Scientists from Lawrence Livermore and Los Alamos national
laboratories, the University of California Santa Cruz and the University of Arizona have
received a $2 million, three-year grant from the Department of Energy to research
supernovae, the cataclysmic deaths of stars.
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 Rob Hoffman, of LLNL and one of the principal scientists for the
project. “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 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 DOE Office of Science grant allows each of the researchers and their
respective institutions to apply their specialties to supernova research.
The grant is funded by the Scientific Discovery through Advanced Computing (SciDAC)
program in High Energy and Nuclear Physics research, which supports the use of
terascale computers to dramatically extend exploration of the fundamental processes
of nature, as well as advance our ability to predict the behavior of a broad range of
complex natural and engineered systems.
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. 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”.
“With this grant, we are trying to understand some of the most challenging issues in
theoretical and computational physics,” Hoffman said. “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”.
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For images of a supernova, go to
www.llnl.gov/llnl/06news/NewsMedia/nova_images.html. For more information on the
SciDAC grants, go to www.science.doe.gov/scidac
Founded in 1952, Lawrence Livermore National Laboratory is a national security
laboratory, with a mission to ensure national security and apply science and
technology to the important issues of our time. Lawrence Livermore National
Laboratory is managed by the University of California for the U.S. Department of
Energy’s National Nuclear Security Administration.
