Helioseismology

Abstract

Sound waves generated in the solar convection zone propagate through the Sun and cause it to vibrate. Resonant vibrations are detected by measuring wavelike motion that they produce on the solar surface; by analyzing them we can learn about conditions in the solar interior. At present we have only limited data but already we have drawn an important conclusion. We have inferred that outside the core of the Sun, and beneath the subsurface convection zone, the abundance of helium is about 25% by mass. It is generally believed that the intermediate region in the Sun is uncontaminated by the products of the nuclear reactions in the core and by material that might have fallen into the Sun from interstellar space. If that is indeed the case, we have measured the helium abundance of the gas cloud from which the Sun condensed some 4.7 thousand million years ago. This information is relevant to our understanding of the early stages of the evolution of the Universe. It is exciting that there are hints in the vibration data that might even enable us to determine whether the Sun really has remained unmixed, and hence whether we have a faithful record of the primordial mixture. There is also evidence in the data concerning the Sun's internal rotation. It appears that the core of the Sun rotate faster than the surface, but by how much is a subject of some controversy. It is very important to settle the matter, because the result is an essential step in one of the most important tests of Einstein's General Theory of Relativity. What evidence we have at present does not contradict Einstein's theory.