Combining more than 1400 observations made over a 21-year period, astronomers in Belgium and Poland have obtained a clearer picture of how the movement in a star’s core affects its evolution. Conny Aerts at the Catholic University of Leuven and colleagues studied a massive B star known as HD 129929 using a technique known as asteroseismology. They hope that their results will help improve current models of stellar evolution (C Aerts et al. 2003 Sciencexpress 1084993).
Stars are made up of several gas layers with different temperatures, pressures and chemical compositions. Asteroseismology allows astronomers to study these layers by analyzing ‘starquakes’ – or the oscillations of a star’s outer crust. These oscillations show up as variations in the star’s brightness and their frequencies can be used to probe the structure of specific internal layers in the star.
HD 129929 has a well-developed core but it is not known how this extends into the more stable upper layers. This movement depends on two mechanisms: ‘core overshooting’, which is the mixing of material from the core to the upper layers, and the rotation of the star itself. These two effects are difficult to separate but are important in determining how a star evolves.
Aerts and colleagues analyzed data from the 0.7 metre Swiss telescope at La Silla Observatory in Chile during three-week periods over 21.2 years. The researchers measured six oscillation frequencies and found evidence for core overshooting that could not be caused by the star’s rotation. This is because it rotates at about 2 km per second, which is too slow.
“The extent of overshooting, which was unknown so far, directly determines the lifetime of the star,” Aerts told PhysicsWeb. As core overshooting is not considered in star evolution calculations, the researchers hope that their results will help to improve these models.