runaway begins.
"Then the entire star blows up. It's similar to a huge hydrogen bomb," Hoffman said. "Amazingly, research suggests the younger star may survive the explosion, although only its dense core would remain. In the process most of the white dwarf is transformed into radioactive nickel, which decays to iron."
About half of all the iron in the galaxy comes from Type 1 supernova explosions. Said Hoffman, "This is the same iron that provides the hemoglobin in your blood with the ability to transport oxygen to the tissues of your body, making carbon-based life possible."—by Anne M. Stark