Kepler mission helps reveal the inner secrets of giant stars for the first time
Subtle oscillation patterns allow scientists to classify red giants by age, so they will be able to compare the fraction of stars that are at the different stages of evolution in a way they couldn't before.
University of Sydney astrophysicists are behind a major breakthrough in the study of the senior citizens of our galaxy — stars known as red giants. Using high-precision brightness measurements taken by the Kepler spacecraft, scientists have been able to distinguish profound differences inside the cores of stars that otherwise look the same on the surface.
The discovery was made possible by observations using NASA's powerful Kepler space telescope, and it is shedding new light on the evolution of stars, including our own Sun.
"Red giants are evolved stars that have exhausted the supply of hydrogen in their cores that powers nuclear fusion, and instead burn hydrogen in a surrounding shell,” said Tim Bedding from the University of Sydney. “Towards the end of their lives, red giants begin burning the helium in their cores."
The Kepler space telescope has allowed Bedding and colleagues to continuously study starlight from hundreds of red giants at an unprecedented level of precision for nearly a year, opening up a window into the stars' cores.
"The changes in brightness at a star's surface is a result of turbulent motions inside that cause continuous ‘star quakes,’ creating sound waves that travel down through the interior and back to the surface," Bedding said.
"Under the right conditions, these waves interact with other waves trapped inside the star's helium core. It is these 'mixed' oscillation modes that are the key to understanding a star's particular life stage. By carefully measuring subtle features of the oscillations in a star's brightness, we can see that some stars have run out of hydrogen in the center and are now burning helium, and are therefore at a later stage of life."
Travis Metcalfe from the United States National Center for Atmospheric Research compares red giants to Hollywood stars, whose age is not always obvious from 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."
Bedding and his colleagues work in an expanding field called asteroseismology. "In the same way that geologists use earthquakes to explore Earth's interior, we use star quakes to explore the internal structure of stars," he explained.
"We are very excited about the results,” Bedding said. “We had some idea from theoretical models that these subtle oscillation patterns would be there, but this confirms our models. It allows us to tell red giants apart, and we will be able to compare the fraction of stars that are at the different stages of evolution in a way that we couldn't before."
"This shows how wonderful the Kepler satellite really is,” said Daniel Huber, a student working with Bedding. “The main aim of the telescope was to find Earth-sized planets that could be habitable, but it has also provided us with a great opportunity to improve our understanding of stars."