p modes and g modes in stars

Today was the first of two 90-minute pedagogical lectures at MPIA by Conny Aerts (Leuven), who is also an external member of the MPIA. I learned a huge amount! She started by carefully defining the modes and their numbers ell, em, and en. She explained the difference between pressure (p) modes and gravity (g) modes, which I have to admit I had never understood. And I asked if this distinction is absolutely clear. I can't quite tell; after all, in the acoustic case, the pressure is still set by the gravity of the star! The g modes have never been detected for the Sun, but they have been detected for many other kinds of stars, and they are very sensitive to the stellar interiors. The relative importance of p and g modes is a strong function of stellar mass (because of the convective and radiative structure in the interior). She also showed that p modes are separated by near-uniform frequency differences, and g modes by near-uniform period differences. And the deviations of these separations from uniformity are amazingly informative about the interiors of the stars, because (I think) the different modes have different radial extents into the interior, so they measure different integrals of the density. Amazing stuff. She also gave a huge amount of credit to the NASA Kepler Mission for changing the game completely.

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