Astrophysics

Convection in Stellar Interiors

Extreme changes in the plasma that happen between a star's core and the photosphere define fluid effects that are responsible for the radial transport of heat and chemicals. A fluid effect example is turbulent thermal convection. In young stars on the pre-main sequence, convective envelopes tend to be large, thermal convection is truly multi-scale, and fluid mixing is efficient. In evolved stars on the red giant branch, layers of chemically different material can mix in the deep interior, altering the plasma's buoyancy and diffusive properties. In evolved stars ascending the asymptotic giant branch, layers where nucleosynthesis occurs can deposit energy into the fluid and change the chemical composition of the plasma, driving vigorous fluid mixing.

The plasma that makes up the interior of stars spans many orders of magnitude in temperature, pressure, and density, much of which corresponds to hot dense matter (HDM). The properties of this material are characterized by opacity measurements and their accompanying equations of state (EOS).

Opacity is behind one of the most dramatic physical effects in stellar physics, an effect that astronomers call the kappa mechanism. The kappa mechanism causes certain stars to pulsate radially, meaning that the outer radius of these stars grows and shrinks in a regular cycle. Such stars are called Cepheids, and changes in opacity drive their instability. A layer of partially ionized gas increases in opacity as it gets squeezed, trapping heat and pressure. It then decreases in opacity as it expands and cools, allowing heat to escape. The period of a Cepheid's radial pulsation can be linked directly to its luminosity through the Leavitt Law, allowing a Cepheid's distance from Earth to be calculated. The precise quantitative description of the internal physics of Cepheids, including accurate opacity tables, holds the key to improving both the estimate of distances and chemical compositions in galaxies beyond the Local Group.

Both stellar evolution and stellar hydrodynamics codes currently use tables produced by the Opacity Project at Livermore (OPAL) in the 1980s and 1990s. Since OPAL measurements are only available for certain values, they are often blended with other theoretical laws to cover the wider range of plasma conditions found in stars. Through experiments at the National Ignition Facility (NIF), LLNL is one of only a few places where researchers can improve on these existing opacity and EOS tables. There remains a vast range of astrophysical plasma conditions for scientists to explore during future experimental campaigns at NIF.

LLNL-developed codes that can simulate fluid dynamics in the HDM regime use the latest opacity and EOS tables, which were also developed at LLNL. The next-generation, higher-order ALE code Marbl is developed as part of the Multiphysics on Advanced Platforms Project specifically to address high energy density physics, including NIF experiments. Marbl simulations of stars have the potential to revolutionize the field of stellar astrophysics through their use of multiphysics fluid models, and by exploiting Marbl's ability to run efficiently on the largest computational resources. These Marbl simulations contribute to the 321D Link, an effort to improve stellar structure and evolution models through 2D and 3D hydrodynamic simulations.

Learn More

• The Marbl code and the Multiphysics on Advanced Platforms Project (MAPP) project at LLNL
• El Capitan revealing hidden worlds in previously unattainable high-fidelity simulations

Lab Astrophysics

LLNL scientists study the entire lifecycle of stars, from the formation of cold gas in molecular clouds, to what might be a rapid, explosive death. To determine a star’s structure throughout the various stages of its life, astrophysicists leverage the National Ingition Facility’s (NIF’s) ability to mimic the temperatures found in stars’ cores (10 to 30 kelvins, or 18 to 54 million degrees Fahrenheit).

For example, LLNL researchers explore the evolution of turbulence in supernova explosions. In a core-collapse supernova, a star with 10 times (or more) mass than our Sun uses up the nuclear fuel at its core, element by element, starting with hydrogen and working up the periodic table. As each fuel is consumed, the star develops an onion-like structure, with layers differing in density and material.

Once the fusion process can no longer compete with the pull of gravity, the star’s core collapses in a few seconds, triggering a powerful explosion that sends a shock wave back through the star. Propelled by the shock wave, fingers of matter from heavier layers penetrate the overlying lighter shells, resulting in Rayleigh‒Taylor hydrodynamic instabilities. The violent collapse produces an enormous number of neutrinos and many complex hydrodynamic effects. The resulting explosion appears as a bright flash of ultraviolet light followed by an extended period of luminosity 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.

Another focus of astrophysics research at LLNL is probing the formation of turbulent collisionless shocks in conditions relevant to young supernova remnants by creating a hydrodynamically scaled version of the shock in the laboratory. Astrophysical collisionless shocks are among the most powerful particle accelerators in the universe. Generated by violent interactions of supersonic plasma flows with the interstellar medium, supernova remnant shocks amplify magnetic fields and accelerate electrons and protons to speeds approaching the speed of light.

However, astrophysical shocks develop turbulence at very small scales—too small to be seen by astronomical observations—that helps accelerate electrons at the shock wave before being boosted up to their final velocities. Researchers conduct laser-driven plasma flow experiments on NIF to probe the formation of turbulent collisionless shocks in conditions relevant to young supernova remnants. They were able to identify the mechanism that allows electrons to be accelerated by small-scale turbulence produced within the shock transition. Their observations provide new insight into electron injection at shocks and open the way for controlled laboratory studies of the physics underlying cosmic accelerators.

Other NIF astrophysics experiments enable scientists to study the state of matter found only in gamma-ray bursts, black holes, and active galaxies.

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• Lab astrophysics research at LLNL
• X-ray and laboratory astrophysics at LLNL
• Space science research at LLNL
• Shock waves created at NIF mimic astrophysical particle accelerators powered by exploding stars
• HED experiments measure supernova magnetic field structure
• Chaotic plasmas give birth to orderly electromagnetic fields

Nuclear Astrophysics

Nuclear astrophysics spans the smallest scales in nature—atomic nuclei, to the largest structures in the universe—stars and galaxies. Key questions include: How do stars shine? Where do the chemical elements come from? How do nuclear reactions shape the evolution and fate of stars? To answer these questions, nuclear astrophysicists at LLNL combine insights from nuclear physics, astrophysics, and cosmology, and they use tools such as particle accelerators, telescopes, and supercomputer simulations.

At the core of their nuclear astrophysics research is nucleosynthesis, the process by which new atomic nuclei are created. The universe started out primarily as hydrogen and helium after the Big Bang, with only trace amounts of a few light elements like lithium. Nearly all of the heavier elements were forged later in stars and stellar explosions. Different environments favor different nuclear reaction pathways. For example, in the cores of ordinary stars like the Sun, relatively slow fusion reactions convert hydrogen into helium and then into heavier elements during later stages of stellar evolution. In contrast, extreme events like supernovae or neutron star mergers produce rapid neutron captures (the r-process) that can build very heavy, neutron-rich nuclei in fractions of a second.

LLNL scientists study the properties of nuclei involved in these reactions. Reaction rates, decay lifetimes, and nuclear energy levels all play key roles when predicting which elements will be produced and their quantities. Many of the nuclei they study are unstable and cannot be found on Earth naturally, so LLNL researchers create them in the lab using particle accelerators and radioactive ion beams. By measuring cross sections and decay schemes, and by developing theoretical nuclear models, they generate data for astrophysical models of stars, supernovae, and compact object mergers.

LLNL researchers also explore the impact of nuclear reactions on how stars live and die. The balance between gravity, which pulls matter inward, and pressure from nuclear burning, which pushes outward, determines a star’s structure and lifetime. As nuclear fuel in the core is consumed and replaced by heavier ashes, the star’s interior changes, setting up new burning stages and, for massive stars, eventually leading to core collapse and explosion. The nuclear energy released, and the neutrinos and photons produced, affect everything from the light curve of a supernova to the formation of compact remnants like white dwarfs, neutron stars, and black holes.

Finally, nuclear astrophysics research at LLNL involves observations across the electromagnetic spectrum and beyond. Spectroscopy of stars and gas clouds reveals the chemical fingerprints of nucleosynthesis in their spectra. Meteorites preserve isotopic clues about early solar system processes. Gravitational wave detections from neutron star mergers, combined with electromagnetic observations, give direct evidence of heavy element production in these collisions. By comparing detailed theoretical models to this growing body of data, nuclear astrophysicists at LLNL help refine our understanding of both nuclear physics and the cosmic history of the elements.

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• Nuclear astrophysics research at the National Ignition Facility
• Scientists probe the conditions of stellar interiors to measure nuclear reactions

Planetary Physics

Planetary interior conditions are characterized by extreme pressures (millions of atmospheres) and temperatures (thousands of Kelvin), which are characteristic of the warm dense matter (WDM) regime—the elusive state of matter between hot solids and cold plasmas. At such extreme conditions quantum effects start to dominate material behavior. The interatomic spacing decreases, and the overlapping electronic band structures and partial ionization fundamentally alter the transport, chemical, and mechanical material properties. Structural phase transformations (melting and re-solidification) can occur, causing materials to assume new forms and to behave in a dramatically different way than on the surface of the Earth.

Understanding the properties of WDM is one of the grand challenges of HED science and finds important applications for planetary science. Advances in astronomical observations led to the discovery of more than 4,000 extrasolar planets (exoplanets), including some that are smaller than Earth and others that are a dozen times more massive than Jupiter. To understand their internal structure and whether they could harbor life, it is critical to study the properties of their constituent materials at the pressures and temperatures existing in their interiors.

Using giant lasers to compress and heat matter, scientists can recreate these conditions in the laboratory for a few billionths of a second and start to unravel the properties of WDM in-situ with ultra-fast diagnostics. LLNL scientists developed instruments that allow them to use the most energetic lasers in the word, such as LLNL’s National Ignition Facility and the Omega Laser at the Laboratory for Laser Energetics, to peer inside the interior of planets of the solar system and beyond. One such instrument enables the collection of x-ray diffraction patterns that provides snapshots of the atomic structure of the material while it is being compressed and heated by the lasers. Other diagnostics include ultra-fast velocimetry and pyrometry measurements that provide information such as pressure, temperature, and optical properties.

Together, these experiments provide unique information regarding the properties of the planetary constituent materials that can be used to constrain models for planetary interior structure, formation, and evolution.

Learn More

• Planetary physics research at LLNL
• Come on in, the water is superionic: Studying the interior of giant planets
• First experimental evidence for superionic ice at planetary interior conditions
• Water at extreme conditions: Breaking the ice mold
• How LLNL scientists make new states of matter in a laboratory (YouTube video)
• Experiments validate the possibility of helium rain inside Jupiter and Saturn
• New way to study how elements mix in giant planets

Supernovae

Supernovae are brilliant explosions that mark the death of some stars. For a short time, a supernova can reach peak brightness of ten billion times that of our Sun, sometimes outshining its entire home galaxy. Because they are visible across vast cosmic distances, they are sometimes used as “standard candles,” objects whose intrinsic brightness can be inferred well enough to measure distances. These measurements help researchers study dark energy and the expansion history of the universe.

There are two primary pathways to a supernova:

Core-collapse supernovae occur when a star more massive than about 8 times that of our Sun exhausts its nuclear fuel and its core collapses. The collapse compresses the innermost region to extreme densities, converting the stored gravitational energy into a burst of neutrinos. That energy drives a supersonic shock into the surrounding material and explodes the star, leaving behind a neutron star or, in some cases, a black hole.

Thermonuclear supernovae occur when, through accretion or merger, the mass of a white dwarf exceeds a critical value known as the Chandrasekhar limit and becomes unstable. This state leads to a runaway thermonuclear explosion that completely burns up the white dwarf, transmuting much of the stellar material into radioactive nickel and other iron-group elements.

LLNL scientists combine laboratory experiments and supercomputer simulations to study the fundamental properties of these explosions. Experiments performed at the National Ignition Facility recreate the hydrodynamic instabilities seen in supernova remnants, as well as the collisionless shockwaves caused by the supernova ejecta colliding with interstellar gas.

In parallel, scientists at LLNL are modeling supernovae at unprecedented detail using the Laboratory’s high-performance computing resources, such as El Capitan, the world’s largest supercomputer. These simulations seek to create a “digital twin” of an exploding star that can reproduce measurements taken by telescopes of real supernovae and predict whether a given explosion left behind a black hole or neutron star.

LLNL scientists also contribute to the development of astronomical instruments for collecting supernova data, such as the Resolve detector on the XRISM x-ray space mission.

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• Experiments expose how powerful magnetic fields are generated in the aftermath of supernovae
• Experiments shed new light on supernovae
• Resolving astronomical mysteries

White Dwarf Stars

White dwarf stars are dense stellar embers, the final state for most stars after their cores can no longer sustain nuclear burning. As the star sheds its outer layers, the core contracts until quantum mechanical degeneracy pressure halts further collapse. Most white dwarfs retain more than half the mass of the Sun, packed into a volume around the size of the Earth. This dense configuration gives rise to a rich array of interesting physical conditions to study, including strong gravitational fields, high-density and strongly-coupled plasmas, and the possibility of thermonuclear instability leading to supernova explosions.

When isolated, white dwarfs are stable and can radiate away their reservoirs of thermal energy over billions of years. LLNL scientists develop physical models of white dwarf cooling that connect observed white dwarf temperatures to their evolutionary cooling history. These models serve as chronometers that are sensitive to billion-year timescales, enabling age estimates of many astronomical systems that include white dwarfs, including binary companions and the Milky Way’s stellar population.

Many white dwarfs have companion stars. Their strong gravity can draw material from a more loosely-bound companion, generating surface instabilities in the white dwarf shell. These instabilities can cause a nova, a temporary brightening by orders of magnitude that repeats at regular intervals. In more extreme cases, this interaction can lead to supernova explosions that destroy the entire white dwarf in one of the brightest phenomena observable in the universe. Studying novae and supernovae supports investigations into the large-scale structure of the universe.

White dwarfs also host the remnants of planetary systems. Their strong gravitational fields can lead them to tidally shred nearby rocky material, which then rains down onto the white dwarf surface. Astronomers use this atmospheric “pollution” alongside white dwarf models to infer the composition of the rocky debris and the geology of planetary bodies that supply the pollution. LLNL scientists are working to develop better physical models for understanding these processes and interpret white dwarf observations.

Models of white dwarf stars rely on detailed plasma physics calculations to provide accurate equations of state (relationships among key variables such as temperature and pressure) and opacities (how easily radiation passes through a material) in high density and high energy density regimes. Researchers use experiments at LLNL’s National Ignition Facility to probe extreme plasma conditions relevant to understanding the physics of white dwarf envelopes and benchmark theoretical plasma physics models. These models help us gain deeper insights into many rich physics problems, from cooling that reveals the age of the universe to the physics that leads to thermonuclear supernova explosions and hypervelocity runaway stars.

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• Record EOS measurement pressures shed light on stellar evolution
• Developing new capabilities to meet future scientific challenges