Helioseismology....very interesting field of study. Looking at the way sound/pressure waves travel through the gases that makeup stars. They can tell quite a lot about the way sound waves behave as they pass through stars....density, temp, elemental abundance, internal physical structure and the processes occurring at depth etc etc etc.
Here's a couple of books to have a look at....
http://www.fishpond.com.au/Books/Sci...elioseismology
http://www.fishpond.com.au/Books/Sci...elioseismology