The Inner Lives of Red Giants

Just as in Hollywood, the age of a star is not always obvious if you look only at the surface. During certain phases in a star's life, its size and brightness are remarkably constant, even while profound transformations are taking place deep inside. For most of their existence, stars shine from the energy released by nuclear reactions that convert hydrogen into helium, but eventually they begin to burn the helium in their cores to synthesize heavier elements, such as carbon and oxygen. In the latest issue of Nature, astronomer Tim Bedding and his collaborators demonstrate a new technique for distinguishing between these life stages, using continuous 'starquakes' to probe the deepest regions, where the changes are most dramatic.

The objects examined by Bedding and colleagues are known as red giants, the bloated fate of stars such as our Sun as they begin to exhaust their primary source of energy -- the hydrogen near the center that powers nuclear fusion. The resulting helium accumulates in the core, forcing hydrogen in a surrounding shell to burn more vigorously than before. About 5 billion years from now, these processes will gradually cause our own star to expand to more than 100 times its present size, becoming a red giant and destroying some of the inner planets in our Solar System. Stars that were born before the Sun, as well as heavier stars (which evolve more quickly), have already reached this phase of stellar evolution.

Like the Sun, the surface of a red giant seems to boil as convection brings heat up from the interior and radiates it into the coldness of outer space. These turbulent motions act like continuous starquakes, creating sound waves that travel down through the interior and back to the surface. Some of the sounds have just the right tone -- a million times lower than the audible range for humans -- to set up standing waves (known as solar-like oscillations) that cause the entire star to change its brightness regularly over hours and days, depending on its size. Inferring the properties of stars from these periodic brightness changes is a technique known as asteroseismology. The sound waves generated near the surface of a red giant can interact with buoyancy waves (rather like the waves in the ocean) that are trapped inside the helium core. Under the right conditions, the two types of waves can couple to each other, changing the regularity of the brightness changes at the surface. These 'mixed' oscillation modes are much more sensitive to structure in the core than are the uncoupled sound waves that sample only the stellar envelope.

The innovation that allowed Bedding and colleagues to distinguish between red giants at different life stages emerged from precise observations by the Kepler space telescope. Launched in March 2009, Kepler stares at a large patch of sky near the constellation Cygnus, monitoring the brightness of more than 156,000 stars with the goal of detecting habitable planets like Earth. The mission has been extremely successful at finding alien worlds, but it is also revolutionizing the study of stellar oscillations by providing many months of continuous data for thousands of stars. Earlier efforts to study red giants from ground-based telescopes were hampered by both the daily interruptions of sunlight and the limited duration of the monitoring.

As mentioned before, the trouble with red giants is that they all look nearly the same on the outside, regardless of their mass and age. Bedding and colleagues sought to determine these properties for the hundreds of red giants observed by the Kepler satellite, to measure precisely when stars of a given mass would shift from burning hydrogen in a shell to helium in the core. The regular pattern of standing waves is insufficient to pinpoint which energy source makes a particular red giant shine, but the mixed oscillation modes exhibit a unique pattern. By deciphering this pattern, Bedding and collaborators demonstrate how the two life stages of red giants can be separated using asteroseismology.

The life story of a red giant theoretically depends not only on its age but also on its mass, with stars smaller than about twice the mass of the Sun experiencing a sudden ignition known as a helium flash. The temperature required to fuse helium is significantly higher than that needed for hydrogen, and in low-mass stars the helium accumulates in the core at very high density until it reaches a critical size and ignites almost instantaneously. In more massive stars, the transition to helium core burning is gradual, so the stars exhibit a wider range of core sizes and never experience a helium flash. Bedding and colleagues show how these two populations can be distinguished observationally using their oscillation modes, providing new data to validate a previously untested prediction of stellar evolution theory.

This extraordinary peek into the inner lives of red giants was made possible by just the first year of observations from the Kepler mission, which is scheduled to operate for at least 3.5 years and might be extended by NASA for a further 2.5 years. The picture that emerges from asteroseismology will steadily improve as the observations continue, so we can expect even better results for the stars examined by Bedding and colleagues, as well as similar measurements for other red giants, in the near future.