Laboratory Astrophysics

Laboratory experiments with intense lasers enable physics studies relevant to the whole lifecycle of a star, from its formation from cold gas in molecular clouds, through its subsequent slow evolution, and on to what might quite possibly be a rapid, explosive death.

To determine a star's structure throughout the various stages of its life, astrophysicists need information on the radiative opacity of plasma that composes the outer stellar envelope and on the equation of state (how density and temperature relate to the pressure or internal energy) of plasma that makes up the star's interior. The radiative opacity of the material in stellar interiors plays a key role in determining how stars evolve: how hot and how luminous the star is while it burns its hydrogen fuel, the neutrino fluxes it emits, etc. Scientists use a set of calculations to predict stellar structures and properties; however, these depend on the opacities at the relevant conditions. Astrophysicists need to conduct direct experiments, using a high-energy facility like NIF, to measure these properties.

The Eagle Nebula, also known as the "Pillars of Creation." Astrophysics experiments at NIF could help answer such questions as: How were the pillars in the Eagle Nebula formed? Do the mechanics that formed the pillars trigger star formation?

Gaining a better understanding of a star's structure during its various stages of evolution might also involve investigating nuclear reaction rates by observing them in a stellar-like plasma. The key to success in studying radiative opacity, equations of state and nuclear reaction rates in stars is the ability to recreate the very hot plasmas that characterize the stellar environments.

Star Formation

Star formation occurs when interstellar gas is dense enough that gravity can pull the gas together to form stars. The required density can be achieved in molecular clouds where complex gas motion (or gas dynamics) is caused by intense ultraviolet radiation, or by shocks from nearby stars. These also determine the peculiar shapes of the molecular clouds, including the famous Eagle Nebula.

Powering Stars

Stars burn hydrogen in their cores for most of their lives at temperatures of 10 to 30 million Kelvin (18 to 54 million degrees Fahrenheit). The National Ignition Facility will achieve a temperature about five times higher than that, so NIF will provide a potential laboratory for studying the nuclear reactions that power the stars. Previously, these reactions have been studied with a series of nuclear cross-section measurements, each initiated by a particle beam with a well-defined energy from an accelerator impinging on the atoms in a very thin target. Stars, however, undergo their nuclear reactions in a thermal environment characterized by ions or even bare nuclei having a much broader energy range than would exist in an accelerator beam. Thus, in order to determine the rate at which such reactions would occur in a thermal environment, the cross-sections must be averaged over the energy range that would be associated with the thermal environments of stars. Furthermore, the effects of screening of the (all positive) charges of the interacting nuclei by the plasma electrons in a star must be calculated. Such effects have never been measured.

At NIF, the relevant nuclear reactions will occur in a thermal environment, generally at temperatures – and thus energy ranges – close to those of stars. This eliminates the need to perform the summing procedure that is required to determine the reaction rates from accelerator-based experiments. And the interations of nuclei in the plasma will obviate the need for electron screening corrections. Several possible reaction studies are described in Nuclear Astrophysics.

A persistent problem for achieving laser fusion is presented by the instabilities that are generated when the fuel pellet is compressed. It is very difficult to maintain a perfect sphere for the imploding pellet throughout the implosion process – a result of non-uniform compression due to imperfections in the pellet and the laser beams. Although the use of the indirect-drive approach to laser fusion using hohlraums has mitigated this problem (see How to Make a Star), it has not done away with it entirely. Astrophysicists who study the hydrodynamics of exploding stars, however, delight in producing and studying these instabilities; they are the same ones that occur in supernovae.

Stellar Explosions

Supernova 1987A, a core-collapse supernova, exploded in 1987 in the Large Magellanic Cloud. The image was taken by the Hubble Space Telescope in 1999.Two basic types of supernovae are important in astrophysics. One is driven by nuclear processes. This involves a white dwarf, a star that is about the size of the Earth but with one and a half times the mass of the sun. Its size is maintained by "electron degeneracy pressure," which is also responsible for maintaining the sizes of the electron distributions in atoms. The supernova occurs when the white dwarf accretes enough matter from a companion star to exceed the maximum mass that can be supported by electron degeneracy pressure, initiating nuclear reactions. The result is runaway thermonuclear reactions that explode the entire star. Because these stars are always about the same mass when they explode, they always produce about the same luminosity, making them invaluable as "standard candles" for use by astronomers in determining their distance from Earth. (View a computer simulation of a white dwarf explosion courtesy of the University of Chicago.)

The other type of stellar explosion is a core-collapse supernova, which is a gravitational collapse that occurs when a heavy star, ten times the mass of the sun or more, has spent all of the nuclear fuel in its core. Normally, the gravitational forces in a star are balanced by a pressure from the heat of the nuclear reactions. As each nuclear fuel is consumed – first hydrogen, then helium, then carbon, etc. – the star develops an onion-like structure, with the interfaces between shells marked by density and composition changes. When the nuclear reactions no longer produce net energy, the core of the star, consisting mostly of iron, collapses under the force of gravity. This catastrophic collapse lasts only a few seconds and triggers a powerful explosion that sends a shock wave back through the star. This is partly responsible for exploding the outer part of the star, although the exact explosion mechanism has not been definitively identified. The violent collapse produces an enormous number of neutrinos and many complex hydrodynamic effects. The resulting stellar explosion appears as a bright flash of ultraviolet light followed by an extended period of luminosity, fueled by nuclear decays, that is initially brighter than the star's entire galaxy. The explosion leaves behind a remnant that is either a neutron star or a black hole.

In both types of supernovae, though, instabilities of the types that have affected laser fusion and that occur when nuclear weapons detonate are also important in determining the evolution of the final explosion event. Indeed, it is now thought that some form of instability may be essential for the supernova to explode. Propelled by the shock wave, fingers of matter from heavier shells penetrate into and through the overlying lighter shells, resulting in Rayleigh-Taylor hydrodynamic instabilities in all three types of events. Thus, NIF will provide an extraordinary laboratory for studying the details of supernovae explosions, nuclear weapon detonations, and laser fusion.

The extreme neutron density at NIF actually exceeds that at which the r-process (for rapid) of nucleosynthesis is thought to occur, albeit for a much shorter time. This condition apparently happens only in a core-collapse supernova or when two neutron stars collide. Recreating this phenomenon could have two consequences: it might allow for study of a reaction in which two ⁴He nuclei and a neutron fuse to form ⁹Be, a measurement that can only be studied at NIF because it requires that the three particles get together in a very short amount of time, 10^–16 seconds, for the reaction to take place. It also might create some neutron-rich nuclei, the study of which would increase the accuracy of our theoretical projections to the nuclei through which the r-process passes, but which are difficult to create in accelerators. (For a more thorough discussion of stellar nucleosynthesis, see "A Closer Look at Nucleosynthesis," Science & Technology Review, July/August 2007).

Properties of Plasmas

Much of our understanding of extreme objects such as black holes arises from observations of X-rays emitted by matter that is pulled into the deep gravitational wells of the black holes. The extreme temperatures of these plasmas produce nuclei in very highly ionized states – most of the electrons that would accompany a neutral atom have been stripped away. The resulting spectra can be quite different from those of the atoms or slightly ionized ions; the identification of these ions is crucial to determining the temperature and density of the environment in which they exist.

Many of the studies in astrophysics with NIF are in their infancy, and are still being evaluated to determine the best experiments or design the details that are required for their implementation. In other cases, significant data already exist from studies done at other high-powered laser facilities. In those situations, the very high energy achievable with NIF will extend our knowledge of the relevant science into new realms.

More Information

Basic Research Directions for User Science at the National Ignition Facility, National Nuclear Security Administration and U.S. Department of Energy Office of Science, November 2011

"Studying Nuclear Astrophysics at NIF," R.N. Boyd, L. Bernstein, and C. Brune, Physics Today 62, no. 8, 60 (2009)

"Preparing for the X Games of Science," Science & Technology Review, July/August 2007

"A Closer Look at Nucleosynthesis," Science & Technology Review, July/August 2007

"Duplicating the Plasmas of Distant Stars," Science & Technology Review, April 1999