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Structure & Evolution of Stars

JILA astrophysicists investigate the Sun, nearby stars, young stars, cool stars, and dying stars. Juri Toomre and the astrophysical fluid dynamics group study the internal workings of the Sun and other similar, but younger, stars. Their goal is to understand the dynamic processes that occur deep within these stars. They want to figure out how the internal motions and rotations of stars help build the magnetic fields that play a role in generating regular cycles of starspots and other phenomena. In their work, they use a powerful combination of supercomputer-based simulations and helioseismology (the use of sound waves produced by the Sun to study its inner workings).

The group believes that working out the details of the Sun’s internal structure and dynamics holds the key to understanding the 22-year sunspot cycle and other regular features such as the Sun’s consistent, but variable, rotation rate. The Sun’s equatorial regions rotate with a period of about 28 days, while the poles rotate with a period of about 35 days. This nonuniform rotation extends down through the Sun’s convection zone, but does not occur in its radiative interior, which rotates uniformly. At the base of the convection zone lies a remarkable layer of shear called the tachocline.

The Toomre group focuses on the interaction of the turbulent convective layer and the Sun’s differential rotation in building magnetic fields by dynamo action. Much of this theoretical work involves major 3D simulations of convection and magnetism within full spherical shells, capturing much of the solar convection zone. This work is complemented by observational helioseismology.

By observing sound waves in the Sun, the group has discovered large-scale meandering motions much like jet streams coexisting with weather patterns such as strong winds, jets, and tornadoes in the Sun's convection zone. These findings inspire and guide supercomputer simulations of convection and magnetism aimed at understanding how this zone of turbulent plasma is able to build the strong magnetic fields that are seen to erupt through the solar surface. The group's models can now largely explain the Sun's differential rotation with strong shear layers thought to be crucial elements in the operation of the solar dynamo.

Toomre has collaborated for more than a decade with investigators around the world in the study of the vibrational modes of the Sun, which are analogous to the vibrational modes of the Earth that can be excited by large earthquakes. These studies confirmed that the upper 29% of the Sun consists of a convective layer that experiences differential rotation. The rotation depends both on depth and distance from the solar equator. Toomre’s group compares its simulations of the Sun with another model of younger Sun-like stars that spin 3–10 times faster than our Sun.

When it was formed more than 4.5 billion years ago, the Sun may have rotated 50–60 times faster than it does today. However, as a star like the Sun ages, it loses material as stellar winds, a process that also causes its rotation to slow. At this stage in its life cycle, the Sun has become a relatively leisurely rotator.  Learning more about the origin and evolution of the elderly Sun’s solar dynamo requires the study of the interaction of heat-driven convection, buoyancy, and electromagnetic forces in much younger stars.

The comparison of circulation patterns in the convection zones of young stars with those in the Sun has yielded new insights into the formation and maintenance of the solar dynamo. For instance, detailed simulations of magnetic fields inside young stars showed that rapidly spinning stars create large-scale magnetic fields throughout their convection zones. These fields are organized into large wreaths with different polarities above and below the stellar equator. These large wreath-like structures (shown at left) are not shredded apart by the young star’s turbulent convection. The evolution of these structures in young stars continues to shed light not only on how the solar dynamo functioned in the past, but also how it works today.

The appearance of the wreaths appears to be a major developmental step in building magnetic fields inside stars. They appear to be the roots of a process that creates the dynamo action observed in the Sun. Because of their simulations of young stars, the Toomre group was able to figure out that the solar dynamo operates simultaneously in the Sun’s tachocline and within its convection zone.

The Toomre group is now exploiting the capabilities of the University of Colorado-Boulder’s new supercomputer Janus for simulations of the Sun and other stars. The researchers are developing detailed simulations of the flows inside F-type stars, which are a little bigger and hotter than the Sun and have an unusual near-surface shear layer. They are also looking at helioseismic data from the Sun obtained by the Solar Dynamics Observatory to analyze real subsurface flows and compare them with simulations. Finally, they are exploring the relationship of buoyant loops in the Sun’s magnetic field to the boundary between the Sun’s convection zone and its radiative interior.

Jeffrey Linsky is expanding his studies of T Tauri stars and planets with data from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope. He continues to investigate the evolution of these very young stars, which are surrounded by accretion disks where planets form. He has discovered that the strong Lyman-a radiation of T Tauri stars photoexcites molecules of hydrogen and carbon monoxide in accretion disks. This process produces bright, fluorescent radiation. Linsky and his colleagues probe this radiation using spectra obtained by COS.

Linsky also studies the coronae (hot ionized plasmas in the outer atmospheres of most stars), chromospheres (regions between photospheres and coronae), stellar winds (analogs of the solar wind), and circumstellar envelopes of stars in the Milky Way Galaxy. He compares these structures among different sizes and types of stars. He also wants to know how such stellar regions are affected inside binary star systems.  He has detected stellar winds for 13 nearby cool stars. (Our Sun is a cool star.)

Using computer modeling, he was able to deduce the relationship between the strength of a stellar wind and the age of a star and the x-ray luminosity of its corona. Understanding the evolution of stellar winds with time allowed him to deduce that a strong solar wind, which appeared about 700 million years after the Sun began to shine, literally blew away the dense atmosphere of the planet Mars. Linsky is now undertaking additional studies of cool stars using data from COS.

Richard McCraywho is now retired, focused for more than 25 years on the death of a single star. He  combined data analysis with theoretical simulations of SN1987A, the brightest supernova visible from Earth since 1604. It occurred when a blue supergiant star exploded in the Large Magellanic Cloud, a galaxy located 160,000 light years from Earth. Thanks to the Hubble Space Telescope and the Chandra and XMM-Newton observatories, this is the first time in history astronomers have been able to witness the evolution of a supernova unfolding in real time.

McCray studied the composition, temperature, and density of the supernova's debris and investigated its circumstellar ring system. He and his colleagues correctly predicted that the ring system would become a thousand times brighter in 1997 when struck by the blast wave from the stellar explosion. Since then, hot spots appeared in the ring system as more parts of it were clobbered by the supernova blast. McCray correctly predicted the 2007 merger of the hot spots. The entire ring system lit up as it was entirely overtaken by the supernova's shockwave.

McCray's observations led him to conclude that the giant star lost most of its outer layers about 20,000 years before it exploded. Next, a high-speed wind blowing off the star's surface carved out a cavity in the cloud. Then, the supernova's shockwave interacted with dense fingers of gas that protrude inward from the edge of the circumstellar gas cloud. As the shockwave rides further into the cloud, it will continue to illuminate the star's past.