Structure & Evolution of Stars

JILA astrophysicists investigate nearby stars, young stars, cool stars, and dying stars. Juri Toomre and the astrophysical fluid dynamics group study the internal workings of stars like the Sun. Their goal is to understand the dynamic processes that occur deep within the Sun and stars like it. To find out, they use a powerful combination of computer 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 turbulent convection and differential rotation to build magnetic fields by dynamo action. Much of this theoretical work involves major 3-D 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 major 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.

The Toomre group has recently compared its simulations of the Sun with another model of young Sun-like stars that spin 3–10 times faster than our Sun. The comparison of circulation patterns in the convection zones of these young stars with those in the Sun has yielded new insights into the formation and maintenance of the solar dynamo. These studies suggest that the solar dynamo operates simultaneously in the tachocline and within the convection zone. The Toomre group is also studying core convection and dynamo processes in A-type stars and is starting similar work on the more massive O- and B-type stars.

Jeffrey Linsky is gaining a better understanding of our Sun’s origins with observations (in the infrared, visible, and X-ray regions) of stellar nurseries such as the Pillars of Creation in the Eagle Nebula. He and his colleagues have identified a Sun-like protostar on the edge of one pillar underneath a very bright star. This protostar is inside an evaporating gaseous globule (EGG), one of 73 such structures visible on the pillars. The EGGs were created when shockwaves from nearby hot stars compressed regions of cold gas inside the pillars, creating dense protostellar cores. Then UV bombardment by massive stars scoured away the surfaces of the pillars, revealing the EGGs. This process either evaporated the EGG core or produced a star with a planetary system. Only four of the EGGs in this region were actually massive enough to form a star. Although these particular EGGs were likely destroyed by a nearby supernova, they are the youngest stars every imaged by astronomers.

Linsky and his group are expanding their studies of young stars and planets with data from the new COS on the Hubble Space Telescope. They continue to investigate the evolution of very young stars surrounded by accretion disks where planets form. They monitor the very bright ultraviolet (UV) light emitted by such stars as gas falls into them. The group is also studying the UV light emitted by failed stars (i.e., brown dwarfs) as they slowly contract. Finally, the group is investigating the UV emissions of Jupiter-like planets in very close orbit to their stars. The atmospheres of these planets (known as "roasters") are in the process of being boiled off by stellar radiation. The researchers are interested in determining the chemical composition of these atmospheres. They want to see if they can detect Aurora Borealis-like emissions from the planets.

The group 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. The researchers compare these structures among different sizes and types of stars. They also want to know how such stellar regions are affected inside binary star systems.

Linsky has also 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 undertaking additional studies of cool stars using data from the COS.

Rosalba Perna investigates the X-ray spectra of highly magnetized neutron stars. She wants to understand the mechanism underlying this emission and derive the temperature distribution on the surface of neutron stars. This information will shed light on the structure of the high magnetic fields of these stars.

Richard McCray has focused for more than 20 years on the death of a single star. He has 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 has 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 have appeared in the ring system as more parts of it have been 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 thousand years before it exploded. Then a high-speed wind blowing off the star's surface carved out a cavity in the cloud. Currently, the supernova's shockwave is interacting 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.