Changes in these elements' abundances generally result from processes like mixing related to convection or rotation, diffusion (the slow drift of atoms to greater or lesser depth inside a star), the slow loss of material from a star’s surface, and magnetic fields. Knowledge of the abundances of these elements can distinguish between physical mechanisms that depend differently on depth. The abundances of these elements also have implications for other areas of astronomy, such as galactic chemical evolution and cosmology.
Importantly, a fraction of the lithium in the universe today was made in the Big Bang; if we want to know how much, scientists need to account for what happens to the lithium in the oldest stars, and knowing the amount of Big Bang lithium is a clue to the density of matter when the nuclei first formed. Moreover, the study of sound waves on a star's surface (known as helioseismology) informs us that standard solar models describe the interior solar structure remarkably accurately, especially when effects due to diffusion are taken into account.
However, the sun has less than one percent of the lithium that it formed with. Deliyannis finds that like the sun, nearby sun-like stars, which comprise approximately 90 percent of the stars in the universe, contain amounts of lithium well below astronomers' standard predictions. These otherwise well-behaved stars have lost much more lithium than standard theory predicts. These data clearly suggest that additional, physical processes not included in standard stellar models must be active inside stars.
In his research Professor Deliyannis uses the abundances of lithium, beryllium, and boron to test the various mechanisms described above, using both stars in the field and stars belonging to star clusters, like the Pleiades or the Hyades star clusters visible in the night sky. Star clusters are especially useful because their ages are known and stars in star clusters formed together during a short period of time, and thus have the same determinable age and overall composition, but different masses. Observations of lithium in many stars in a single cluster provide astronomers information about how the lithium abundance depends on stellar mass, and studying clusters of different ages and compositions informs us about how the lithium abundance changes and how it depends on composition.
This research requires observations of at least dozens of stars in each cluster, and notably, Deliyannis’ research takes advantage of the Hydra Multi-fiber Spectrograph on the 3.5-meter WIYN telescope at Kitt Peak in Arizona, operated by a consortium of which Indiana University is an active partner. A robot positions optical fibers at the locations of specific stars in each target star cluster, and the optical fibers carry the stars’ light from telescope to a spectrograph, which records the spectrum of each individual star.
This efficient process allows many stars to be observed simultaneously. Analysis of the spectra then provides insight into the lithium abundance as well as other information about the stars.
Deliyannis' research on the lithium abundance in star clusters tells us that the loss of lithium is mostly due to the way stars gradually spin down, rotating more and more slowly as they age. Tops become unstable as they spin down; stars, which are giant balls of gas, become even more unstable. Increasingly, stars rotate with different speeds in different layers in their interiors, which triggers mixing between the layers. And so, knowing just how fast the lithium disappears helps astronomers understand how that instability affects the stars’ interiors.
“To put it simply, I want to understand what makes stars tick,” said Professor Deliyannis. “And the more astronomers know about stars, the better we can understand our own star, the sun. And because scientists know the sun is capable of highly energetic outbursts, driven by processes from deep in the interior, understanding the sun better helps us predict, and prepare for, such outbursts, which are capable of bringing down communications and power distribution systems world-wide.”
Deliyannis has received funding for his research from the National Science Foundation and NASA. He has been on the IU Astronomy faculty since 1997. He received a B.S. in physics from Illinois Institute of Technology, M. Phil. and M.S. degrees in physics and a Ph.D. in physics from Yale University.
