Ann Boesgaard: Looking for Lithium

Ann Boesgaard uses the rare elements, lithium (Li), beryllium (Be) and boron (B) to probe the invisible interiors of stars. Lithium is familiar to us as a key element in cell phone and camera batteries.

The gases near the outer edges of many stars circulate up and down due to convection. As the gases move down beneath the surface of the star they heat up. If their temperature gets to above 2 million degrees K, then any lithium in the gas is destroyed. The destruction of beryllium and boron happens in somewhat deeper layers, where the temperature is higher.

By studying the abundances of Li, Be, and B spectral lines at the surface of a star, we can therefore learn something about the temperature structure in its hidden interior and the complicated process of convective gas circulation in stars.

Starting in 1983, Boesgaard’s team used the telescopes on Mauna Kea and later the Hubble Space Telescope to study Li, Be, and B in different-sized stars in the young Hyades cluster. The latter two elements required more challenging observations, since their destruction occurs at a greater depth than Li.

The abundance of lithium in stars in the Hyades cluster shows shows a strong minimum at stellar temperatures of about 6700K.

The team made many important discoveries. The destruction of these elements depends on how heavy the stars are and how old. Stars a little more massive than the sun show severe effects of depletion, but only when they get older; it takes time for the destruction process to become efficient. The depletion is strongest for Li and less for Be and B, because the convective circulation barely reaches the hotter layers needed for the destruction of these elements.

“The light element Li was formed during the Big Bang, so measuring its abundances helps us  understand the physics of the early universe.”    —  Ann Boesgaard